Foundation Design and Practice. an Economic View 9780231882514

Table of contents :
Acknowledgments
Contents
Figures, Tables
Introduction
Part One. The Preliminaries
1. A Quick Survey
2. Choosing the Site
3. Investigating the Chosen Site
4. Core Borings
5. The Demands of the Superstructure
6. Breakdown of Design and Specification Cost Factors
Part II. Foundation Types
7. The Elimination Table
8. Spread Footings, Alone and in Combinations
9. Wood Piles, Untreated and Treated
10. Cast-In-Place Uncased Concrete Piles; MacArthur, Simplex, and Other Types
11. Cast-In-Place Uncased Concrete Piles: Western Foundation Corporation Types
12. Light-Shelled Cast-In-Place Concrete Piles
13. Composite Piles
14. Precast Concrete Piles
15. Medium-Wall Steel Pipe Piles
16. Structural Steel Piles
17. Open-End Pipe Piles to Rock
18. Caissons
19. Drop-Shaft or Free-Air Method of Forming Caissons
20. Drilled-In Caissons
21. Grouping in General
22. Selecting the Bid Group
Part Three. Laboratory and Field Testing
23. Field Testing: Soil-Load Tests
24. A Method of Pile-Load Testing
25. What a Designing Engineer Should Know About Tools, Plant, and Dynamic Driving Formulas
26. Methods of Installation Other Than Driving
27. Specifications
28. Foundation Design Based on Performance or Functional Criteria
29. Load by Test: Purpose and Uses
30. Choice of Working Load Per Pile
Part Four. Specifications and Contracts
31. After the Bid Group
32. Legal Requirements
33. The General Clauses of a Specification
34. Contract Forms Other than Specifications
35. Writing a Group Specification
Part Five. Building Codes
36. Using The New Building Codes to Reduce Cost
37. Explanation and Discussion of Certain Descriptive Terms
Part Six. Basic Design
38. A Five-Force Approach
39. Dock Faces, Railway Trestles, Buildings, And Retaining Walls
40. Bridge Piers
41. Graphic Method Of Selection Of Foundation Types
42. Basis Of Load-Carrying Capacity Of Foundations
Index

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FOUNDATION DESIGN AND PRACTICE AN ECONOMIC VIEW

Foundation Design and Practice AN ECONOMIC VIEW

By J. H. Thornley

COLUMBIA U N I V E R S I T Y

PRESS

Morningside Heiglits New York 1959

Library of Congress Catalog Card Number: 59-7233 Copyright © 1959 Columbia University Press, New York Published in Great Britain, Canada, India, and Pakistan by the Oxford University Press London, Toronto, Bombay, and Karachi

Manufactured in the United States of America

ACKNOWLEDGMENTS

FOR permission to reproduce illustrations of their products, I am grateful to the following companies: MacArthur Concrete Pile Corporation, for Figure 2, Improved MacArthur Pedestal Pile; Union Metal Manufacturing Company, for Figure 10, Union Metal Two-Section Monotube Pile; and Armco Drainage & Metal Products, Inc., for Figure 11, Armco Spiral Butt-Welded Foundation Pipe. I am also grateful to the editors of Civil Engineering and the editors of Engineering News-Record for their permission to reprint in Chapters 39 and 40 material originally written for their respective publications. J . H. THORNLEY

New York City March, 1959

CONTENTS

Introduction

1 PART ONE. THE PRELIMINARIES

1. 2. 3. 4. 5. 6.

A Quick Survey Choosing the Site Investigating the Chosen Site Core Borings The Demands of the Superstructure Breakdown of Design and Specification Cost Factors

5 8 10 15 28 33

PART TWO. FOUNDATION TYPES 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

The Elimination Table Spread Footings, Alone and in Combinations Wood Piles, Untreated and Treated Cast-In-Place Uncased Concrete Piles; MacArthur, Simplex, and Other Types Cast-In-Place Uncased Concrete Piles: Western Foundation Corporation Types Light-Shelled Cast-In-Place Concrete Piles Composite Piles Precast Concrete Piles Medium-Wall Steel Pipe Piles Structural Steel Piles Open-End Pipe Piles to Rock Caissons Drop-Shaft or Free-Air Method of Forming Caissons Drilled-In Caissons Grouping in General Selecting the Bid Group

39 52 55 61 72 81 88 101 108 116 118 123 129 131 133 138

CONTENTS

viii

PART THREE. LABORATORY AND FIELD TESTING 23. Field Testing: Soil-Load Tests 145 24. A Method of Pile-Load Testing 150 25. What a Designing Engineer Should Know About Tools, Plant, and Dynamic Driving Formulas 159 26. Methods of Installation Other Than Driving 163 27. Specifications 170 28. Foundation Design Based on Performance or Functional Criteria 175 29. Load by Test: Purpose and Uses 180 30. Choice of Working Load Per Pile 185 PART FOUR. SPECIFICATIONS AND CONTRACTS 31. After the Bid Group 32. Legal Requirements 33. The General Clauses of a Specification 34. Contract Forms Other than Specifications 35. Writing a Group Specification

193 195 201 209 211

PART FIVE. BUILDING CODES 36. 37.

Using the New Building Codes to Reduce Cost Explanation and Discussion of Certain Descriptive Terms

217 223

PART SIX. BASIC DESIGN 38. A Five-Force Approach 39. Dock Faces, Railway Trestles, Buildings, and Retaining Walls 40. Bridge Piers 41. Graphic Method of Selection of Foundation Types 42. Basis of Load-Carrying Capacity of Foundations Index

231 234 247 261 278 287

FIGURES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

Cross Sections of an Uncased Type of Pile When Driven in a Group of Two or More 62 Improved MacArthur Pedestal Pile (Type Two) 65 Western Compressed Pile 74-75 Western Pedestal Pile 76 Button-Bottom Pile 85 Sample Soil Profile for Splice Specification 93 Driving Steel Pipe Composite by Projectile Method 96 Steel Pipe Composite with Slip-Joint 97 Steel Pipe Composite with Locked-on Joint 97 Union Metal Two-Section Monotube Pile 108 111 Armco Spiral Butt-Welded Foundation Pipe Swage Bottom Pile 113 Drilled-In Caisson 132 Loading and Gauging Assemblies to Be Used in Making a Soil-Load Test: I 146 Loading and Gauging Assemblies to Be Used in Making a Soil-Load Test: II 147 Loading and Gauging Assemblies to Be Used in Making a Soil-Load Test: III 148 Loading Devices in Pile-Load Test 155 Measuring Devices in Pile-Load Test 156 Typical Application of the Control of Heave for the Protection of Adjacent Property: I 166 Typical Application of the Control of Heave for the Protection of Adjacent Property: II 167 Railway Bridge Pier 232 Anchored Bulkhead 234 Design for Bulkheads and Dock Walls 235 Standard Layout for Diaphragm Wall 236 Basic Design Layout for Diaphragm Wall 236 Detail of Basic Design Layout for Diaphragm Wall 237 Driving of Four Exactly Similar Piles in Exactly Similar Soils 237 Battered Pile "Borrowing" Needed Vertical Load to Develop Axial Reaction 238 Addition of Tensile Anchorage 239 Vertical Piles Eliminated 240 Piles Used as Earthquake Protection 243 Reinforced Concrete Gravity Type Wall with Counterforts 244 Retaining Wall Design Based on the Use of Battered and Anchored Piles 245 Three Typical Bridge Pier Designs 247

X

35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

FIGURES 248 Application of Battered and Anchored Caissons to a Suspension Bridge Design Three Possible Failures in a Standard Gravity Type Freestanding Pier 249 One Possible Failure in a Standard Gravity Type Freestanding Pier 249 Adding Tension Caissons at Right 250 Adding Tension Caissons at Right and Left 250 Caissons Placed on the Batter 250 Setting up Compression and Tension with Vertical Caissons 251 Increasing the Efficiency of Compression and Tension by Changing from Vertical to Battered Caissons 252 255 Battered Piles Acting in Axial Compression Pile Fully Restrained at Ground Line 256 Triangle of Forces 257 Typical Soil Profiles 262-63 Twenty Foundation Types in Commercial Use 264-67 A Failed Pile 282 Effect of Friction as a Force Resisting Movement of a Pile under Load 284

TABLES 1. 2. 3. 4. 5.

Structures Classified According to Permissible Foundation Settlements Elimination Table Foundation Cost Comparison Based on End-Bearing Piles Comparative Foundation Costs Due to Change in Pile Load Capacity Comparative Foundation Costs for a Typical Heavy Building

31-32 40-45 187 188 189

FOUNDATION DESIGN AND PRACTICE AN ECONOMIC VIEW

INTRODUCTION

ALL that the layman sees of any completed building project is the superstructure and the site on which—to him—it sits. Engineers know that it doesn't "sit." It has been "planted" deep in the soil, and between the soil and the superstructure is a rootlike link on which its continued existence depends—the foundation. The building of a foundation, from the inception of the idea to the finished structure, requires a variety of skills, much experience, and a considerable amount of study of the various solutions, both good and bad, which have been tried out by others. There are many questions to be answered and problems to be solved, but they can be divided into two broad groups: engineering problems and economic problems. The engineering problems resolve themselves into questions of safety and suitability. The first question is that of the loads which the superstructure will impose upon the foundation—and this is not always the simple problem of dead and live vertical loads. The loads may also be those imposed by seismic disturbances, by heavy or high-frequency vibration, or by horizontal thrusts due to wind, water, or ice action. The second question the engineer must answer concerns the permissible tolerances of movements which govern the type and use of the structure under design. A foundation which would be entirely suitable for a hotel or an office building might be disastrously bad if used for a building housing machines or gauges operating to a high degree of accuracy.

Having studied his loads—all of them— and having determined the permissible tolerances, not only for settlements but for all movements under forces liable to act on the foundation, the engineer will then turn to the third problem—an analysis of the soil types and conditions to be encountered on the site. The next step is to link up the two sets of facts, above and below ground, and to choose the type or types of foundation which will satisfy the demands of the structure and the soil. Up to this point, the problems have been primarily engineering problems. But the fifth step takes us into another field, where the problems become primarily economic. Any one of several solutions may show the way to a perfectly safe design which will meet fully all requirements of the superstructure and the soil, but of all these possible designs, generally only one will give the desired safety and suitability at the least possible cost. Only that one design will, therefore, be economically correct. Few if any of the engineers who design foundations would claim to have all the information they need to solve the engineering problem stored in their heads. But they do know where to get it, because a great many excellent books have been written, covering practically every phase of structural design and soil analysis. Very little has, however, been written about the bearing of design and construction methods on the economics of the foundation problem. The reason seems obvious. The man who should know the most about the effect of

2

INTRODUCTION

design and specifications upon the cost of a foundation is the contractor who will translate the blueprints and specifications into steel and concrete. He spends his life betting on his ability to assess costs correctly. He is apt, however, to be a pretty busy man, and he rarely gets around to writings books.* It is the primary purpose of this book— written by an engineer who is also a contractor—to mark out the road to the type, design, and finally the construction of the foundation which will meet the engineering necessities at the least possible cost to the owner. In general the sequence followed in this

book has been that of the growth of the foundation itself, starting with the owner's decision to build and concluding with the payment of the last bill. The point of view has been primarily that of the designing engineer. • Of course there are a few exceptions. The two very practical and authoritative books Underpinning (2d ed., 1950) and Cofferdams (2d ed., 1956), both written by Edmund Astley Prentis and Lazarus White of the contracting firm of Spencer, White & Prentis, Inc., are excellent examples of the useful treatises which only master builders could write. While underpinning and cofferdams might reasonably be classified as foundations, this book has avoided the subjects entirely, since the author feels they have already been well handled.

PART ONE • THE PRELIMINARIES

1 A QUICK SURVEY

T H E prospective owner of any proposed structure has the final word within the limits of soundness and safety—because he pays the bills. The engineers work for him, whether they design, purchase, or install. If he likes what they do and continues to like it, they make money. And the best way to make him like it is to provide what he wants at or below what he thought it would cost. The problem is as simple as that—to state. But to solve it is a full-time job for the best engineering and construction brains. Though this book is addressed primarily to engineers, the author hopes that at least a few prospective owners will read it. To them he would say: "If there is any choice as to the site of your next project, pick out your designing engineer when you decide upon your real estate broker. You may be your own best judge as to the transportation facilities, the availability of public utilities, and many other factors which affect the desirability of a site, but there is at least one important item on which engineering advice is certainly needed—the subsurface soil conditions." After determining the structural loads the first step toward the selection of the site is to decide on the nature and extent of the preliminary soil investigation. The decision will, of course, have to bear a proper relationship to the size, nature, and cost of the proposed structure and to the number of sites to be checked. But it is usually possible to make, quite cheaply, investigations that will give warning of any subsurface conditions likely to cause serious trouble later. Such trouble

entails unnecessary expense, which the owner, in the last analysis, will have to pay. The subject of this book is foundations— not the first section of the project to be designed (because no foundation can be designed till the working loads and settlement tolerance have been determined), but the first to be installed and basic to all the rest, structurally and financially. Yet the owner and too often the engineers are less up-to-date in their thinking about foundations than about almost any other part of the job. My intent and purpose is to set up a logical, modern method of answering one basic question: "What are the studies to be followed to assure that the foundation of any proposed structure will be correctly chosen, properly specified, and well installed, and all at the least possible cost?" What engineering data is needed? First, we need full data on the purpose to be served by the proposed structure and, as a corollary, the loads to be transmitted to the foundation and the permissible tolerances as to net, gross, and differential settlements. We also need to know the desired permanence of the structure and the necessary resistance of the foundation to vibration and impact, if these occur. At least a general, tentative design of the superstructure must have been made before the information necessary to the foundation design will be available. An approximation of all facts in this category should be readily obtainable. Second, we must have all pertinent facts

6

THE PRELIMINARIES

as to the nature of the supporting soil to which the superstructure loads are to be transmitted. These facts may include results of test pits, soil load tests, laboratory analysis of soil samples, the driving of test piles or other probes, the loading of test piles, and reports as to the behavior of adjacent foundations. This information is generally not so readily available as that required under the first category. But we cannot start thinking about foundations until we have it in hand. Having obtained these two sets of facts, the engineer's next step consists in "marrying" the data from the first category to data from the second. Usually the result would be several possible engineering solutions, equally satisfactory in that cach would meet all engineering requirements. The evaluation of the comparative costs of the various solutions which have qualified under the engineering study follows. The answer to the question of cost is not so definite or so easily obtainable as that of engineering qualifications. However, the study of a table showing approximate comparative costs of various types of foundation leads to the elimination of some of those previously qualified under the engineering study. This may still leave several general types or classifications ranking so close to each other that it would be unwise to eliminate any of them definitely. Their theoretical advantages, however, could be minimized by several other factors, not strictly engineering or economic, though they often affect the cost. The most important of these is the effect of the local building code, if the proposed structure is in a territory where a code governs. A second consideration, sometimes of importance, is the effect of the rulings and scales of pay of local unions. These affect both the cost and the time of completion of the job. They are discussed further in Chapter 27. The final step is then obvious: specify and ask bids on the several types of foundation

which remain on the list of possibles. The dollars-and-eents answer is conclusive. But here there is a joker. All too frequently insufficient attention is paid to the details of design and the form of specification and contract which will assure engineering sufficiency and the lowest consistent cost for each type of foundation. Hence, although the bulk of this book is concerned with engineering and economic facts, together with the best methods of arriving at and later making use of them, information regarding various forms for requests for bids—specifications and contracts —is also included. It may not be amiss to cite a few of the wasteful and expensive errors frequently met in foundation design. Here are some of them, along with suggested correctives: 1. In wood pile jobs, obsolete requirements for material and size are too often used instead of American Institute of Lumber Construction grades and specifications which reflect the modern market. If these obsolete requirements are insisted upon, they involve unnecessary expense. Also, pile inspection at the site is called for, instead of inspection and acceptance by an inspection company in the field. This will save transportation cost on the culls. If adequate subsoil information has not been obtained beforehand, piles may be ordered in extravagant lengths, which must later be cut off. A knowledge of the merits of cast steel removable coned hoods, particularly for the driving of creosoted piles, should be part of the designing engineer's equipment. 2. In concrete pile jobs, cap specifications often seem to have been written by Rip Van Winkle. "Punching shear" dates back to 2,000pound concrete. Cap design should be moving forward to the circular cap, in which the coiled metal strip takes circular shape automatically, if backfilled on the outside, so that concrete pressure is no problem.

A QUICK SURVEY Again, if an adequate soil investigation program has not been carried out in advance and constructive thinking based on it by the designing engineer, reliable data on which a competent contractor can give approximate figures for the cost of the job will not be forthcoming. If the contractor has to add in a lump sum to make up for the vagueness of the data, his figures will naturally be high, the cost of the job will be boosted, the owner will be out of pocket, and the designing engineer will not have added to his reputation. Note. The engineer should be in position to submit general characteristics—¿hat is, type, number, and length of piles, soil profile, variation of cutoffs—to competent contractors for approximate figures. Let him then add a test program and approximate cap costs. If he allows % yard to 1 yard of reinforced concrete capping per pile on piles of 30 to 60 tons capacity and, as a very rough check, allows excavation, forms, reinforcing, and concrete at $35 to $50 (depending on locality) per cubic yard, he will at least have a basis for the choice of the most efficient foundation unit loading. The per pile quantity of concrete in caps will increase with the number of piles in the group to be capped. The load per pile will have little effect on the cap yardage per pile but will cause some increase. (All of this is gone into in much greater detail in Chapter 30.) 3. If caissons, mats, or spread footings are considered, all possible information as to soil and water conditions should be in hand as a result of preliminary soil-boring investigations.

7

A contract for boring should be so written as to require information regarding the elevation of ground water, and, if caissons are a probable solution, the rate of water flow at various elevations and also the number of blows on the casing immediately after each spoon sample should be required. If this information has not been obtained and given to the foundation contractor estimating the job, trouble is likely to occur, together with unnecessary expense and delay. 4. What does the code say? Admittedly, fine-combing a governing code to dig out requirements is no after-dinner task. But, nevertheless, it may be extremely rewarding, economically. Many of the newer codes express the latest engineering opinion, and so may point the way to savings. But whether the code is new or obsolete, a knowledge of its prohibitions and permissions is certainly essential, and the lack of such knowledge is frequently costly. In the following chapters the author has attempted to make each of the foregoing researches, and many others, easier for busy engineers who may specialize in different sectors of the construction field. There are a number of classes and uses of foundations which have been omitted from discussions in this book simply because it is impossible to include all of the variants correctly belonging within the scope of one book. Among these omitted types, classes, and uses are the following: (a) pier and dock work, drydocks; ( b ) locks, canals, and dams; ( c ) embankments, jetties, flood and tidal control works; and (d) bridges and roads.

2 CHOOSING THE SITE

AS pointed out in Chapter 1, numerous factors have entered into the owner's selection of a site. Frequently they have been considered and found decisive even before the detailed design of the structure itself entered into the picture. The location of a bridge, lock, dam, grade elimination, power plant, for example, may have been dictated by economic considerations which are usually outside the field of the designing engineer. However, at times there is a possible choice among a number of sites. In such cases, the factor of foundation cost should be considered and might be decisive. If the owner has, as was suggested, selected his designer at the same time, or even before, he chose his real estate agent, then the engineer has had the opportunity to make at least a preliminary investigation of the soil and foundation conditions of the several sites considered before the purchase of one of them has been completed. That this elementary precaution is often omitted will never cease to astonish the man in the field. Rarely a month passes in which foundation contractors are not asked for rush figures to meet subsoil conditions which were not anticipated by the owner or the builder, but which could and should have been known to the engineer. Usually a few dollars spent on the simplest sort of soil investigation would have revealed the danger. Even if, for other reasons, the site had been selected in spite of bad soil conditions, considerable money and usually vital time still could have been saved by originally designing the foundation to meet the facts.

Even on relatively unambitious projects, the following steps should be taken: 1. Investigation should be made by inquiry in the locality. If adjacent structures are present, they should be studied for cracks or other indications of settlement. It should be noted that very considerable settlement may occur without evidence of cracking. This is particularly true of reinforced concrete structures where the loading is approximately uniform. Such structures may settle a foot or more without damage, and the lack of observable cracks should not be taken as proof that no further investigation is needed. Often too little attention is given to the factor of uniform loading. In the case, for example, of grain elevators and various types of silos and storage bins, some parts of the structure may be fully loaded for considerable periods, while others are carrying only the weight of the structure. Soil conditions which should cause no worry for a hotel might be fatal for a grain elevator. 2. Unless the engineer is thoroughly conversant with the soil conditions in the immediate neighborhood of the site, he should make a check on the geology of the district. This can generally be done at little or no cost, other than an hour or two of time, by consulting the professor of geology at the nearest university; or, if the designer himself has sufficient general knowledge of geology, he can often get the needed information from a study of the geologic maps on file at the public library of the nearest large city. The principal value of a geologic study is that it will indicate the distances within which

CHOOSING THE SITE worth-while foundation information may be obtained from the study of the foundations of existing structures. Out in the prairie country, information about a building a mile away might have considerable significance, while in most parts of New York City the foundation type and behavior of a building across the street certainly could not be considered final evidence and, indeed, might be highly misleading. One caution might be noted: studies should not be encumbered with the observations of "the oldest inhabitants." They generally "locate" for you "the old swimming hole" or "the public dump" in use fifty years ago, but if you ever find these landmarks, they probably will not be within a mile of where you expected them. 3. The next inexpensive method of investigation is the taking of rod soundings. There are many elaborate forms of rod-sounding outfits, for example, those used by the State of Ohio Road Department. The results obtained by the use of these plants are somewhat more accurate than those of simpler apparatus, but the plant and skills required would eliminate their use under most conditions. ADVANTAGES AND LIMITATIONS OF ROD SOUNDINGS

As a means of preliminary soil investigation, rod soundings, in conjunction with shallow test pits, develop a fairly reliable picture. There are, however, these limitations: 1. The anticipated depth to good bearing materials should not be in excess of 50 feet. The inertia of the rods, plus the friction on the rods as related to the impact of a sledge or other light hammer, generally indicates good bearing in almost any soil at depths of 55 to 60 feet, even if no such bearing exists. 2. In a dried-out clay soil, rods are of little use. Wood piles have been driven for 20-ton loading at 40-foot depths on the South Side of Chicago, in dry clay soils which showed absolute rod refusal at various depths less

9

than 15 feet below the surface of the soil. 3. Rod soundings will give irregular and unreliable results when gravel with boulders running to 3 or 4 inches or more in diameter is encountered. The above cautions do not add up to the conclusion that rod soundings are generally useless—far from it. The condition which most frequently needs checking is the possibility of a stratum, not seen at the surface, of bog, peat, organic silt, or soft clay. In the presence of such conditions, the rods will show a sharp fall-off in resistance which will indicate clearly the need for further and more positive methods of investigation. Dangerously soft strata at or near the 50-foot depth may not show up very clearly in rod soundings, but weakness at such a depth would not be serious unless the load over a considerable area would be great enough to cause area settlement,* even though individually loaded footings might show very little movement. Up to this point the studies suggested are primarily intended to eliminate or warn against unsuitable sites. The choice having been narrowed to a single site, further tests in the form of soil borings should be made. The number and depth of these borings will be dictated by three factors: (1) by the general nature of the soil as indicated by the tests already made; (2) by the importance of the proposed structure; and (3) by the governing code, where one controls. • T h e total load of a structure divided by the builtover-area of the site would be described as the unit area loading at the ground surface. Similarly, at any plane below the ground surface, the area load intensity would be the area load at the ground surface plus the soil load, divided by the loaded area at that plane. Except occasionally in mat foundations, under tanks or the like, the actual intensity of loading is rarely uniform and therefore rarely quite the same as the theoretical area load intensity. However, the area load intensity, while an approximation, is important because it sets up the first limiting factor in the consideration of the suitability of any particular type of foundation to any given site.

3 INVESTIGATING THE CHOSEN SITE

I T would seem obvious that the study of soil conditions on the site of a proposed structure should always precede the detailed design of the foundation. But, for some obscure reason, this procedure is frequently reversed. In such "Alice-in-Wonderland" sequences, the soil-bearing tests, the soil-boring program, and the driving and loading of test piles will, any or all of them, be included in the general contract for the construction of the foundation. The bidding will be based on complete and detailed drawings and specifications—all carried out without benefit of test programs. Foundation design is supposed to be a more or less scientific procedure based on known facts, ascertained by accepted methods of investigation. Yet it seems that in many instances the designer depends on clairvoyance, since his facts are developed long after his design is completed. A vast amount of foundation money could be saved if a complete soil investigation contract were to be let in advance of the designing of the foundation. This procedure has been used in a few instances by departments of the Federal government, but its possibilities for moneysaving have been mostly overlooked elsewhere. Load

Testing

However, during the past few years, the use of field load tests for all types of pile foundations has increased greatly. This is a natural accompaniment to the increased loading on materials and structures now allowed. Under the usual foundation specifications this full-

scale field testing is included in the general foundation contract, and therefore the results are not available before the design has been completed and the general contract let. The possibility of designing on the basis of increased unit loading with substantial savings to the owner has been lost, even if the tests ultimately prove that the higher loading value would have been fully justified. The only useful purpose served by the load tests under such circumstances is to prove that the engineer's original assumptions were conservative —or otherwise! Letting a separate field load testing contract prior to the bidding of the general contract and prior to the making of the final foundation design is by no means a new idea, but it could be used more frequently, with great advantage. One warning, however, is in order. Recently one of the Federal bureaus had a very large foundation in prospect and decided to run a series of predesign tests on four or five different pile types and loadings. But the contract for the test work was let, on the basis of competitive bids, to a small contractor who was properly equipped for the driving of only one of the types on which information was wanted. As a result the testing was a fiasco. The best method of letting such work would presumably have been on a fee and rental basis to a selected contractor. But if some competitive element was unavoidable, then the bid request should at least have been written so as to be open only to those who could prove that they possessed the necessary plant and experience to assure that they

INVESTIGATING THE CHOSEN SITE could install all types required for testing and up to the stipulated maximum loadings. Examples of wasteful specifications could be multiplied almost indefinitely. The importance of knowledge of the new codes, of their increased allowances and the restrictions surrounding such allowances, is more fully treated in Chapter 36. However, it should be mentioned here that on work bid by our firm in New York City during the six years in which its new building code has been in operation, it has been found that there are very few important foundations on which major savings could not have been made by design changes, entirely safe and within the requirements of the new code, though in many cases excluded by the previous code. These possible savings were not small; they ran into millions of dollars. T o the designing engineer who is called upon only infrequently for special foundations, it may seem that there are a bewildering number of types and methods among which to choose. Further, if he reads the advertising in the technical journals, he will learn that while there are numerous types which obviously differ widely in the theoretical basis of their supporting power, yet each claims to meet all soil and load conditions with equal and always top efficiency. This is too much like old patent medicine advertising to be entirely convincing. However, when one boils down the list of types and methods to those which have been demonstrated and proved by wide commercial use and by acceptance under Federal, state, and city codes and specifications, the list is no longer formidable. Fortunately, the new codes and specifications have largely eliminated the need to judge the validity of the various claims on a basis of engineering theory. They have made this change possible by basing load-carrying capacity on test programs instead of on abstract theory. Many specifications are still written on the old basis of theory, or perhaps it is better

11

said, on the basis of prejudice. Even when designing engineers have caught the idea of "load by test," they are still, in many cases, merely adding the expensive, and properly conclusive, test programs on top of old specifications which have been built around clever advertising arguments, questionable theories, and patented gimmicks that reduce or eliminate competition. The test-load specification was looked upon as final and sufficient by the code writers, but with the so-called "safety clauses" added, the intent of the new type code, which was to open the foundation business to all legitimate competition and thus reduce the present unnecessarily high cost, has been largely destroyed. THE VALUE OF A SOIL-LOAD TEST PROGRAM

Generally the value of soil-bearing tests is not in the knowledge they give as to the ultimate bearing value of the soil, but rather in the information provided regarding the magnitude of settlement under varying loads and on the uniformity, or lack of it, of probable settlements throughout the area of the site. Differential settlements under working loads are often destructive where much greater settlement, if uniform throughout the foundation area, could be safely neglected. Some soils, such as non-consolidated loose sand or sandy, silty soils, may be capable of sustaining high ultimate loading for an indefinite period, but still be totally unsuited to even low-unit loading (say 1 % tons per square foot, or less), because a settlement of several inches may occur before the soil reaches a stabilized condition under load. Such a settlement, even if uniform throughout the site, would rarely be permissible under any structure having connections (such as sewers or water mains or party walls) to other structures. Where there is some question as to the proper choice between spread footings or mats, on the one hand, and some form of deep

12

THE PRELIMINARIES

foundation on the other, soil load-bearing unsightly destructive cracking could have tests would be indicated. In subsequent chap- been avoided at no extra cost in the constructers details, illustrations, and approximate tion. costs are given for such tests. Our firm has recently completed fourteen ERRORS IN SOIL-BEARING TESTS load tests on a site in New York City. These A common error in soil-bearing testing is tests were made on piles driven into a loose to make the test on too small an area, somesand deposited by wind and wave action. times only 1 square foot. Unless the soil is Practically every test reached stability when very firm, there will be a "heave"—a minialoads were at or around 100 tons, but settle- ture "mud wave"—set up around the periphment under test before coming to final rest ery of the loading plate. The apparent settleshowed 2 to 5 inches. While the movement un- ment, due to the penetration of the bearing der spread footings would undoubtedly have plate into the loaded surface, will be much been of less magnitude, it would still have greater than would occur when the larger been such as to require an area of footings areas of the actual building footings are put which would have been greater than the avail- under load. able load-carrying spacc under the structure. A loading plate 4 square feet in area seems The use on this site of spread footings without to give reasonably accurate results, but this soil-bearing tests—or indeed of any type of of course would vary somewhat with the type foundation without extensive load testing— and condition of the soil. Regardless of the would have proved disastrous. The problem size of the area tested, it is a good precaution was finally solved by the driving of concrete to establish ground check points about 6 pedestals to compact the soil and then the inches away from the plate to check on possidriving of concrete piles into this compacted ble local heave. stratum. A second frequent error involves a misunIn Chicago's North Shore district near Lake derstanding of shear in soil. Where a soil to Michigan dozens of lightly constructed apart- be loaded is stratified with layers of soft matement houses show settlement cracks. All these rial below those directly tested, or where the small buildings are on spread footings, and loaded stratum partakes of the nature of a certainly no other type of foundation would crust, tests must be made on areas of sufficient have been economically possible. But the magnitude to preclude the possibility that settlement cracking could easily have been when larger areas are loaded, shear in the soil avoided if the cheapest sort of soil load test will not occur around the perimeter of the had been made and the size of the spread loaded area. An increase in the loaded area footings designed to keep the unit loads will not result in a proportionate increase in within limits which would hold the differ- the area of supporting shear. ential settlement to, say, 1 to % of an inch. This effect of shear versus direct surface In most cases, this would have meant re- load or compression was exemplified by the ducing rather than increasing the area of failure of a large warehouse in Milwaukee, some of the footings. By introducing some which was subsequently underpinned at a flexibility into the water and sewer connec- cost to the owners that was only a little less tions, a general settlement in the range of, than the primary cost of the entire building. say, 3 inches could have been allowed in The soil underlying the site showed, from these comparatively unimportant buildings, the surface down, about 2 feet of hard-packed provided that the differential settlement had cinders, overlying 6 to 8 feet of medium-firm been controlled by design. In this way, the clay, which was in turn underlain by 60 to

INVESTIGATING THE CHOSEN SITE 80 feet of very soft river mud, overlying hardpan. The architects' design originally called for the use of concrete and wood composite piling to carry the loads to the hardpan. The piling job, for which our company had a contract, had already started when a bright young engineer saw the work, spent a few moments in thought, had an idea, and went to see the owner. He had observed the hard surface of the site and the few blows of stiff driving needed to start the piles. He asked the owner for and received permission to make soilbearing tests with a view to eliminating the piles and with, of course, a very large resultant saving. He then proceeded to make some thirty 1-square-foot tests. Every test carried 4 tons with only minor settlement. On the strength of these tests, the pile drivers were ordered off the site and a new design made on the basis of spread footings. Before the start of actual construction, this design was again changed to one calling for a mat covering the entire site. It seemed that no one in the design department stopped to consider that while the 1foot-square soil test load of 4 tons was largely supported by 4 x 2 = 8 square feet of shear in the cinder crust (and was spreading the test load by means of its cone of pressure over a considerable area by the time the load had reached the extremely soft soil about 6 feet down), the average 12 x 12 footing would have only 48 x 2 = 96 square feet of shear to carry a load of 576 tons—a loading equivalent to the 4 ton per square foot on the shear area. (I am assuming, for the sake of simplicity, that the shear in the soil would react as a direct, vertical punching shear, as it probably would at the failure point.) Both loads—the 1-square-foot test load and the equivalent load on the 144 square foot spread footing—are assumed to have developed all their bearing capacity by shear in the upper stratum, though in actual fact the soupy soil below must have exerted some small direct pressure on the bases of the

13

loaded areas. The neglected factors would tend to further aggravate the error caused by the too small test area. The proved shear value for the test load would have been 0.5 tons per square foot, whereas the required shear for the 12 x 12 footing would have had to be 6 tons per square foot, an obviously excessive loading. Of course, by changing to a mat design, the danger from shear failure reached its maximum, and catastrophe, short of a miracle, was absolutely assured. No miracle occurred, and before the building was completed, it was settling so rapidly that the cracking could actually be heard at frequent intervals, and in many instances the glass in the windows shattered under the wracking action of the continuous irregular settlements. Underpinning was a long-drawn-out, intricate job. The rate of settlement precluded the possibility of completely stabilizing any small area without causing it to punch its way upward through all the floors above it, and the slowing down of movement at one point caused erratic and at times inexplicable variation of stresses at other points. LOADS MUST BE TRACED TO

BEARING

While the case history just told recounts a more lurid building tragedy than is often seen, still it may be stated without fear of contradiction that most foundation failures result from one cause. That is the neglect on the part of designing engineers to trace the structure loads all the way to the point at which they will finally be carried without the possibility of further settlement. In the case of pile foundation failures, this may be traced, at least in part, to the erroneous assumption that "friction" is in itself an ultimate loadcarrying factor, when, actually, it is only a method of load-transfer of some part of the load from the pile to the soil. The soil must ultimately develop resistance to this load as direct compression. A corollary to this mistaken view of friction

14

THE PRELIMINARIES

as in itself an ultimate load-carrying value is the often repeated statement that a tapered pile has a higher unit friction value than a cylindrical pile. This does not mean that the bearing value of tapered and cylindrical piles having the same surface area and delivering a part of their loads by friction will necessarily be the same. As a matter of fact, they rarely will be. For one thing, as long as piles must have structure and bulk, a pile without some direct bearing is an impossibility. This question of the effect of the shape of a pile on its bearing capacity is discussed more fully in Chapter 41. To sum up: soil load tests and test pits are

the primary means of evaluating mats, spread footings, or piers (which may be defined as extensions in depth of spread footings), as a solution of the foundation problem for a given site and structure. One or more borings should be taken as a protection against the possibility of deep-seated strata of poor bearing value lying below the bearing stratum carrying the spread footings, even where soil load tests and pits indicate that spread footings may safely be used. If spread footings can be used, they will practically always offer the cheapest solution, and it will be unnecessary to work up designs and costs for other methods.

4 CORE BORINGS

ASSUMING that the site has been chosen and that preliminary studies have ruled out the use of spread footings, the next logical move would be to investigate subsoil conditions by means of core borings, or, if a few have been made during the preliminary studies, of additional borings. The correct design of deep foundations, and indeed of all special foundations, including piles, mats, floats, caissons, and mechanically, electrically, or chemically consolidated soil foundations, is becoming progressively more and more dependent upon the analysis and interpretation of soil borings. Yet too often the results hoped for by the designing engineer and paid for by the owner are not achieved. Selecting

a Core-Boring

Contractor

If, as is usually the case, the designing engineer is responsible for letting the contract for borings and sample-taking—and the contract for the testing and analysis of these samples—his first consideration should be the choosing of a soil-boring contractor who is widely experienced and of whose complete honesty he is well assured. Occasionally my company is asked to bid on, and is awarded, work based on soil-boring information which subsequently proves to have little or no relationship to the conditions actually encountered. The records would appear to have been made at the nearest bar rather than on the site. While such faked data is comparatively rare, there are many cases in which the information submitted to

the foundation bidders is of little use because of the ignorance or carelessness of the drilling contractor. In a still larger number of cases, while the results given prove to be reasonably accurate, much valuable additional information—such as the elevation of the water table, rate of water flow, etc.—which could have been obtained at very little or no added cost, has been omitted. How can a designer, who requires such services but rarely, be sure he is getting a qualified boring contractor who will give him all the data he needs? Certainly not by opening the Red Book and picking the first drilling contractor who lists soil boring among his activities. Sweet's Architectural and Engineering Catalog may be of some assistance, but there are probably ten competent soilboring contractors who do not appear in Sweet's to one who does. Nearly all soil-boring concerns, however, will be found in the Red Books of such cities as New York, Chicago, Philadelphia, and Boston. Procedure, then, might be as follows: First, pick out two or three names from cities relatively near the job under consideration. Second, send to each a letter setting forth the work required and asking for a statement of work of similar nature which they have done, say within the past eighteen months, with particular reference to the clients for whom they have worked. Familiarity with the standing of these clients and the nature of the jobs may then take the place of firsthand knowledge of the concern itself. Even the best selection will not take the place of

16

THE PRELIMINARIES

knowledge on the part of the designer as to the information he wants and can get, provided the work is properly specified and properly handled. How Best to Specify

for Core

Borings

1. It is a "must" that there be a sufficient number of borings to assure that the probable range of soil conditions will be ascertained. Where soil conditions are known to be erratic, as in the delta of a geologically old river, no economically possible number of borings on the site could give absolute assurance that some freak pocket of material will not pose a special problem for one or more piers. The only reasonable way to handle such an unusual condition is to solve the problem if and when the digging or driving uncovers it. Generally, in any site of one-half acre or more, at least five borings would seem to be the minimum requirement. These should be located as follows: one in the center of the site of the proposed building or group of buildings, and each of the other four near a corner, on the diagonals of the site. Not infrequently the corner borings are placed within a very few feet of the corner, but since information is wanted about the site and not the surrounding territory such placing is inefficient. When the shape of the site approaches a square, the borings may be on the diagonals and approximately one quarter of the distance from the corner toward the center, so that the area represented by each boring will be as nearly equal as possible and the weight given to each will be the same. Where the site is not square, and it rarely is, a simple method of locating the borings is to take a scale drawing of the built-over area, locate on it points at which a general observation would suggest that the permissible number of borings would most nearly cover the site if their influence circles were equal, and then using those tentative locations as center points, scribe a circle of equal radius around each. The uncovered or over-

lapped areas will be evident. By changing one or all of the proposed locations and also the radius of the influence circles, the boring locations most nearly offering a uniform coverage for any shape of site can closely be approximated. Of course, the above arrangement of borings may prove impossible because of obstructions such as existing structures which will not be moved till after borings have been made, or may be undesirable by reason of unusually concentrated loading on some sections of the site. But, in most cases, it provides a mark at which to shoot. 2. Borings, when taken for design purposes, should always be carried to some possible "ultimate bearing stratum." The cone of pressure or the bulb of pressure, regardless of how it receives its load, must finally deliver its load at some depth by direct compression. Our knowledge of the angle of spread of load in various soils in their natural beds is scanty. Much greater use of soil pressure cells should be made to gather information on this matter. 3. A water-table study adds little to the cost and may be of great value to both engineer and contractor. A record of the rate of flow as well as the elevation of the table is desirable. The Boussinesque Formulas, though based on assumptions which are rarely even approximated in soils, offer the best theoretical means of ascertaining soil pressures at varying depths below different surface load patterns. Where the soil has been or will be modified by the driving of piles, they are probably more dangerous than useful. A typical case of the requirement for extra boring information would be that of a projected heavy building under which material of doubtful bearing value might be encountered at a greater depth than could be economically reached by any type of deep foundation. Under these conditions, it would be advisable to obtain large-diameter undisturbed samples

CORE BORINGS to such depth as would assure the dispersion of the load to a safe low-unit bearing on the deeper soils. Careful calculations of the probable area settlement over a considerable period of years should also be made by a good soil mechanics laboratory. Full-scale multiple pile tests would give the final answer and would be indicated where the magnitude of the job would warrant the expense. The

Limitations of Core

Borings

It is important that the designer should have a clear view of the limitations as well as the uses of soil borings. The following are the most important limitations. PLUMB

Core borings, even in reasonably uniform soils, and even when taken by well-trained and highly experienced operators, cannot be depended upon for plumbness. Our engineers recently had occasion to excavate around a boring to a depth of 30 feet. It was over 3 feet out of its initial position at that depth, and this is by no means an unusual condition. At a 100-foot depth, there would be no certainty that the bottom of a boring would be within 10 feet of the location of the top of the same boring. When the foundation is to be a piled mat, or where the purpose of the boring program is to make a study of possible deep area settlements, considerable divergence from the plumb might be of no great import. The error in location, however, could be serious where caissons or small pile groups would be required. GRAVEL OR BOULDERS

Where borings pass through heavy gravel or numerous boulders, there may be—in fact there generally will be—very deceptive results. The small-diameter casing pipe used in the usual boring will frequently "wind" past boulders, even though the boulders are of considerable size. On the other hand, a small boulder hit squarely by the sampling spoon

17

will suggest a falsely high bearing value for the soil in general at that depth. Where even a few boulders have to be drilled to allow advancing of a 2%-inch pipe, a pile 12 inches or more in diameter will probably find rough going and will hit many boulders. The areas of a 2Y>-inch boring and a 12-inch diameter pile are 6.28 inches and 144.0 inches respectively, which vastly increases the probability of hitting boulders with the pile where few or none have been indicated by the boring. Many times the contractor, estimating on the basis of borings as submitted for bidding, claims deception because the borings have shown little indication of boulders, whereas the driving or digging runs into very many, with resultant unexpectedly high costs. If boulders are indicated in the borings, it might be well to call the foundation bidders' attention to them in the request for a bid in order to avoid a possible claim for extras. FRICTION

Another fundamental weakness of borings in general is their failure to indicate properly the friction value of the soils penetrated. Blows on the casing pipe, as well as blows on the sampling spoon, should always be recorded. But even when this is done the result is frequently misleading, because the smalldiameter pipe may be carrying a plug of soil with it and the blows to drive the pipe will then represent not only the friction on the outside of the pipe but also the much higher friction of the plug of soil on the inside of the pipe. When the plug reaches a certain compaction it may be impossible to drive it further into the pipe, thus presenting a condition in which the resistance of the pipe to driving represents the full closed end-bearing resistance of the pipe, as well as the total external friction. A very good indication of at least the comparative friction value of the soil at varying depths would be obtained if the specifica-

18

THE PRELIMINARIES

tions required that the casing be driven immediately after the taking of each spoon sample and that an accurate count be kept of the blows on the pipe for the first 6 inches of this drive. Before taking a spoon sample the pipe is cleaned out to the bottom. Next, the sample is taken out as a core. Under these conditions very little end pressure will remain for at least the first 6 inches of the casing drive and for at least a few minutes after sampling. Suppose samples are taken and the 6-inch casing drive is made every 5 feet as usual, then the increase of the casing resistance as between each 5-foot check point will give a reasonably accurate record of the build-up or breakdown of the friction on the pipe. The results of such a test would only have meaning if flush-joint casings were used. Again, assuming that flush-joint casing is used, the casing may be jacked up 6 inches or a foot at several of the check points. By using a hydraulic jack and gauge, the actual friction value of the pipe in pounds at these check points would be known, and a comparison could be set up between the blows required to drive down and the equivalent friction value shown by the pulling jack. It is not suggested that such data should be taken on more than a few of the test borings on any site. Piles are constantly being classified as "friction-bearing" or "end-bearing," and the newer codes give substantially differing permitted loadings based on this classification. But unless a pile stops suddenly on hard bottom, we may be reasonably sure that its resistance to load is neither purely in friction nor purely in endbearing. More information on this feature would be very worth while, and in cases where the pile could, after such tests as those suggested, be classified as end-bearing, could result in an increased loading of as much as 33% per cent. This would mean a corresponding reduction in the number of piles required and would save money for the owner.

Unfortunately, after going to the expense of carrying out a boring program few engineers seem to have any idea as to the meaning of the data they have obtained. For example, how many blows on the sampling spoon through how long a distance would indicate the probability of developing, say, a 50-ton working load pile of any particular type and dimensions? How would the answer be affected by the general classification of the soil? If 30-spoon blows per foot would be required in a fine-grained sand soil, would the same answer be correct if the soil were around 50 per cent sand and 50 per cent clay—or silt and gravel? The answers to such questions cannot be stated positively but unless the engineer requiring the collection of the data has some idea as to its meaning, why go to the expense of collecting it? The "Art" of Core

Boring

A logical approach to the determination of the nature and extent of the boring and sampling programs to be set up would certainly begin with a careful listing of the facts regarding the soil which it is desired to determine. No money should be spent in gathering a mass of information merely because it is possible to get it. Sometimes specifications for building site investigations call for needless expense because they require the acquisition of information which is not essential to the work on hand. However, such information might be invaluable where the contemplated work consisted of, say, the study of soils with a view to their use in road fills or as the base for the runways of an airport, or for reservoirs, drainage projects, proposed deep excavations, flood control works, route studies for proposed tunnel work, the blocking out of bodies of ore, or for any one of numerous other forms of work. The following are the requirements which would generally justify their cost where the purpose of the investigation is the choice or the design of the foundation for a structure.

CORE BORINGS They should be stated in the first request for bids on borings. Further and more elaborate methods might finally be necessary, but such necessity would be indicated by the results of the minimum requirements listed and should not be included in the work unless they are so indicated. THE M I N I M U M

REQUIREMENTS

Tests should locate the "ultimate bearing stratum" to which the building load must finally be delivered, regardless of how it is originally delivered to the soil. Investigations for this purpose may be limited in the large majority of cases to standard 2%-inch dry sample borings, supplemented by diamond drilling where rock or boulders or other normally impenetrable material is encountered. SPOON SAMPLING

Spoon sampling—usually at 5-foot intervals—the taking of rock core samples, and the checking of the elevation of the water table and of the rate of flow may have very little to do with the question of foundation stability, but such information will frequently be of vital importance to the party who does the work of excavation or the placing of the piles or caissons and will therefore affect the choice of type when the problem of economy is studied. Where the foundation units will be carried down to impenetrable material, there would seem to be no sufficient reason for the employment of continuous or undisturbed sampling or for the analysis of soil samples from ground surface to the bearing stratum. Where end-bearing on rock or other impenetrable material offers the obvious solution, the core-boring program will be governed by three principal purposes: 1. To ascertain that the thickness of the impenetrable stratum is sufficient to safely carry the proposed loads, that is to say, that the bearing stratum is not underlain by poorer strata within a depth which could permit of

19

area settlement. Even after obtaining information as to the thickness and nature of soil strata underlying the firm bearing stratum which is under consideration as an "ultimate bearing stratum," the problem of the sufficiency of any underlying stratum to carry the foundation load without area settlement is directly keyed to the angle of spread of pressure through various classes of soils in their natural beds. Some information on this point can be developed from the testing of undisturbed samples, but a great deal more testing by means of soil pressure cells seems to be needed before we can be at all sure of the answers. 2. To ascertain for bid purposes the probable length of the foundation units. 3. To provide the designer and the bidders with information regarding the conditions existing between the surface of the soil and the top of the bearing stratum as they will effect progress and therefore costs. In a very substantial number of cases where end-bearing may still be the obvious answer, the situation is somewhat complicated because the bearing stratum in its natural bed is not truly impenetrable even from the standpoint of an end-bearing pile, but would become so by a limited "packing up" by means of piles or by the use of concrete pedestals. The most common form in which this problem is presented is the case where the necessary resistance to develop the required pile-load capacity can be obtained by driving the bearing unit into the bearing stratum rather than by setting it on that stratum. This penetration into the bearing stratum may be no more than a few inches or it may be some feet. There is no fixed rule as to the depth into the bearing stratum at which a pile would cease to be "end-bearing" and would become a "non-end-bearing" unit or at which it should be considered as a friction unit. After the job has reached the point at which full-sized piles are being driven, this question is readily resolved by making load-bear-

20

THE PRELIMINARIES

ing tests followed by tension tests. End-bearing will be eliminated when tension tests are underway. The tension value mav generally be considered as the friction value. (This question of friction-bearing value is studied at great length in Chapter 7.) However, the question at the moment is not how to determine possible friction-bearing values after the construction of the foundation is underway, but rather to see what information may be obtained by the designer and the bidders from core borings and sample analysis. If the core boring data is to be used, as it very frequently is, to determine the length of pile which would be required to develop a given unit load in a firm but not impenetrable soil, then disappointment is most likely to follow. The reasons are quite obvious. First, the spoon blows: the casing blows and the samples, no matter how they are taken, indicate the soil conditions before the soil has been disturbed by pile driving or caisson digging. Whereas the reaction of the pile itself is dependent on the conditions of the soil after it has been changed by the previously driven part of the foundation work. The addition of each pile creates a new condition to be met by the following piles. Most frequently the added piles increase the soil compaction and decrease its permeability, but on occasion the reaction may be just the reverse; the soil becomes more malleable as the work progresses. The result is progressively lessened resistance to loading and therefore progressively increased depth is required. This is one side of a complicated problem; the other side, to which even less attention is generally given, arises from the wide variety of shapes and sizes of piles. Certainly the resistance to pile penetration into any given stratum will be a function of the volume of the pile and the frictional area in contact with the soil. If various sizes of cylindrical piles are to

be considered, the prejudgment of pile length to carry any determined load—on the basis of core borings and sample analysis—approaches the insoluble. It is still further complicated if the pile is tapered, since every inch of the added depth will change the volume, the frictional area in the bearing stratum, and the pressure between the soil and the surface of the pile—and therefore the intensity of the friction. It is a puzzle why anyone should assume that piles in a penetrable soil will be of the same length regardless of shape, volume, and friction area, but as of the present most requests for bids are written on that assumption. To anyone who has not had the opportunity and occasion to make a wide study of the matter, it might seem that it would be easy to develop a simple formula which could be used to equate the blows per foot on a sampling spoon with the bearing value of a driven pile, or to similarly equate pressure required to advance a sampling tool with the bearing value of a driven pile. If such a formula has been developed and proved dependable by comparison with field results, then after diligent search over many years I have failed to find it. My own research on the subject, and that of others to which I had access, never even reached the point of attempting to write a formula. It seemed obvious that the first move must be to demonstrate that a definite relationship existed between light hammer blows on a smaller diameter testing spoon, delivered through a flexible rod or pipe, and hammer blows a hundred times as heavy, delivered through a vastly more rigid pile or pile-forming apparatus having an entirely different ratio of weights of driving member to driven member. I never discovered any uniform relationship between the two even where conditions seemed closely similar. At the very start one meets the basic objection to all proof by the use of scale models,

CORE BORINGS that is, the requirement that all of the factors entering into the value of the full dimension design must be scaled down to the same degree. It is not sufficient that the linear dimensions are scaled in the same proportion. The weights of all moving parts must also be scaled down to the same ratio. This might be fairly simple. But the big problem arises from the necessity of first ascertaining and then scaling down all of the vital soil characteristics. If the soil in which the testing apparatus and the driven unit are to be placed should chance, by rare good fortune, to be uniform throughout the full depth of the bearing stratum, the grain size would still have to be scaled down. The porosity and, if it played a part, the cohesion would also have to be adjusted. And certainly the rigidity of the members of the full-sized and the scaled-down model would require some compensation. If all of these requirements could be compensated, there would still be the two problems mentioned when considering the possibility of predicting the bearing value and depth which would be required of any type of foundation by means of a purely theoretical approach based on soil characteristics as shown by the analysis of samples. Where the use of driven piles would be considered, these all but insurmountable obstacles are ( 1 ) the constantly changing condition of the soil during the progress of the driving of successive piles, and ( 2 ) the necessity of deducing different answers for each different size or shape of unit to be considered. It would seem that in order to arrive at any usable link between the information given by the core borings and the reactions of the full-scale foundation units the only possible path would be by means of some empirical formula. A number of engineers are working, though slowly, toward this end. The greatest obstacle to progress is the necessity of finding a group of jobs in all of which ( 1 ) the soil profiles are the same or at least very similar (the acceptance of "similarity"

21

must, for the most part, be based on laboratory analysis of soil samples throughout the full depth of the bearing strata); ( 2 ) the same type of piles must have been used on each of the test cases; and ( 3 ) full-scale load tests must have been made and recorded on each of the jobs. Other similarities would of course be very desirable—as, for example, that the water conditions be similar—but if the three conditions above had even been approximated in, say, a dozen cases, it should be possible to begin to find some vague pattern in that strictly limited field. Unfortunately, so far I have to admit that I for one have not found the needed data. It should be easier to trace a relationship in a strictly limited field between blows on the spoon and blows of some standard hammer. But is that effort worth while when we know that pile-driving formulas offer no dependable information as to the bearing values of driven piles? It seems more realistic to make the bolder effort from the start and seek to set up a direct relationship between the information available from a boring- and soil-testing program and the bearing value of the full-scale driven units. Even limited progress in arriving at any general formula will be slow unless some engineering school or widely known soil laboratory can and will collect information from many different sources and then correlate it, as a continuing operation. It may seem that up to this point I have been crying down the value of core borings and soil analysis in general. That is the reverse of my intention. I am merely trying to point out that at the present stage of our knowledge opinion based on wide experience and careful consideration of all available data is often the only guide. This will continue to be true until full-scale testing can provide the answers. Although the solving of foundation bearing problems may seem to be as much an art as a science, this does not mean that the services of those best qualified in the field should not be

22

THE PRELIMINARIES

sought. Because the value of an architect's dream cannot be fully assessed by the use of a slide rule would anyone be fool enough to suggest that the architect should throw away the tools of his profession? Of course not. In the field of foundation analysis as with any other field, one should seek the services of the man (or group) with the widest experience and best record of performance and pin his faith there. Again and again I have asked several engineers and contractors to look over a set of boring and soil reports and tell me the approximate depth to which piles of a given type carrying a certain load would be required to go. After study each has given his answer, and the surprising point is that rather generally their answers were closely similar and not far from the mark when the work had been completed. I have asked the same men whether they had anything in the way of a formula from which to work out those answers; none did. Perhaps they sound like dowsers who locate water wells with "divining rods." But the fact is the system works, sometimes with excellent results. While trying unsuccessfully to find some consistency of relationship between blows on the spoon and hammer blows on a full-scale pile, it has frequently been observed, though of course not in all cases, that a nearer approach to consistency could be traced between blows on the casing and on the pile-driving hammer than was shown between spoon and hammer. Any deductions made from a study of blows on the casing of a core boring would benefit from the use of flush-jointed casing. The variation of the friction of soil in place can also be gauged by the use of a flush-joint casing jacked up by a registering jack at successive check points. By using a plug below the tools lowered to the bottom of the casing, comparative over-all bearing values can also be obtained. The deduction of the previously observed friction value from the combined values obtained by jacking the

plugged unit downward would give a comparative value for the direct bearing. Turning the comparative friction and direct bearing values obtained on flush-joint pipe into corresponding values for large diameter piles would seem to be a possibility. All that can be said at present is that observation of blows on the casing shows at least a vague pattern, which is more than can generally be noted for blows on the spoon. Blows on the casing should always be required in addition to those on the spoon, and where available without added cost the flush-joint casing is to be preferred. When stipulating the information which is to be obtained and recorded by the boring contractor, it is important to include a classification of the rock, both in natural bed and when composing drifts and boulders. Two large jobs came out for bids, one based on the use of large diameter open-end pipe piles, the other on H-beams with an alternate of openend pipe. The borings for both show a substantial number of boulders or drifts, but it is now impossible to obtain from either set of boring reports and core samples any idea as to the composition of the boulders. Judging from the geology of the sites, the boulders may be merely drifts consisting of mica schist, shale, and limestone, or they may be hard heads from a terminal moraine. Perhaps these fragments described as boulders can be broken up by a few blows on the pipe or, at worst, by spudding, but if they happen to be hard heads, survivors of thousands of years of rolling and grinding, it will be necessary to drill them out, possibly even to blast them. The cost of removing these obstructions might change the total cost of placing the piles by 50 per cent. There is nothing unusual about omissions of this vital information. If the boulders have been large enough to require diamond-core drilling, samples will probably be available somewhere from which the contractors may make their own deductions, but in a sur-

CORE BORINGS prising number of cases such samples either were not taken or have been lost. Samples of bedrock are generally preserved. It should certainly be required in all boring contracts that information as to the nature of all obstructions be stated on the log. Technical

Specifications

for Core

Borings

The taking of samples—and the keeping of a full record of these samples and of all pertinent information, such as the number of hours worked and the name of the driller in charge—and the temporary storing of samples shall be the responsibility of the contractor even where an owner's representative is continuously present and even though he may keep a duplicate log of the operations. Soil samples must be so protected that they will not become heated beyond 90° F. or chilled below 40°. All samples will be delivered within 24 hours after they are taken to the address specified. Or, alternately, samples will be collected from time to time by the engineering laboratory making the soil analysis. Bottled samples are to be placed in the bottles immediately after they are taken. Each bottle is to be labeled to show the boring and the depth from which it was taken. The continuous log of each hole to be kept by the driller will identify the samples by number and will give the driller's description of the soil sampled together with pertinent information such as the loss of drilling water, change in color of soils otherwise similar, whether drive casing is changed in size or omitted, and special tools used, if any. The point at which gas or sulphur water or artesian flow of water or any other unanticipated condition is encountered should be carefully described and noted, and if the owner's or engineer's representative is not present at the time, this information shall be sent to him as quickly as possible. All samples shall be properly packaged for shipment. It shall be the responsibility of the contractor to assure that they are suitably packaged to meet the anticipated method of shipment. The cartons, boxes, or other containers shall be fully marked for identification. Where samples have been insufficiently protected so that they reach the laboratory in un-

23

usable condition due to improper packing and handling by the contractor, the work covered by such samples shall be repeated at no cost to the owner. Blasting may be employed where other standard methods are unsuitable. If blasting is to be done, it shall be the responsibility of the contractor to make the arrangements for the transportation, storage, and use of the explosives with the proper authorities and the owner's representatives. The size of the charges used shall be such as to avoid injury to adjacent property. If required by the owner, the contractor shall take out added insurance or bond to cover possible damage or personal injury, charged at cost to the owner. Where undisturbed samples or continuous sampling is required, the diameter of the casing shall be . Where two or more casing diameters are required to pass obstructions, the casing through the critical strata about which information is needed shall where possible be 2 % inches, except that where undisturbed samples are to be taken, it shall be of such larger diameters as are required under special conditions. Unless otherwise specified the soil pipe or surface drive casing shall be at least 10 feet in length. The hole may be partly cased or cased the full depth of the boring, providing samples satisfactory to the owner's representative are obtained and also providing that data desired as to the water table and water head can be obtained. The contractor shall be responsible under all circumstances for the employment of such tools and methods as prove necessary to obtain the information required under the contract. DRY SAMPLING THROUGH WASH BORINGS

The procedure for obtaining dry samples through wash borings shall be in general as follows: 1. A soil pipe or casing shall be driven openend into the ground at a point shown on accompanying drawings. The diameters, driving weights, and free fall shall be (except when otherwise stated) : drive casing, 2'^-inch diameter and in 5-foot lengths; hammer for drive casing, 300 pounds falling 30 inches. 2. Wherever possible the full 5-foot casing section shall be driven without cleaning. If boil occurs

24

THE PRELIMINARIES

during the cleaning of a 5-foot section, one or more additional 5-foot sections shall be added until cleaning can be accomplished without causing inflow of soil. 3. When the cased (or uncased) hole has been cleaned of soil down to the bottom, the sample spoon shall be employed to take an 18-inch sample. Similar samples shall be taken at intervals of 5 feet and in addition (where no casing has been used) at each point of change in the nature of the soil. All samples shall be taken with a split type sampling spoon. The length of the barrel of the spoon shall be such as to permit of taking a full undisturbed soil sample at least 18 inches in length and iy 4 inches in diameter. The sampling spoon shall be driven by means of a 140-pound weight falling 30 inches. The sampler shall be driven 18 inches and the blows per 6 inches (or per foot as required by the engineer) shall be recorded. During all operations of driving, washing, and sampling every precaution shall be taken so that a full head of water up to the top of the drive pipe is maintained at all times. Samples must be placed undisturbed and sealed in glass containers immediately after removal from the spoon and must be labeled, packed, and shipped as required. Procedure in taking of Shelby Tube samples shall be as follows: Samples shall be taken with a Shelby Tube sampler in tubes 30 inches long. The tube shall be forced into the soil with a registering hydraulic jack. Both ends of the tube shall be sealed immediately after the tube is removed from the sampler. Detailed records of each step of the operation shall be taken and shall form a part of the log of the hole as kept by the driller. The use of a driving hammer to force the sampler into the ground shall be avoided unless absolutely essential. Continuous undisturbed sampling consists primarily in a series of Shelby Tube or similar undisturbed samples in which each sample is a continuation of the previous sample, with no boring or washing operations taking place between samples, except to the extent needed to clean out any slough which may have oc-

curred d u e to the taking of an earlier sample. The 2 %-inch hole used for standard dry sampling is frequently increased to a S c inch hole for continuous sampling. In unit prices add continuous sampling (per foot) in the 2%-inch hole and also in the 3%-inch hole. The taking of satisfactory Shelby Tube or similar samples, whether singly or continuously, is a fairly simple matter in cohesive soils. However, such sampling in noncohesive soils is another matter, requiring operators who are experienced in such work if any degree of faith is to be put in the information obtained. One procedure commonly used is specified as follows: 1. The drill hole down to the point at which the sampling is to be started shall be drilled by the method commonly in use for rotary drilling of oil and gas wells through both cohesive and noncohesive soils and through sedimentary rocks. Unless the soil to be penetrated is preponderantly clay, the hole is filled with a slurry containing some stable viscous admixture such as bentonite or aquatek. In oil-well drilling it is generally desirable to have drilling slurry as heavily weighted as possible, since the heavier the slurry the greater will be the protection against caving of the walls of the hole. Also, when a very loose soil such as gravel must be penetrated, it is of advantage to have the slurry penetrate into the surrounding soils as much as possible. The entry of high pressure water will also be sealed off more effectively by a heavy slurry, though not if the fluidity of the mix has been seriously reduced by the demand for a heavily weighted mix. However, in soil-boring operations a consideration is encountered which is not present in oil-well drilling. That is the necessity of maintaining the soil pressure undisturbed as nearly as possible during the taking of the samples and also preventing any influx of drilling slurry into the sample itself. As long as the core boring is through a

CORE BORINGS cohesive soil and a properly balanced drilling slurry has been used, the casing may sometimes be omitted, even where it is desired to wash the bottom of the hole until only clear water will remain in it. However, in cohesive soils and especially in noncohesive soils the use of a casing for the full depth is desirable if undisturbed samples are called for. The omission of casing should be at the contractor's risk. 2. As with all other samples, Shelby Tube samples, whether taken singly or continuously, shall be taken only after all loose soil has been removed from the bottom of the boring down to the maximum depth drilled, cored, or jetted. The responsibility for a clean hole before sampling is started shall be that of the contractor, except where it may be impossible to remove the drilling slurry without causing a "blow." In loose noncohesive soil a piston-type sampler will be required. This type of sampler is frequently used in taking undisturbed samples in soils of any class. The length of a piston-type sampler should be at least 30 inches so that a certain amount of disturbed sample may be discarded and still leave enough for testing. 3. Care shall be taken to avoid pushing the sampler, regardless of the type or use, to a greater depth than that required to almost but not entirely fill the barrel of the sampler or the contained thin wall sample tube. If the sampler is jacked or driven downward after the barrel or tube has been filled, the reaction will be to compress the sample, which will then show greater density than the soil from which it was taken. 4. There are on the market a number of samplers for the taking of ordinary dry samples and also for the taking of undisturbed samples under varying conditions. The contractor shall state in his bid the type or types which he proposes to use on each of the sampling operations covered by the request for bid. All samplers used must meet the approval of the owner's representative or engineer. (Even when the work is done by experts, widely experienced and well equipped, the taking

25

of undisturbed samples in loose dry sand or clean gravel may prove impossible.) 5. Where core drilling in rock, boulders, or hardpan is called for, the diameter of the recovered core shall be 1 3 / l s of an inch, if not otherwise specified. The depth of each run in rock or hardpan at the base of each exploratory core hole shall be at least 5 feet. Recovery shall be reported as a percentage of the total drilled. In case greater depths of drill holes are required they shall be paid for at the price stated in the unit bids in the proposal for diamond drill cores. Recovered rock cores shall be placed in soundly built wooden boxes at least 5 feet in length having wooden strips equal in depth to the diameter of the core taken and spaced about % of an inch wider than the diameter of the samples and run lengthways of the box. The samples shall be placed in these slots so that the recovered core taken from a 5-foot drill hole shall occupy one full 5-foot slot. The recovered core shall be laid in the slot as nearly as possible in the position it occupied in the rock before the drill hole was made. The length and positions of the recovered core shall be noted on the edge of the spacers so that in case they are disturbed they can be replaced approximately in their original positions. After a sample box has been filled, packing shall be used above the samples so that when the wooden lid is screwed down for shipping there will be no movement of the samples. A note shall be written on or firmly attached to the inside of the cover giving the number of the core hole from which the samples were taken and their positions. Any special conditions, such as the nature and the quantity of water seeping through cracks in the rock or the presence of gas in or immediately above the rock, shall be noted in the driller's log of the boring. The boxes shall be supplied and the packing of the samples done at the expense of the contractor. THE DRILLERS LOG

The accuracy, completeness, and clarity of the driller's log are matters of prime importance which are altogether too frequently neglected. The log should be kept in a stiffbacked notebook similar to that used by a

26

THE

PRELIMINARIES

land surveyor or instrument man working on layout. The boring number of each core boring, sounding, or drilling should be keyed to the plot plan on which the borings are located, and the elevation of the site of the boring noted. The date and time and the name of each operator working on the boring should be stated. If one or more false starts occur and are abandoned because of obstructions, these must be recorded and the exact changes in location and depths to the obstructions must be noted. If spudding, churn drilling, diamond drill coring, or blasting are used to pass an obstruction, information as to the tools and methods used should be recorded in detail. (Any information or opinion as to the nature and thickness of the obstruction which the driller may obtain from his observations and experience should be noted. In particular, if a boulder is drilled an effort should be made to discover whether it is a "hard head" or a local drift. Such information may be a determining factor in the choice of the type of foundation.) If information as to the elevation of the water table and the rate of flow is required by the specifications, then the specifications should clearly define the degree of accuracy, the time to be allowed, and the number of borings in which water observations are to be made. The driller should be familiar with these requirements as well as all other requirements of the specifications under which he is operating. (Unfortunately, where the hole is put down with either a partial or a full casing which is kept full of water or drilling slurry, it is entirely possible to case off a thin water-bearing stratum, which under reduced head might deliver a considerable volume of water, without knowing that it has been passed.) Any degree of accuracy in judging water

conditions requires the opening up of at least a few feet of hole, say 5 feet, in advance of the driving of the casing and also the bailing down of the water in the casing at frequent intervals so that the rate of rise of the water in the hole may be measured. Maintaining a head by introducing water to raise the level in the hole above the natural water table to varying elevations gives a check of the porosity of the soil. Where pack up of the soil may be used to increase its bearing value, as in the case of pedestal piles, this information will be important. Not infrequently drilling water is lost in a dry or low pressure stratum. Where this occurs the depth should be noted in the log, and also the depth at which drilling water is again held. If gas or sulphur water or oil is struck, the depth and, if possible, some approximate measurement of the flow should be recorded (at small extra cost if ordered by the owner's representative). The diameter and type and the depths to which the casing or casings are carried should be stated for each hole. The weight and the stroke of the drop used for driving of the casing and the sampler should also be noted. It is not claimed that the information given in this chapter fully covers the field of core borings, even as it normally applies to building foundations only. The primary purpose of these studies, as stated at the beginning of the chapter, is rather to point out ways and means whereby the total cost of soil investigations may be held within reasonable proportions without sacrificing safety or the requirements of sound engineering design. Briefly summarized, the conclusions that have been reached are: First, set up a preliminary program of soil investigation only after cataloging the basic requirements of the structure to be supported. This may sound like a statement of the obvious, but very frequently altogether unneces-

CORE BORINGS sary and expensive investigation procedures are specified and carried out, though the matters they cover have no bearing on the needs and cost of the building to be supported. Second, require first a minimum program of investigation, but make the request for bid and the technical specifications such that after the minimum obviously necessary work has been performed wholly or in part, it will be possible to expand the investigation along

27

such lines as may be indicated. It is not suggested that the specifications and unit bid prices shall cover the multitude of special investigations which might be undertaken to meet any load requirements under all conditions. Such a request will automatically eliminate all but an extremely limited number of completely equipped and experienced bidders, even though the possible requirements for the particular job may be well within the capacities of many of those ruled out.

5 THE DEMANDS OF THE SUPERSTRUCTURE

SO far in our attempt to solve the problem of choosing the correct foundation to fit a particular project we have for the most part considered the bearing values available in the soil. Perhaps a more logical approach would have been to give precedence to the structural demands. However, load requirements and soil bearing values are so closely knit and so subject to accommodation the one to the other, that neither could be treated without frequent reference to the other. Let us now consider the demands of the superstructure. Is the project to be a cathedral or a national monument planned to stand a thousand years? A mammoth hotel or apartment house of luxury type which will start becoming obsolete before the ink is dry on the leases? A low-income housing development erected on marginal land? Or a grain elevator? A tank? A factory full of heavy machinery? Even if such superstructures were to be built on identical sites with identical soil conditions they would pose totally different foundation problems. The following questions about any particular structure must be studied and answered before even a start can be made on the design of the foundation. What is the required permanence of the structure? At one time the Ford Motor Company instructed its designing engineers to plan all buildings for a life expectancy of 100 years. Safety factors naturally went up on such structures. When the Jefferson Memorial in Washington was under design, it was the declared intent of the engineers to plan a structure that would remain in perfect condi-

tion for 1,000 years. Some of the great cathedrals were no doubt designed with similar life spans in mind. At the other end of the line are the so-called "taxpayers," buildings ready to be demolished almost as soon as they are erected. And in between lies the great bulk of construction, with buildings dotted along the expected time-scale like beads on a string. What special demands of the superstructure, other than its required permanence, call for extra precautions in designing the foundation? Does the structure lie in the path of high winds, hurricanes, tornadoes? Is it situated in the earthquake belt? Is the owner abnormally concerned about nuclear bombs? It may be argued that if the foundation is designed to take care of 100 per cent of the maximum anticipated loads, it will neither be better nor more permanent if designed to have a 300 per cent safety factor. Possibly the key to the over-design of the foundations of many important structures lies in the word "anticipated." In any case, insurance against possible, as well as probable, hazards leaves plenty of room for widely differing answers. The degree of protection, when it goes beyond standard practice and design features required by codes or other authorities, is a matter which the owner should decide in the light of the fullest information his engineers can give him. Does the particular problem have to take into account the effect of vibration and/or impact? The foundation of a factory in which heavy machinery will be operating may show considerably less theoretical design loads

THE DEMANDS OF THE SUPERSTRUCTURE than that of a ten-story hotel. The hotel may represent a greater expenditure of money than the factory. But the factory foundation should certainly be designed with a substantially greater factor of safety than would be financially justified for the hotel. This calls for less net settlement under overload tests, even though the limit of settlement under working load test may be the same for both structures. It may be that as more testing is done the effects of heavy vibrating loads on settlement will be more accurately known, but to date very little has been done in the development of vibration load testing. In fact studies of the effect of vibration on various types of structures are still in an embryonic stage, so it is difficult if not impossible to set measurable limits to foundation vibration. Here is plenty of room for some postgraduate study. The question of impact is closely related to that of vibration. Since it would be difficult to draw a sharp line between the effects of the two forces, I have classed buildings housing machinery setting up impact under the same category as those with vibration. Has the factor of duration of application of load a bearing on the problem in hand? If so, it should be thoroughly considered, since numerous foundation failures have demonstrated this factor to be sadly underrated. Both tests and observation on completed structures show that live loads, such as wind or the presence of crowds of people, generally have little effect on settlement—a hurricane or a tornado may destroy the building but it is rarely found to have had any effect on the foundation. A slow but wide variation of load, such as takes place when the bins of a grain elevator or cement silo are alternately full-loaded and empty, will set up soil stresses which seem to have almost the effect of a continued "driving" of the foundation units. This reaction is largely, if not wholly, dependent on the bearing qualities of the soil and the nature of the transfer of the load,

29

that is, by direct bearing or by friction. But since it is impossible to tabulate all combinations of soil, methods of load distribution, and variation in loading, storage silos are shown in a high classification. PERMISSIBLE SETTLEMENTS

NET, GROSS, AND

DIFFERENTIAL

Where a no-settlement foundation is impossible, either physically or financially, it is necessary to decide upon the permissible limits of gross, net, and differential settlements for foundations underlying various types of superstructures. The writer has submitted the question of permissible settlement to a number of topflight designing engineers, and while he cannot say that Table 1 represents anything like unanimity among those consulted, yet, averaging the answers, it represents the best approximation he can offer at present. Such uncertainty and wide variation of opinion seem strange. One would think that the choice of type and design of foundation must always start with a decision as to the kinds and the limits of settlement which may be safely allowed. If such information is not somewhere available, further studies on the subject are surely urgently needed. Tolerance to settlement, as specified by various authorities for soil-load testing, pileload testing, and caisson testing, seems to indicate considerable vagueness in the minds of many designing engineers as to the nature, as well as the degrees, of critical settlements which should be allowed for different classes and types of structures. Critical settlements, for which values should be set, would include the following (a) ultimate total gross settlement under working loads; ( b ) differential settlement under working loads, that is, the difference in amount of settlement at various points throughout the foundation which may occur simultaneously as a result of varying intensity of loading between maximum and minimum load conditions.

30

THE PRELIMINARIES

If the allowances for (a) and (h) are properly chosen, and if they are accurately assessed by tests, then the probability of distortion and cracking of the structure can be held within proper limits. Also the probability of injurious reactions in instruments or tools, where such are to be supported, can be eliminated. However, two further tolerances should be set up: ( c ) total gross settlement under some predetermined degree of overloading—usually set at 200 per cent of the working load; and ( d ) total net settlement under some predetermined degree of overloading—usually set at 200 per cent of the working load. Factors ( c ) and ( d ) represent safety measures. They would scarcely be needed if full dependence could be placed on the methods used to prove ( a ) and ( b ) . But the overload net settlement requirements, which presumably would be judged by overload tests, serve two very important purposes. Many soils, particularly clays and silts, or admixtures of sand with clay or silt, may reach stability under load only after a considerable lapse of time. It is usually prohibitively expensive to permit working load tests to remain on a foundation for a period of weeks or months in order to make certain that the unit under test has finally come to rest. However, the behavior under substantial overload will give accelerated test results. Movements under, say, 100 per cent overload will be many times as rapid as they would be on the same unit under working load only. Too few experiments have been made to set up any direct time relationship between settlement under 100 per cent and 200 per cent of working load. However, it can be stated that damaging settlements over a long period of time under working load have rarely if ever been noted where the units under test had come to substantial rest (say a n i n c h o v e r a 48not more than Viooo hour period) under a test load of double the working load.

The gross settlement under overload is made up of the following factors: ( 1 ) the net settlement under overload; ( 2 ) the elastic deformation of the shaft; and ( 3 ) the elastic deformation of the bearing soil. Factors ( 2 ) and ( 3 ) are frequently considered as if they would be operative over the entire length of the pile. This is not necessarily correct because the length affecting these factors will be modified by the degree of transference of load by friction on the shaft of the pile. Recovery after release of load will rarely equal its theoretical value where the deformation has been figured for the full length of the pile. The last two factors can be individually evaluated only by rather expensive field tests. It is possible, however, by measurement of the recovery after removal of the test load, to ascertain the combined value of factors ( 2 ) and ( 3 ) . The amount of elastic recovery should be carefully studied in cases where the live load will be in large part of a vibrating or impact nature. The net settlement of two possible types of foundation may be the same, but if one type meets theoretical requirements by virtue of a large recovery factor while the other meets the net settlement requirements without benefit of recovery, the second type will be far superior for vibrating loads; the first type may not be usable at all. The smaller the spread between gross and net settlement, the smaller will be the amplitude of the vibration waves and therefore the greater will be the "dampening" effect of the foundation upon injurious vibration set up in tools or instruments. HOW TO USE PERMISSIBLE TABLE

(TABLE

SETTLEMENT

i)

From the foundation design standpoint, the cost of a structure is not the determining factor in assigning it to a particular class in Table 1. The class of a structure may best be judged by its permissible tolerance to settlement. For example, a heavy duty, long-span structural steel or suspension bridge would

THE DEMANDS OF THE SUPERSTRUCTURE

31

TABLE 1. STRUCTURES CLASSIFIED ACCORDING TO PERMISSIBLE FOUNDATION SETTLEMENTS CLASS I Requirements

Structures

1. Differential settlements under working load must be held within a maximum limit of

a. Monumental structures, with interiors or exteriors of marble or other material in which cracking is readily observable. b. Cathedrals and large power plants, steam and hydraulic. c. Foundations for heavy machinery, without severe vibration or impact. d. Structures designed to remain in serviceable condition for exceptionally long periods, such as monuments. e. Grain elevators, storage bins, and other structures subject to wide changes in loading lasting over considerable periods. f. Large concrete tanks. g. Office buildings, hotels, and stores, all of 10 or more stories in height and all of reinforced concrete construction or structural steel and concrete construction. h. Warehouses of multiple-story, heavy-load type of reinforced concrete. i. Retaining walls.

2. Gross settlement under working load, maximum As an indication that these requirements will be met— 3. Gross settlement under 200% working load, maximum w 4. Net settlement after application and removal of 200% working load, maximum limit Foundation must develop full capacity by direct bearing without dependence upon transference of load from structure to soil by friction.

CLASS 11 Requirements

Structures

1. Differential settlements under working load, maximum

a. Foundation for machinery causing heavy vibration or impact. b. Buildings housing delicately balanced instruments, such as automatic telephone exchanges, telescopes, testing equipment. c. Concrete arches for bridges, hangars, and the like.

2. Gross settlement under working load, maximum Vi" As an indication that these requirements will be met— 3. Gross settlement under 200% working load, maximum %" 4. Net settlement after 200% overload released, maximum Vi" Foundation should be capable of developing load capacity by direct bearing without dependence upon friction for transference of load.

CLASS 111 Structures

Requirements 1. Differential settlements under working load, maximum Yi" 2. Gross settlement under working load, maximum %" As an indication that these requirements will be met— 3. Gross settlement under 200% working load, maximum

a. b. c. d.

Bridges, structural steel or suspension types. Steel frame buildings. Steel tanks. Piers and docks.

32

THE PRELIMINARIES TABLE 1. STRUCTURES CLASSIFIED ACCORDING TO PERMISSIBLE FOUNDATION SETTLEMENTS (cont.)

4. Net settlement after 200% working load has been applied and released, maximum 1" These structures may be of equal money value with those of Class I and Class II but will be less affected by settlement. They should develop their bearing at least principally by direct loading in or on a noncompressible stratum not underlain by materials of lesser bearing value. CLASS TV Requirements

Structures

1. Differential settlements under working load, maximum 2. Cross settlement under working load, maximum As an indication that these requirements will be met— 3. Gross settlement under 200% working load, maximum 2" 4. Net settlement after 200% overload released, maximum Loads in these foundation units may be transferred to bearing strata either directly or by friction, or by a combination of both.

a. Factories, b. Stores. d. e. fa g.

Apartment buildings. Hotels of less than, say, 15 stones, steel-frame type. Churches. Schools. Warehouses of medium load capacity.

Machine shops not housing massive machinery or extra-heavy cranes or gauging devices of high degrees of delicacy. ¡_ Recreational buildings. j Highway structures, grade eliminations.

CLASS V Requirements

Structures

Permissible settlements vary too widely to tabulate.

Temporary structures of all types, such as military bridges, false work for concrete arches, wood frame buildings, etc., would be included in this class.

usually suffer no injury even by very substantial settlement of its supports, while a c o n c r e t e arch bridge of modest size would require foundation settlements to b e held to a minimum. Again, a foundation under heavy machine tools must be of a higher class than would be required for the foundation of even a monumental public building.

is designing. In classifying structures on the basis of permissible settlements, I do not attempt to cover all types of superstructure, though almost any type could b e fitted into T a b l e 1 by analogy. Settlement factors ascertainable by direct load-testing a r e given the greatest weight, but they are not and should not be the only factors considered. Other factors are, as previously stated, t h e importance of the structure, the d e g r e e of insurance desired, the effect of vibration a n d / o r impact, and t h e duration of application of live loads.

T h e designing engineer using Table 1 would first have to decide the maximum settlement—gross and differential, under working load conditions—which would be allowable for the type and use of the structure he

6 BREAKDOWN OF DESIGN AND SPECIFICATION COST FACTORS

T H E designing engineer is probably mentally favoring some particular solution to his problem. But before deciding definitely, it would be wise for him to survey the entire field with an open mind. Sometimes, it would seem, the designing engineer omits this precautionary run-through of all general foundation types together with the special types included under the general classifications, and while he probably will eventually choose a sound solution so far as engineering sufficiency goes, he may miss the economically correct answer and burden the owner with unnecessarily heavy cost. There are many factors which enter into the cost of a foundation. A failure to understand and give due weight to any one of them must eventuate in an addition to the total cost of the work. Sometimes the opportunity for saving which is missed, or the unnecessary cost which is added, may run to a third or more of the whole cost of the foundation, but even if the net saving which can be effected is small, there is still good reason for making it. If any saving is passed up, it should at least be with full knowledge and not from ignorance of the fact that the chance of saving existed. In Table 2, the Elimination Table, which prefaces the detailed study of the sixteen leading foundation types, comparative costs of these types have been noted in a general way. There are several factors which do not appear in that table, but which none the less have an important bearing an cost.

The first of these is the question of the most efficient load per pile (or less frequently the most efficient load concentration to be employed on caissons or piers). This factor is of prime importance since it may easily result in an increase or decrease of the cost of the entire foundation by 25 to 30 per cent or more. Where a single factor may have so great an effect on total costs, it is surely essential that it be studied on a logical basis and not be guessed at. The high load pile, say 60 tons working load per pile and upwards, has been very little used to date in this country except in the case of the open-end steel pipe pile. Frequently it has not entered into the design studies. Non-end-bearing piles with working loads up to 120 tons have been used freely elsewhere throughout the world. This factor may have a strong bearing on the selection of the type, since some piles which are competitive where high unit working loads can be used would not even be considered where 30 or 40 tons would be the working load limit. A detailed study on the choice of load per pile to produce the highest efficiency is developed in Chapter 30. The second factor, which will come first in point of timing, is basic to the first. This second factor is that of soil studies. It bears directly on the engineering problem and indirectly but very importantly on costs. Up to a few years back many foundations, including some with heavy loads and difficult soil conditions, were designed without benefit of dry core borings; wet samples were con-

34

THE PRELIMINARIES

sidered sufficient, and subsoil studies and soil analysis in good soil mechanics laboratories were thought to be unnecessary refinements. This type of guesswork design is still met with occasionally, but fortunately it is becoming rare. However, there is still quite a way to go before all foundations, or even all important foundations, have the benefit during the design stages of scientific laboratory and field soil studies and load tests. The economic impact of this factor comes after the superstructure is completed and the cracking starts. The third factor is the failure of the specifying engineer and the designing engineer to understand the contractor's limitations and difficulties. The engineer may find it simple to put a line on paper which cannot be reproduced in building materials except at wholly unnecessary cost. The contractor might sometimes stand the loss, but if he knows his way around the chances are that he has protected himself in his bid—and the owner pays. The effect of specifications on competition should perhaps be included as the second rather than the fourth factor. It is probably as important as the first factor, though not subject to as close a cost analysis. Free enterprise and human nature being what they are, a bid based on no competition or on competition so hamstrung that it is little more than window dressing will inevitably raise the bid price up to a figure limited only by the danger of not letting out the work at all. Even where the foundation types or subtypes have been well chosen from the engineering angle and would normally leave full room for competition, the specifications may be so written as to stifle competitive bidding, even where no such result was intended. Advertising propaganda is often mistaken for engineering fact. Naturally, the engineer cannot be an expert on all the endless details entering into many modem structures. The road to safety in this dilemma would seem to be the

same road for foundations as has been taken in the specifying of most other technical requirements—that is, the substitution of performance specifications for the older type of specifications based on theory. Emphasis on performance may seem to be a frequently recurring refrain. The writer feels that such reiteration is necessary because foundation engineering has to date lagged far behind other branches of the science of construction in stressing what ought to be the final criterion— the test. Even with performance proved by fullscale tests as the final touchstone, we do not by any means escape the necessity of preliminary theoretical studies to point the way to the types which may reasonably be chosen for the tests which will determine acceptance. It is obvious that if the suitability of all types had to be determined for each site on the basis of full-scale performance tests, the task would be impossible both from the standpoint of cost and of time consumed. The method of selection by elimination based on the results of sampling of core borings, and of soil studies carried out on these small-scale samples, is an indispensable requirement and prerequisite of the full-scale performance tests. Throughout Part One I have dealt primarily with the setting up of the foundation problem, which can be defined as that of fitting together the needs of a structure and the values of a given supporting soil. The structure requirements would consist of the nature and intensity of the loads to be carried—the total and differential settlements permissible under the action of those loads; the permissible limits of reaction to vibration and shock; and the degree of importance and permanence required. With the requirements of the structure clearly defined, the second step consists in the choice of a site and the subsequent soil studies necessary to prove that the site con-

BREAKDOWN O F COST FACTORS ditions could or could not meet the building requirements and, if they could, the soil characteristics upon which dependence would have to be placed. The basic information as to the soil and the loads to be carried having been obtained the next step would be to weld the two sets of facts together in such a way as to obtain the

35

required foundation at the least possible cost. This is the subject of Part Two. While basic problems can be simply stated their correct solutions will of course be affected by a great many related and sometimes very distantly related issues. Part One set up the problem, the following parts undertake to answer it.

PART TWO • FOUNDATION TYPES

7 THE ELIMINATION TABLE

IT cannot be stated too often or too strongly that the author's object in compiling the Elimination Table is not to provide a short cut to a final choice of foundation type, but merely to assist the designing engineer in narrowing his field by excluding those types which, for one reason or another, will not solve his problem. Having done this excluding, he arrives at a limited group—say three or four types or subtypes of foundation within which the final choice should lie. It then becomes advisable to get a thorough understanding of each of these types, to learn the engineering conditions, both of superstructure and soil, which favor their use and also the conditions which might preclude their use. These factors are outlined in a very general way in the Elimination Table, but at this point it becomes necessary to have a closer look. One of the basic requirements for the use of untreated wood piles is that the pile cutoff must be below the permanent water table. That requirement might seem to be simple and definite. But one should not skip the study which must be given the word "permanent." The presence of an adjacent river, a lake, or even the ocean is by no means proof that the low level in such a body of water is necessarily the "permanent low water level" to be applied to wood pile cutoffs. Far from it. Failure to carry the study far enough on this question of pcrmanence of water table has been—as is shown more fully in Chapter 9—one of the major causes of wood pile foundation failures. Each type of the residual group of types to

which the engineer must now direct his attention must be studied separately. This study may require added tests similar to those already outlined in earlier chapters—core borings, soil load tests, probings, etc.—and may also need other and more specialized forms of testing and investigation. There are some sixteen types or subtypes of foundation mentioned in the Elimination Table, but a designing engineer may need special knowledge of only two or three of these. There may be a considerable mass of detailed foundation information on the remaining types he may never want to study. For this reason the author has tried in the following chapters to pull together the practical information available on each type. The Elimination Table indicates the particular chapter in which each type is discussed. Each of these sections contains: 1. A very brief statement of the general nature and history of the type, and, where needed, a description of the method of installation. 2. A discussion and elaboration of the controlling superstructure and soil conditions as given in the Elimination Table, and a statement of other and less general conditions having a bearing on the suitability of the type. 3. An outline of additional tests and studies which may be required. 4. Remarks on the probable cost effect of 1, 2, and 3 on the type being considered. Other factors are then added, such as the effect of code requirements and union rules and rates for the locality of the work.

40

FOUNDATION TYPES T A B L E 2. E L I M I N A T I O N T A B L E

II

FOUNDATION TYPE

GOVERNING SUPERSTRUCTURE CLASS FOR WHICH GENERALLY ACCEPTABLE (See Table 1)

III REQUIREMENTS PERMISSIBLE CONCENTRATIONS OF LOADING

Spread Footings (Chapter 8 )

If on impenetrable material, all classes. Otherwise Class IV only.

Such as would require not more than 150 sq. ft. of area of the unit bearing value of the bearing stratum.

Wood Piles Untreated Treated (Chapter 9 )

Classes III, IV, and V.

Wood pile loading has been variously limited by codes at from 12 to 25 tons each. Irregularity of shape and straightness eliminates use of test programs. Load concentrations requiring not more than 40 piles on 2-ft.-6-in. centers on 20-ton per pile loading.

Note: Mats, not in the order of costs, may vary widely.

Piles Cast-in-Place Uncased Concrete Piles (Chapters 10 and 1 1 )

Caisson Piles (Chapter 1 1 )

Same conditions as for Spread Footings with two further requirements: ( 1 ) The load distribution of the structure must be reasonably uniform, i.e., load intensity under working conditions should not vary more than 2 5 % from point to point. ( 2 ) T h e bearing value of the soil under the mat should be sufficiently uniform so that settlements (indicated by soil load tests) would show less than % in. variation (or whatever settlement tolerance is allowed for the class of structure) under loadings equivalent to the maximum loadings under working conditions. If carried to impenetrable material and length does not exceed 60 diameters, all classes. Otherwise Classes III, IV, and V.

Up to 1,000 tons where individual piles loaded to 5 0 tons or more on basis of test program are used.

Note: "Caisson Pile" is a trade term applied to heavy load capacity, large diameter ( 2 4 in. to 48 in.) piles. Usually driven to hardpan or rock; cleaned out and seated; casing withdrawn. If carried to impenetrable mateUp to 1,600 tons where individrial, all classes. ual piles loaded 100 to 4 0 0 tons. Otherwise by special design only, including belling or pedestaling.

41

THE ELIMINATION TABLE T A B L E 2. ELIMINATION T A B L E

(coni.)

IV

V

VI

DEPTH TO BEARING STRATUM ( Might prove economical within the following depths providing bearing stratum has minimum value stated. )

LIMITING WATER CONDITIONS

LIMITING CONDITIONS FOR SOIL TO BE PENETRATED TO REACH BEARING STRATUM

Bearing stratum has minimum 1 ton per sq. ft. working load value at maximum 10 ft. depth. This depth is the depth for an acceptable stratum not underlain by poorer material.

Unwatering for placing footings can be handled by 2 0 0 gal.-portable pumps.

Can be dug with clamshell. Boxing may be required but no driven sheeting.

65 ft. below cutoff in the Eastern section of the country. 100 ft. below cutoff in the West. Based on 20-ton bearing per pile.

Untreated—excavation for caps not more than 8 ft. dry excavation or 6 ft. wet excavation. Treated—no effect.

Untreated—Hard woods, cypress, and yellow pine stand pretty rough going. Some other woods break easily when dry, e.g., spruce, jack pine. Treated—large obstructions bruise and tear creosote jacket. Also if treated by empty cell process, piles break easily.

Note: Mats may be placed directly on soil or may be supported by uniformly distributed piles or caissons. If pile or caisson supported, comparative costs will be governed principally by supporting elements and the soil requirements will be those of the supporting members as shown in this table.

Bearing stratum can develop twice the proposed working load without exceeding permitted settlement. Pile test loads can be reached within 65 ft. depth.

Hardpan or rock. Encountered within limit of 50 ft.

length

Sands and silts having less than 2 5 % clay admixture. Such as can be penetrated by maximum driving of 6 blows of No. 1 Vulcan hammer per inch of penetration ( 1 5 to 35 blows on standard spoon test).

Unimportant

Unimportant. Forming casing pumped bailed. Concrete deposited tremie.

or by

Resistance above final bearing stratum such that blows on standard sampling spoon not in excess of 25 per foot.

42

FOUNDATION TYPES TABLE 2. ELIMINATION TABLE (cont.) 11

FOUNDATION TYPE

Light-Shelled Cast-in-Place Concrete Piles (Chapter 12)

GOVERNING SUPERSTRUCTURE CLASS FOR WHICH GENERALLY ACCEPTABLE (See Table 1)

111 REQUIREMENTS PERMISSIBLE CONCENTRATIONS OF LOADING

If driven to impenetrable material and length does not exceed 60 times minimum diameter, all classes. Otherwise Classes III, IV, and V.

Up to 1,000 tons where individual piles loaded to 50 tons or more on basis of test progTam. Without tests end-bearing piles to 40 tons. Other than end-bearing 30 tons.

Composite Piles (Chapter 13)

Limited to the lowest classes shown for either section.

Limited to the lowest concentration shown for either member.

Precast Concrete Piles (Chapter 14)

All classes if properly designed to withstand handling and driving and field tested for damaging cracking.

Limited to individual pile loads 30 to 40 tons unless qualified under load test program, then up to 100 tons when designed for handling, placed by jetting and tested for cracking and also for water penetration.

Fluted Tapered Steel Piles (Chapter 15)

(Union Metal Co. Pile). Thickness of steel 9 gauge or thicker. Driven without supporting mandrel light-walled pipe piles. Cylindrical, driven with or without supporting mandrel, 9 gauge or heavier. Light pipe driven with mandrel We-in. wall or more.

Light-Walled Steel Pipe Piles (Chapter 15)

If carried to impenetrable material, all classes. Otherwise Classes III, IV, and V.

Up to 1,000 tons where individual piles are loaded to 50 tons or more on the basis of a test program. Load on individual pile under test program 60 to 100 tons. Without test program, 40 tons where load delivered by direct bearing. 30 tons when load delivered other than by direct bearing.

Structural Steel Piles (H-Beam Piles) (Chapter 16)

Not suitable unless carried to impenetrable material. Classes I, III, IV, V.

Up to 1,500 tons where individual piles loaded to 100 tons or more.

THE ELIMINATION TABLE

43

T A B L E 2. ELIMINATION T A B L E (coni.) TV

V

V/

DEPTH TO BEARING STRATUM (Might prove economical within the following depths providing bearing stratum has minimum value stated.)

LIMITINO WATER CONDITIONS

LIMITING CONDITIONS FOR SOIL TO BE PENETRATED TO REACH BEARING STRATUM

Unimportant unless exceptionally high pressures are indicated, in which case the shell type and thickness may have to be modified.

Such as can be penetrated by maximum driving of 6 blows of 15,000 ft. lbs. or more where jetting or spudding can be used ( 1 5 to 35 blows on spoon).

Limited to the lowest concentration shown for either member.

Limited to the lowest concentration shown for either member.

Where the elements of the combination are properly chosen and correctly driven, can be used to penetrate soils which neither element alone would penetrate. Can be used in any soil where piles can be driven, jetted, or spudded into position.

Rarely competition in price with cast-in-place piles except for bridge approaches, trestles, and viaducts where piles will extend above surrounding ground or water.

Unaffected by water conditions.

Expensive and hard to place through rough soils, e.g., boulders, heavy rock, or rubbish fills. Serious cracking may occur below ground line, where it cannot be detected.

100 ft.

Unimportant.

Such as can be penetrated by driving 10 blows or more of a No. 1 Vulcan or equal ( 1 5 to 50 blows on standard spoon test).

Rock at depth of 150 ft. or less.

Unimportant.

Toughness of soil of little importance. Not suitable where large boulders would cause deflection which cannot be checked.

100 ft.

44

FOUNDATION TYPES TABLE 2. ELIMINATION TABLE II FOUNDATION TYPE

Open-End Pipe Piles to Rock (Chapter 17)

(cont.)

GOVERNING

SUPERSTRUCTURE CLASS FOR WHICH CENERALLY ACCEPTABLE (See Table I)

III REQUIREMENTS PERMISSIBLE CONCENTRATIONS OF LOADING

All classes.

Not less than 500 tons. Up to 1,500 tons. For underpinning and other special work may be economical for load of 50 tons or even less.

All classes,

Not less than 500 tons. Up to any load.

Caissons with Working Chamber in Free Air Drop Shaft (Chapter 19)

All classes,

Not less than 500 tons. No top limit.

Caissons Compressed Air Type (Chapter 19)

All classes.

Not less than 500 tons. No top limit.

Drilled-In Caissons (Chapter 20)

All classes.

Any concentration over 200 tons.

Caissons Chicago Wells. All classes ( Belled out to hardpan or directly bearing on rock. ) (Chapter 18)

a. b. c. d.

Soil Consolidation Vibro Compaction Sand Pile Compaction Chemical Compaction Compaction by Electric Current

Composite Piles (Chapter 13) Cased Concrete and Steel Pipe Piles Cased or Compressed Concrete Top Section and Wood Precast Concrete and Wood

Where proved suitable by fullscale tests. Spread footings or mats may be carried by the compacted soil and would then be required to meet same conditions as Spread Footings.

See Spread Footings and Mats.

Note: In general the governing characteristics of the lower section of the pile, as given above, would govern use-load and soil conditions. The upper section acts largely as a column to transmit the structure load to the soil bearing value developed by the lower section. The critical point in all composite piles is the splice between the two sections. To produce a good splice the two sections must be held in perfect alignment. The splice must be designed to develop the full bending value of the weaker section of the pile.

THE ELIMINATION TABLE T A B L E 2. E L I M I N A T I O N T A B L E

45

(coni.)

JV

V

V/

DEPTH TO BEARING STRATUM (Might prove economical within the following depths providing bearing stratum has minimum value stated.)

LIMITING WATER CONDITIONS

LIMITING CONDITIONS FOR SOIL TO BE PENETRATED TO REACH BEARING STRATUM

Rock at depths to 150 ft.

Unimportant.

Any soil not requiring drilling operations.

Rock or hardpan up to 150 ft.

Must permit of bottoming out in the dry.

Must be such as will stand open for minimum 3 ft. at all depths to be penetrated.

Rock or hardpan any depth.

Such that flow can be contained using 2 ft. 6 in. pulsometers or equivalent. No quicksand.

Costly except in clean going. Boulders, tree trunks, running sand, difficult to handle. High friction soils very hard to penetrate without hanging up.

Up to 100 ft. head of water.

Such as can be met with air at lbs. or less.

Rough going, such as large boulders, poor grade rock, or the like, makes this type prohibitively expensive where any other type is feasible, except for large diameter caissons such as those under bridge piers.

Any depth, rock bearing.

Unimportant

Load applied at surface of compacted material.

Purpose of compaction to modify soil conditions.

None.

Vibro method usable in sand or highly sandy soils. Sand pile (compaction type) principally in sand. Can be used where soil pressures induced will not be relieved by heave. Sand wick piles are used in speed up drying out of any very soft soil, organic silt, peat, very loose sand or inorganic silt.

FOUNDATION TYPES

46

5. Remarks on the possible advantages of a preliminary load-by-test program. These several detailed studies of possible types may sometimes provide an answer that is obvious, and a single type will be the solution. More frequently there will still remain a question as to the comparative cost of two or more types or subtypes. In either case, the next move on the part of the designing engineer, or the general contractor, where one has been chosen in advance of the letting of the foundation work, will be to ask for bids by contractors or subcontractors on the type or types to be considered. The contracting authority will require from the specifying engineer, usually the same person as the designing engineer, the following: (a) A formal specification for each type to be bid. ( b ) A form of request for bid. Sometimes on public work, this may be a very elaborate form. On private work, in many cases, it will be in the form of a letter or phone call, stating how plans and specifications may b e obtained, ( c ) A form of bid. ( d ) Finally, to be used upon acceptance, a form of contract. While these forms have no bearing on the choice of type, the specification and the form of contract may vary considerably with the type chosen. Short forms for (a), ( b ) , ( c ) , and (d) which can be adapted to each of the sixteen types and subtypes have been grouped together in later chapters. With preliminary soil investigations finished, data in hand and analyzed—and with a clear understanding of the demands of the proposed structure—we are prepared to begin choosing the foundation. Organization

of the Elimination

Table

The Elimination Table consists of six columns, and before proceeding to a detailed study of the table the following points about these columns should b e noted.

FOUNDATION

TYPE

T h e order in which the general types and subtypes are given in Column I is that of their usual comparative cost, assuming that the necessary structural and engineering requirements will be met. Thus, if spread footings will meet all requirements, they will almost certainly provide the most economical solution. SUPERSTRUCTURE CLASS

Column I I lists the class of superstructure for which the foundation type in Column I is generally acceptable, on the basis of the permissible settlement (see Table 1 ) . It is obvious that in setting up settlement tolerances which might be expected from various foundation types, I am making a generalization which is based on experience under average conditions and which should be used only in the broadest way, even for purposes of elimination. Important factors such as the length of a pile or caisson, the compressibility of the bearing stratum reached or strata penetrated, the refusal to which the driving is carried, the possibility of the reduction of the load at the point of the pile by reason of load transferred by friction on the shaft—which may be the equivalent of shortening the column with a consequent reduction of the compression factor in the working load settlements—all these are variables which are not taken into account in the Elimination Table. These factors are discussed in detail and methods of evaluating some of them are suggested in the discussions of special types in Part Three, which deals with pile load tests and general test programs. PERMISSIBLE CONCENTRATIONS OF LOADING

Column I I I deals with permissible concentrations of loading—that is, column, pier, wall or area loading—which generally and under average conditions might indicate a limita-

THE ELIMINATION TABLE

47

tion to the usability of the foundation type. There is probably not one limitation shown in this column which has not frequently been exceeded where it should not have been exceeded, and has also been exceeded where special conditions have indicated that such excess was justified. For example, many buildings in Mexico City have been built on mat foundations in which most of the requirements here indicated were not met or even approximated. But the choice lay between a foundation failing to meet the proposed limits and no foundation at all—because no building. Economically none of these structures could justify the cost of what might be considered a "proper" foundation. Incidentally, it might be noted that at least a hundred of these structures would already be classed as failures, and many of them condemned as unsafe, in most other countries. Perhaps the answer should have been, "no foundation and no building."

tum will have insufficient bearing value to meet the load concentration to be transferred to or through it, by the delivery of load in or on the bearing stratum. Where a limit such as 75 feet for wood piles is suggested, local conditions should be considered when applying it. For example, wood piles of a length over 75 feet and meeting the usual tolerances are high in price and difficult to find in the East and Central areas of the United States, while piles up to 110 feet and more are readily procurable in the North Pacific Coast territory. For a job in New England or the North Central States, if other conditions indicated that cast-in-place concrete piles and wood piles would both meet the requirements (except that wood pile length would be in excess of 75 feet) then, tentatively, wood piles should be dropped from consideration.

The limit on the size of a spread footing or the number of piles in a group is an economic limitation based on the assumption that other types of foundation could qualify. If, however, none of the other types do qualify, then obviously a further study of the engineering and economic possibilities of non-qualifying types must be made, assuming loading concentrations in excess of those here suggested.

Column V deals with the factor of "free" water in the soil to be penetrated in developing the required bearing capacity. The depth of wet excavation is a vital factor in the cost of spread footings and most types of caissons. In considering any type of foundation, the possible changes in the elevation of the water table should be considered. A clay soil which may show ample bearing value when dry may be worthless for foundation purposes when saturated. Any one of the combinations—clay and sand, clay and silt, clay and gravel— where a considerable amount of clay, say 25 per cent or more, is present may lose a large part of its bearing value when wet. The reverse may be true where the deletion of water in sandy or silty strata may cause soil shrinkage, settlement, and, depending on circumstances, either increased or decreased bearing value. Any large loss in the bearing value of a stratum to which the working load has been partly or entirely transferred may pose a dou-

DEPTH TO BEARING STRATUM

Column IV treats of another factor which has considerable bearing on the suitability of a foundation type to the soil and superstructure conditions—depth to a usable bearing stratum. This bearing stratum is not meant to be judged entirely by the shear and compression value of the stratum on which load could be directly applied, as indicated by the hardness of the stratum and the load to be carried, but must also satisfy the requirement that no stratum below that described as the bearing stra-

LIMITING WATER CONDITIONS

48

FOUNDATION TYPES

ble threat to the integrity of the foundation, first, by loss of previously developed bearing, and, second, by a reversal of friction which will result in throwing the weight of upper strata, which settle under the changed conditions, onto the pile by friction as an added downward load. This reaction has been the principal cause of a number of failures where extensive pumping or another unforeseen cause has lowered the water table existing when the building load was first applied. LIMITING SOIL CONDITIONS

Column VI deals with the nature of the soil to be penetrated—other than the moisture content, covered under Column V. As in the case of the other factors, the effect on the choice of foundation type of the soil to be penetrated will vary widely as between different types of foundation. Sharp, strongly bedded obstructions—such as occur in fills made of rough blasted rock, or wreckage from massive concrete structures, or thin layers of limestone, sandstone, slate, schists, or shales —may rule out the use of creosoted wood piles and add greatly to the cost of cast-inplace concrete thin-shelled types, where the shell is exposed directly to the soil while driving the pile. Such obstructions will also add greatly to the cost of any hand-dug type of caisson. Our firm took over a job where heavy subway rock spoil had been dumped in to make the principle fill, after which the surface was covered with ordinary soil fill. Several types of pile, including open-end steel pipe, had been tried and abandoned. The final solution proved to be the advance drilling on every pile location of pilot holes of approximately the diameter of the piles to be formed. Needless to say, this solution was extravagantly expensive. Had the site been investigated and the conditions exposed before the first abortive attempts at piling were made, the foundation cost could have been greatly reduced. The abandonment of the site, or an alteration in

the designed concentrations of loadings, might have been the best solution of all. Where the number of standard spoon blows is used to indicate the bearing value of a stratum, a wide leeway has been given (such as 15 to 35 blows). This is necessary, because the correct deduction to be made from a given number of blows per foot, on say a 2-inch sampling spoon under the impact of a 140pound weight falling 2% feet, will be governed in large degree by the nature of the soil to be penetrated. Where an assumption is made as to the probable length of a pile, indicated by a certain number of spoon blows, not only the type and nature of the soil producing the resistance but also the size and shape of the pile must be taken into account. If, for a foundation type, only one governing requirement is missing, then the elimination of that type is far from certain. For example, if rock could be reached by 12 or 15 feet of dry excavation which would not require boxing or shoring and all requirements for spread footings with this single exception were met, then spread footings would still warrant consideration, in fact they would almost certainly be the answer. Check every tentative conclusion carefully. Grouping It is sometimes possible to save time and effort by a short cut which will permit elimination of a "group" of types at one time rather than studying and eliminating individual types and subtypes one by one. In any case some knowledge of the broader "groupings" will at times be useful in the writing of the final specification and in assessing the truths or fallacies in the advertising claims of one type or another. A few of these groupings in common use are determined by basic characteristics, for example, the grouping by method of delivery of load to bearing stratum. Frequently, but often inaccurately, this de-

THE ELIMINATION TABLE livery of load is defined as being by one of two methods, by "end-bearing" or by "frictionbearing." Conditions may permit a choice of either method, but where, as often happens, the borings eliminate one or the other entirely, then a considerable number of entries may be scratched in the elimination handicap and time saved. For example, if no end-bearing can be reached within 1,000 feet, as is the case in many areas of the Mississippi delta, there is no use wasting time studying any of the many strictly end-bearing foundation types. However, it is just here that it becomes important to set up a clear definition of "end-bearing" and to substitute for the very inadequate term "friction-bearing" some other description which will offer a better line of demarcation. To use the terminology of the New York City Building Code, the first class includes all piles which deliver their loads principally in direct bearing on rock, hardpan, or boulder gravel directly overlying rock. The second class includes all other bearing piles, that is, all piles which do not develop their loads principally by direct bearing on rock, hardpan, or boulder gravel formations directly overlying rock. This somewhat clumsy definition was adopted by the writers of the New York City Code to avoid the oversimplified but frequently used definition which classified all piles as "endbearing" or "friction-bearing." It is impossible for any driven pile to avoid elements of both friction and direct bearing, and it is difficult in many cases to even approximate the proportion of the load-carrying capacity which is delivered in usable form by end-bearing and by friction. It has been attempted in a few important foundations where both friction- and endbearing elements would obviously be capable of developing considerable bearing values to measure the ratio between the two, directly and in the field. The method used consisted in the driving of a pile down to or slightly into the end-bearing stratum; load testing the pile to incipient failure; and then pulling the

49

pile while measuring the maximum stress needed to produce an upward movement. The amount of this tension was assumed to be the friction value of the pile, and the differences between the total downward load at failure and the tension pull necessary to start the pile was assumed to be the end-bearing value. Such tests are expensive and they would only be justified where gross settlements or the possibility of even minor differential settlements would be of vital importance. Such conditions might be anticipated where the piling would be required to support long-span concrete arches of bridges or hangars or the like. There are two possible errors in the deductions made from the tests described. The first is that downward friction and upward friction on a driven pile are not always necessarily the same. This is particularly true where the pile is driven through a varved clay, the strata of which may bend slightly with the passing of the pile shaft but may act as "toggles" clamping the pile against upward movement. The second error is in the assumption that both friction-bearing and end-bearing can operate simultaneously. The soil in which the friction-bearing is developed will generally have a low "modulus of elasticity," and considerable movement of the pile shaft may be necessary to develop the maximum frictionbearing value, which will be indicated by the pull test. If the lower end of the pile is bearing on or in a bearing material which is virtually impenetrable and possessed of a high modulus of elasticity, only a very slight movement of the pile may be possible under load without partial failure of the shaft or of the stratum in which the end-bearing is developed. Little or no friction may have entered into the downward load indicated by the test and the value shown may be practically all end-bearing. This combination of friction- and endbearing is frequently assumed by designers, but where a pile presumably could develop

50

FOUNDATION TYPES

both end-bearing and friction values, the only safe assumption, regardless of test, would seem to be that all of the bearing value under working load will be in the form of end-bearing and that the pile shaft must be designed accordingly. There are conditions under which the choice of a non-end-bearing pile would be preferable even though bearing on rock could be obtained by the use of a somewhat longer end-bearing pile. One such case was observed in the design of a foundation for a building intended to house gauges of the utmost delicacy. Tests were made on both steel H-beams carried to rock at a depth of about 100 feet and on piles which were driven into and pedestaled to compact a fairly resistant stratum at a depth of 40 to 50 feet. Tests were run to ascertain the settlements under anticipated loads and the possible effects of vibration. The net settlements of the two types of piles were approximately the same. But due to elastic deformation of the shaft the gross settlements were considerably higher on the long rock-bearing piles. As would be expected, the effect of the piles upon the deadening of vibration was greater with the short piles driven in well-compacted soil than it was with the long piles subject to relatively great gross settlement.

Another subgroup is that which contains piers, caissons, and end-bearing piles. These three types of foundations are not very clearly distinguished. Some books and codes base the distinction between piles and caissons solely on the diameter of the shaft, others on the method used in forming. If the unit is placed primarily by machine and includes the operation of pile-driving hammers, it is classed as a "pile." If the principal method of placing is a digging operation, whether by mechanical means or by hand power, it is classed either as a "pier" or a "caisson." A universally accepted dividing line between pier and caisson and between pile and caisson is lacking. The line between piers and caissons is generally drawn on the basis of the ratio between average diameter and depth, but no agreement exists as to the ratio which is to govern. Technical terminology is certainly of importance, but lacking a generally accepted rule, I have here, more or less arbitrarily, used a combination of the maximum working load and the minimum diameter as the deciding factors and offer the following rules. Any load-bearing unit belonging to a class or type of units customarily carrying working loads of 300 tons or more and having a miniOne might generalize that the effect of piles mum diameter of 20 inches will be classed upon vibration is a function of gross rather as a "caisson"—except that: than net settlement. Any load-bearing unit placed in a preLoad-bearing piles, whether they classify as excavated opening and having a ratio of end-bearing or non-end-bearing, may be fur- height to average diameter not exceeding 4 ther divided according to the materials used to 1 will be classed as a "pier." or the methods of forming the pile. The most Any load-bearing unit having a diameter frequently used of these groupings, each of 18 inches or less and normally used in lengths which contains a number of variants are: (a) whose diameters exceed 10 inches will be wood piles, creosoted or plain; ( b ) precast classed as a pile. concrete piles, of many and varied shapes and The setting up of a grouping developed on designs; (c) cast-in-place concrete piles, shell- the basis of some obvious similarity of its memencased or compressed concrete of various bers is of doubtful value and may be highly shapes and sizes; (d) steel pipe piles, open misleading. Sooner or later any careful foundaor closed end; (e) composite piles. These tion designer must set up for himself a group groupings are of doubtful value. of foundation types any member of which

THE ELIMINATION TABLE might be the right solution to his particular problem. A group based on similarities which may or may not be vitally important may merely lead him into a maze from which he

51

will find no exit. The purpose of the Elimination Table is to provide a logical means of setting up this essential group covering all of the "possibles."

8 SPREAD FOOTINGS, ALONE AND IN COMBINATIONS

T H E spread footing is probably the oldest and certainly the most widely used foundation type and, where conditions permit of its installation, the cheapest. Tests to ascertain its suitability have been outlined in Chapters 3 and 4. These investigations may include rod soundings, test piles, soil load tests, and borings. If all the requirements listed in the Elimination Table for spread footings are met, the engineer is saved many headaches and the owner much cash. Spread footings can be used for temporary or very light structures under practically all conditions, except where the soil is covered with water or where the surface soil is semifluid. Bearing soils may vary from semifluid to impenetrable. The field of use expands with the increase of firmness of the bearing stratum until finally all types of loads are covered, except those benefiting from anchorage characteristics. No attempt will be made here to cover the subject of structural design of spread footings (or of mats, which are merely expanded, possibly eccentrically loaded, spread footings). The proper design of mats and continuous footings requires considerable knowledge and skill, but most designing engineers have been well grounded in this line of study. Where the building column loads are sufficiently great to require grillage in addition to billet plates, it might sometimes be wise to call in a good consulting engineer familiar with such work, if the designing engineer has not himself had considerable experience. The

cost of a single grillage, where single or multiple combined loads run into thousands of tons, may be several thousands of dollars. I have been able on occasion to suggest redesigns showing savings up to 50 per cent. This can mean a substantial sum in structures having a large number of heavily loaded columns. This matter of grillage and cap design may determine the comparison of costs for a given site and loading, as between spread footings and groups of short piles. The piles permit the concentration of bearing capacity more nearly below the concentrated loads of the superstructure to such an extent as sometimes to warrant the use of piles even where spread footings would give a structurally adequate answer. This is indicated in the Elimination Table, which suggests maximum size of spread footings beyond which they might not be economical, even if other limiting conditions were satisfied. It has been claimed that in many cases the use of short high-capacity (100 tons per pile or more) uncased concrete piles where spread footings would normally be used can show a saving. The saving would in many cases be due in part to the effect of compacting the soil and increasing its bearing value. For example, if the natural surface of the soil would be good for a working load of 1 ton per square foot, and a 20-inch diameter heavily pedestaled pile would carry 100 tons with a length of 15 feet, the spread footing would be using approximately 10 cubic yards

SPREAD FOOTINGS, ALONE AND IN COMBINATIONS of reinforced concrete requiring excavation and forms, where the pile would require only 2 y2 cubic yards of unreinforced concrete. Obviously the method of placing concrete as a pile would be much more expensive on a per-yard basis than the pouring of concrete in the form of spread footing; also some sort of cap would be required to transfer the column load to the pile. However, assuming that the spread footing concrete would be worth $50 per cubic yard, including excavation, forms, reinforcing, and concrete, the 10 yards required would represent an item of $500, which normally would be well in excess of the cost of a 15- to 20-foot uncased pedestaled concrete pile and a small cap. The writer has assumed that one cubic yard of concrete could be driven out in the pedestal, which is probably excessive. SPREAD FOOTINGS PLUS SAND PILES

There is a combination of spread footing in the form of a mat plus sand piles which has rarely been used in this country but with which our firm has completed a job in the California Desert. The method was conceived and designed by the Stone and Webster Corporation. The problem was to "dampen" excessive vibration which seriously interferes with the operation of large compressor plants used in the pumping of gas from the Texas oil fields to Los Angeles. The solution was the driving of about a thousand highly compressed sand piles and the placing of a 3-foot thick reinforced concrete mat, not on the piles but on the highly compacted soil. Many thousands of "sand piles" have been used for drainage purposes in numerous jobs —mostly road work—on both the East Coast and West Coast. In many cases they have been very successful in speeding soil consolidation by acting as wicks or vents to permit the rapid release of the water from the soil. The purpose and the method of forming these piles, however, as also the soil types in which they proved their use, bear little relationship

53

to the sand piles used in the compressor foundation. The desert soil is a fairly loose, medium-tocoarse sand containing but little moisture, and the purpose of the piles was not to drain it but to close up the voids by means of compaction. To accomplish this the sand was placed through an apparatus consisting of a 16-inch diameter, 1-inch thick wall-casing equipped with a tight fitting core used to drive out the added sand into the existing natural sand deposit. The success of the process under the conditions encountered may be judged by these facts: (1) The driving was started with a 40-foot apparatus easily driven the full length by the use of a No. 1 Vulcan hammer, which served also to force out the sand. (2) Before the job was a quarter done, it was necessary to change from the No. 1 Vulcan to a No. 0 Vulcan having approximately twice the driving power. (3) A short time after the change to the heavier hammer, it became impossible to reach the original depth, and the spacing of the piles was increased. (4) Before the job was half complete, it became impossible to reach the 40-foot depth even with the increased power and spacing, and the pile length had to be reduced to 25 feet. I think we may conclude that the pack-up process was a complete success. When the mat was placed on the soil, vibration was reduced to the ultimate minimum. I leave open the question as to whether this foundation is a spread footing or a pile structure. OTHER

COMBINATIONS

There are other "hybrids" of a simpler form —the foundations in which the light loads, or the sections of the foundation under which soil-bearing conditions are best, are carried on spread footings, while the heavier loads or poorer bearing conditions are met by the use of piles of some type. There is a rather widespread objection on the part of experienced foundation engineers to the combination on a single site and undei a

54

FOUNDATION TYPES

single building of any two radically dissimilar types of foundation. Up to a certain point this would seem to be a well-founded prejudice— if it is a prejudice—and there certainly are soil and load conditions under which some foundation types would be so badly mismated as to assure trouble from the start. However, where the economic advantages to be obtained, or the difficulties to be overcome, are such as to warrant some time, trouble, and expense, it would be sound business to go in for a thorough test program—a series of trial marriages, so to speak—which would almost certainly point to a combination of types which would be dependable and economical as well. I have seen some very bad results due to insistence upon the use of a single type of foundation under a single structure in spite of changed soil conditions in some part of the site, but have yet to see any trouble result from the use of two or even more types, where

the difficult conditions have been realized and the combination chosen and installed intelligently. The usual combination is that of the spread footing with some type of caisson or of piles. The danger is, of course, that of differential settlement under working-load conditions. This is often solved by substantially underloading the spread footings by increasing their areas, though sometimes the opposite course is indicated by a proper test program. It is suggested that in many cases the use of a pedestal pile offers the best solution, because by varying the size and drive-out resistance of the pedestals, almost any desired reduction of differential, working-load settlement may be accurately gauged and developed even in very variable ground. One word of caution: pedestals are generally of very little use in pure clay soils or where clay is the preponderant element.

9 WOOD PILES, UNTREATED AND TREATED

L E T us assume that the tests and studies which should lead to a decision on the suitability of spread footings as the link between the known foundation requirements of the structure and the observed qualities of the soil have been completed. If it seems obvious that spread footings will fill all requirements, both engineering and economic, one would generally be justified in accepting the problem as solved and turning at once to the detailed design of the footings. If, however, the requirements for the acceptability of a spread foundation have been barely met, it would be advisable to inquire into one or even two of the solutions which would normally come closest in the order of cost. In such case there would be no point in studying the possibilities of, say, an air caisson. There would be no chance that such a solution could be right from an economic point of view. But if short, untreated wood piles could meet the needs of the situation from an engineering point of view, it might be well for the designing engineer to find out, by a process of actual bidding if possible, or at least by making a sketch layout and getting estimating figures from competent contractors, whether in this particular case the order of cost, as between spread footings and wood piles, might be incorrect by reason of some special requirement of the structure or peculiarity of the soil. This is no more than a repetition of the warning, given several times before, that apparently fixed rules governing foundations must always be used with caution and can

only be considered as statements of average and reasonably probable conclusions, particularly when applied to the economic side of the problem. Not only do many basic factors which are hard to evaluate enter into all construction costs, but there are also "intangibles" which can be decided only by the taking of reliable bids. Perhaps the greatest of these intangibles is the comparative ability of the bidders. A contractor trained in the use of some special method or type may be able to offer a bid figure which will beat others bidding a normally and usually cheaper type of foundation. But suppose we change the picture slightly. Instead of assuming, as above, that spread footings are still in the running and that the study of other foundation types is more or less in the nature of a precautionary measure, let us now assume that the specifying authority has concluded that spread footings are definitely out. His next move would then undoubtedly be to study other types of foundation in the order of comparative cost, as set up in the Elimination Table. This would bring him to wood piles, untreated or treated. Untreated Wood Piles The history of the plain wood pile foundation, like that of spread footings, stretches back into the far reaches of antiquity, when prehistoric man sought safety by building crude huts supported by slender sticks forced into the mud of the swamps. As a matter of fact, there is much similar construction still

56

FOUNDATION TYPES

in use for the same purpose today, beyond the edge of civilization. Sometimes, when observing current methods and designs of pile foundations in such supposedly civilized countries as the United States, one feels that a few of the original designers and users of wood piles must still be with us. Certainly no ether type of foundation is so often misused. Here are two questions that must be answered at once in wood pile foundation design. Is the soil chemically inert? Presumably everyone knows that wood when buried in chemically negative, permanently saturated soil will be safe from decay for an indefinite period, certainly for hundreds and in many observed cases for thousands of years. The trouble starts when the designer "assumes," instead of knowing, that the soil is chemically inert. Where a wood pile foundation is used under an important structure, a chemical analysis performed on a soil sample from the site, taken below the proposed pile cutoff depth, would seem advisable and would cost but little. It would certainly be worth the price and trouble where the site had previously been occupied by any type of plant which might have required the disposal of chemical wastes. Is the water table permanent? Nine tenths of foundation failures on untreated wood piles can be traced directly to the failure of the designer to estimate correctly the constancy of the water table on which he places his dependence. For example, a hundred or more buildings in the principal ports of the Great Lakes—Milwaukee and Chicago occupying the star positions—had to be underpinned when the water level of Lake Michigan fell to an all-time low in the 1920's. This slow fluctuation of the water table of the Great Lakes over periods of many years (the usual cycle is approximately 25 years from high to high) was well known to many engineers who designed wood pile foundations that ultimately rotted out. But it was equally well known to them that the cost of cutting off

piles in loose sand 5 feet or so below the existing ground water levels would take a lot of the dollars the unsuspecting owner had planned to spend above ground. Result: many a wood pile design called for pile cutoffs at, or often a foot or two above, the existing Lake level, even after the Lake cycle had clearly passed its peak. The writer may speak with feeling on this point, because at that time he and his associates were fighting an uphill battle to persuade the architects and engineers to substitute composite (concrete and wood) piles for the cheaper all-wood design. The cutoff of composite piles can be lowered many feet below the water table at small added cost. The permanence of the elevation of the water table may be affected by man-made changes as well as those produced by nature. Wood pile failures, due to the rotting out of the pile tops, have occurred many times in New York City, as in most other port cities, as a result of the building of sewers, tunnels, subways, underpasses, and adjacent deep foundations which had not entered into the plans of the designers. In most cases such structures could not have been considered when the designs were made because the need for them had not then been conceived. Such failures have been so common that the present New York City Building Code virtually excludes the use of untreated wood pile foundations within the city limits for any but temporary or second-class structures. It reads: For temporary structures of a minor character as approved by the superintendent and for lightly loaded class 4 and class 5 structures, as defined in sections C26-242.0 and C26-243.0, located over submerged or marsh land, untreated wood piles having minimum diameters of four inches at the point and eight inches at the butt shall be permitted above high tide level provided the top five feet of each such pile remains exposed for visual inspection. Wood piles not impregnated with an approved preservative shall not be used unless the cut-off

WOOD PILES, UNTREATED AND TREATED or top level of the pile is below permanent water table level. The permanent water table level shall not be assumed higher than the invert level of any sewer, drain or subsurface structure, existing or planned, in the adjacent streets, nor higher than the water level at the site resulting from the lowest drawdown of wells or sumps. [Italics mine.] Some builders have complained that this is too drastic and have supported their position by pointing out that many structures requiring major foundations have been, and undoubtedly will continue to be, built at locations adjacent to the Hudson River, the East River, and the Harlem River. The argument has been that in these locations permanent ground water cannot vary substantially, since its elevation must depend upon the level of the ocean, and that even the drainage system of a subway could have no more than a very localized effect. The important factor, well known to those who work below ground, is that a body of water such as the East River will silt up its bed so efficiently that it is to all intents impervious to water, even under a considerable head. I have seen a large open excavation, 30 feet deep, within a hundred feet or so of the East River, remain so dry that a 3-inch pump would carry off the seepage. Obviously, there would be no safety in the use of untreated wood piles in such a locality unless it was certain that no deep excavation would ever be made in the vicinity. It should also be remembered that the cause of trouble need not be a permanent drainage. Deep neighboring excavation for any type of structure, even if the water table would ultimately be restored, may exhaust soil water for some months, a sufficient time to cause decay in piles which have been submerged over a long period. WOOD PILES ARE NEVER

"UNIFORM"

Another reason for caution in the use of wood piles, whether treated or untreated, is the fact that vegetable growth does not at

57

any time produce a uniform product, in size, shape, or quality. No doubt it could be said that no material of construction—steel, concrete, plastic or the like—can claim absolute uniformity. But most of them have a far greater degree of uniformity than can be claimed for wood piles. The realization of this fact has caused the writers of specifications and codes to severely limit permitted loadings on wood piles and to exclude them from the high loads allowed on the "load-bytest" basis permitted to practically all other construction materials. Load tests on selected wood piles, properly driven, at suitable locations, might show a safety factor of 200 per cent or more, at double the highest loads presently allowed. But on such an uneven product there could be no certainty that the results shown by the test would be found throughout the large number of piles needed for the structure. In fact the certainty would be just the reverse— uniformity of result would never be present. HOW

TO

SPECIFY

FOR

WOOD

PILE

FOUNDA-

TIONS

In Part Four there will be found suggested forms for (a) request for bid, ( b ) contractor's proposal, and ( c ) formal contract for the work. But, at this point we may want to know a few of the important "whys" as well as the "whats" necessary to correct procedure. Not being familiar with both may cause confusion, delay, extra expense, and, all too often, ultimate failure. So here are three crucial specifying clauses, with a bit of amplifying explanation: 1. The piles shall qualify as Class B under the specifications of the Lumber Institute with a point dimension of inches and a dimension of inches at 2 feet from the butt.

Using Institute specifications gets us at once off two hooks—specifying species of wood no longer available, or unavailable in the necessary lengths except at unwarranted expense.

58

FOUNDATION TYPES

2. Inspection is to be done in the field by a qualified inspection company. Not at the site, because transportation costs to and from the site on culls will run up needless expense. Not by the architect or engineer, who are not specialists familiar with all the tricks of the wood-pile suppliers trade. 3. To obtain the desired penetration while minimizing the danger of injury to the piles, driving shall be done with a single-acting hammer having an impact of not less than 15,000 foot-pounds per blow, nor more than foot-pounds per blow. The hammer selected could be heavier, where the soil studies showed that no hard driving would occur till, say, one third of the pile length would be embedded in the soil. When driving Nlin. 8-inch point yellow pine, cypress, Douglas fir, or hardwood piles, a No. 0 Vulcan hammer or one similar to it may safely be used where hard driving at the surface will not occur. When hard surface driving is anticipated, the heaviest hammer allowed should be a No. 1 Vulcan or similar hammer. It might seem that such matters as the weight of hammer, use of spudding, use of jetting, and similar technical points could be left to the judgment of the bidding contractors, since the specifications will state that piles cracked or broken in driving will be replaced at the contractor's expense. The trouble is that injury to wood piles in driving may not be detected or, if believed to have taken place, can often be proved only by the pulling of the pile in question, which is a delaying and expensive process, to be paid for by the owner if no injury to the pile is shown. Too much detail in specification requirements can raise the bid price, but too little detail may result in second-class work and may also discourage the conscientious bidder from bidding at all. I have seen specifications for wood pile driving restricted to the use of a No. 2 Vulcan hammer, "or one of equivalent power," which would inevitably raise the bid price by slow-

ing progress. By inviting the use of a highvelocity double-acting hammer under permission of the clause "or one of equivalent power" might seriously jeopardize the safety of the job. In Chapter 25 it is pointed out that the "or equal" as applied to pile hammers of different types and different velocities at the impact point may be "equal" theoretically, but no known means of ascertaining the equality of result in the matter of the final bearing value of the pile presently exists. Treated

Wood

Piles

The subject of treated wood piles may be, for all practical purposes, boiled down to that of creosoted wood piles. There are other preservative treatments used for structural wood, but rarely for piling. The following is quoted from the introduction to an excellent article written some years ago by Mr. J. F. Seiler and published by the Century Wood Preserving Company: The use of pile foundations for heavy structures erected on compressible soils is such an ancient practice, and one so universally used, that a discussion of the subject seems superfluous, to say the least. On the contrary, there are few subjects about which so little scientific knowledge exists, despite its great importance in the field of engineering. Because of this, the practice has been heaped with abuses, beginning at the drafting board, and ending only with the completed foundation. Particular emphasis is laid on the structural efficiency and economy of timber piles. The durability of timber piles when submerged is well understood by all engineers. It is known that under such conditions wood can never deteriorate, nor can its strength become impaired. The permanence of creosoted timber when completely buried in the ground, though not submerged, is not so well appreciated, because of unfamiliarity with service records of this kind. After reading half a dozen brochures and the applicable chapters from various books on creosoted wood piling, the writer's chief

WOOD PILES, UNTREATED AND TREATED impression is that no one has taken the trouble to amass sufficient definite data to answer the two primary questions: What is the life span of a creosoted wood pile under various conditions of use? What should be the limitations of load on a creosoted wood pile? L I F E SPAN

Properly creosoted piles, say of 10 pounds retention or better, when entirely buried in close-grained soils with the tops sealed by embedment in concrete caps, will last indefinitely even though partly or wholly above permanent water line. If, on the contrary, the soil surrounding the piles is loose-grained sand, gravel, or the like, and might therefore be subject at some point to high temperatures (say 90 to 100 degrees or more), the creosote might leach out in time. Piles above permanent water line and exposed to the air might have a life of ten to thirty years or more, depending principally on climate. WORKING LOADS

Most foundation engineers agree that there is no reliable mathematical solution of the question of bearing value of any type of pile which can be based on the reaction under the driving hammer. This has been mentioned at several points in this book, but it is again emphasized because, in spite of all argument and opinion, we still frequently meet with specifications and codes in which some formula is stipulated to govern working loads. This is particularly true where wood piles are to be considered. Even the specification of some final minimum resistance to driving has little meaning when applied to any wood pile, treated or untreated, because of the irregularity of shape and of the resilience of wood piles. The deduction of permissible loadings on wood piles from one or more pile-load tests is subject to the same objection—there is no

59

regularity of size, shape, and resilience. The load value shown by test can govern only where all piles are at least approximately of uniform nature, and this would be an impossible requirement for wood piling. There is one factor concerning creosoted piles which is rarely mentioned: the process of creosoting seems to seriously weaken the bending strength of the pile. Our firm has driven many thousands of creosoted wood piles, scattered through several states and in a number of locations, and we have invariably had a much higher percentage of piles broken in driving, and in many cases broken in handling from the railway cars, than we have experienced in the driving and handling of untreated piles. Our conclusion would be that maximum permitted pile loads on creosoted piles should be held to fairly low limits —at a very maximum 25 tons per pile. Working within a 20- to 25-ton limit, the creosoted wood pile will still, generally, prove to be the cheapest answer to the smaller foundation jobs, say up to 150 piles, running 50 feet or less in average length. These suggested job-size and -length limits would, of course, vary widely from one part of the country to another, and also with the concentration of loading. SUGGESTIONS AND PRECAUTIONS

1. Avoid driving with light, fast hammers. The resilience of wood piles, treated or untreated, is so great that the energy of a highvelocity hammer stroke may be largely absorbed in compression of the pile itself, without causing penetration of the pile as a whole into the soil. It should be remembered that under most building conditions the heavier the drop of the hammer and the greater the impact, the less the driving will cost. A change in hammer may often speed a job up by 50 per cent or even more, therefore the use of a heavy hammer should not be ruled out without a very definite reason.

60

FOUNDATION TYPES

2. If there is a greater variation of pile lengths observed when driving the job than would seem to be reasonable in the light of the soil borings, then several piles should be pulled to make sure that the lack of uniformity does not arise from broken piles rather than from irregular soil-bearing conditions. 3. Another consideration which requires study where treated piles are to be used is the effect of sharp-edged material in the fill or the natural soil through which the piles are to pass. If the creosoted protective layer of the wood is punctured, the effectiveness of the treatment may be entirely lost. Most specifications for creosoted pile foundations stipulate that the piles must be handled with rope slings and that every precaution be taken to avoid rupture of the cresoted "jacket" of the pile. Yet the same specifications may call for the driving of these treated piles through fill containing jagged-edged pieces of broken concrete or thin layers of slate or shale which,

when broken under the action of driving into knife-edged fragments, are far more dangerous to the pile jacket than the use of tongs or cant hooks. To appreciate the probability of puncture of the creosote wall under such conditions, one should remember that effective creosoting of many of the most popular species of wood used for creosoted piling in the East and South depends for protection almost entirely on the impregnation of the sap wood. This sap wood may be much softer than the heart wood and therefore much more easily punctured. Wide use is being made of the creosoted pile, and this use is, and should be, increasing rapidly. As with every other type of foundation, the results will be satisfactory only if the type is properly understood. It should be chosen only where it suits the soil and structure requirements, and even then only if it is rightly specified and the installation performed in accordance with the specifications.

10 CAST-IN-PLACE UNCASED CONCRETE PILES MACARTHUR,

SIMPLEX,

T H E uncased concrete pile was introduced to the engineers and contractors of Europe and America approximately fifty years ago. This was at about the same time that the lightshelled cast-in-place concrete pile, which is next in probable cost in the Elimination Table, was introduced to the same designers. European engineers have continued to choose the uncased type almost to the exclusion of the light-shelled. To the very considerable extent that cast-in-place piles have been used in Asia, Africa, South America, and Canada, the same trend towards the uncased pile has been the rule. In the United States, however, the use of the light-shelled permanently cased type has far exceeded that of the cast-in-place uncased concrete type (or compressed concrete type, as it also called). If three fourths of the world chooses one type and the United States alone backs a different one, it would seem that American foundation designers should give very careful study and consideration to the type they have largely neglected—the compressed uncased pile. Undoubtedly the correct answer will not favor the complete elimination of either the cased or the uncased type. Soil conditions and, even more important, economic conditions will be controlling factors. In one area the cost of concrete might be low and the cost of steel high, while in another the comparative cost factors per ton of bearing capacity might be reversed. If the costs of all

AND OTHER

TYPES

materials of construction are comparatively high while the cost of labor is very low, the type which would be correct in one country or locality may be completely wrong in another where labor rates are high and material costs low. Even if the engineering conditions of two sites were identical, the economic conditions might rule out one or the other. Before condemning or accepting a basic type of foundation the designer should be very sure of his economic and engineering facts. The mere knowledge that a certain type has been dominant or completely rejected in a certain area is not in itself convincing proof that the choice has always been the correct one. Perhaps a look at the history of these two dominant types in the cast-in-place concrete pile field may help in explaining their present degrees of acceptance. In the United States the earliest entry in the cased type field was the Raymond pile. While the first form of any important invention will be progressively improved, even the earliest Raymond piles showed a sound and reasonably secure answer to most foundation requirements. On the other hand, the earliest Simplex and MacArthur piles, which were the first uncased cast-in-place concrete piles offered to American engineers, had serious shortcomings which limited their use to certain soils. The result was the preference given to the cased over the uncased pile throughout most of the United States.

62

FOUNDATION TYPES

In Europe the original cast-in-place concrete piles were introduced at about the same time as were the compressed and the lightshelled types in the United States. The lower labor rates outside the United States undoubtedly had a strong influence upon the wide acceptance of the compressed pedestaled types, almost to the exclusion of the shelled types, but unless the solution had proved sound the uncased type would unquestionably have suffered the same fate in Europe as it did in the United States. Considerable space will be given here to analyzing the virtues and faults of compressed concrete piles for just one reason: where the compressed shaft pedestal pile will meet the engineering requirements of the soil and the loads, it will usually offer a satisfactory solution to the foundation problem at a substantially lower cost than that of any other foundation, except spread footings. Today, the situation in the United States is changing, and the uncased pile is becoming increasingly important. MacArthur's Type Two long ago ironed out most of the weaknesses of the original Type One. Western, which has always believed in uncased as well as cased piles, continues to install uncased piles in increasing numbers, with increasing loads (up to 80 tons or better), in accordance with those permitted under the load-by-test programs of the newer codes. A study of castin-place uncased concrete piles is therefore a "must" for the designing engineer of today, and we will examine some of the various types in two chapters, here, in Chapter 10, and in Chapter 11. Crushing

Strength

of Pile

Shafts

Before entering on a detailed discussion of these types, it may be advisable to give some consideration to a danger factor common to the entire uncased range, that is, the crushing strength of pile shafts. Since the underlying theory seems to be little understood by engineers who are not primarily foundation

men, the following paragraphs are offered in clarification. Figure 1 is a diagrammatic series of cross sections of an uncased type of pile when driven in a group of two or more piles. Pi, P 2 , and P 3 signify the plasticity of the soil. This plasticity represents the resistance to deformation and is closely related to the soil pressure and the soil density. It is usually, but not always, directly proportional to either pressure or density or a combination of both. Step 2

Step 3

Step 4

Figure 1. Cross Sections of an Uncased Type of Pile When Driven in a Group of Two or More Step 1 represents the soil condition before any pile has been driven and formed. The natural undisturbed soil has a reasonably uniform pressure, density, and plasticity (within small areas such as might be covered by a pile group), with an intensity represented by Pj. Step 2 represents the conditions in the same plot of soil as that shown in Step 1, after the forming casing has been driven and before its withdrawal. At some small distance from the pile, the soil is still at or near its original condition of Pi. Immediately surrounding the casing, however, a new condition of pressure, density, and therefore plasticity has been created by the packing up of the soil due to driving outward the particles originally occupying the space A, with a resulting annular ring of a new density surrounding the drive casing. This soil state or condition may be called P 2 .

MACARTHUR AND O T H E R UNCASED C O N C R E T E PILES If the casing were to be withdrawn at this stage, there would be every probability—in nearly all soils a certainty—that the compacted soil surrounding the air space A would squeeze into that space. Step 3 represents the condition as to soil pressure, density, and plasticity which would prevail if, before pulling the casing at location A, it had been filled with wet plastic concrete. This area within A would then have a pressure-density-plasticity factor which would differ from Pi or P 2 ; let us designate this condition P3. If at this stage P s is greater than P2, which must be greater than Pi, then there will be a stable condition which will preclude any probability of the surrounding soil under condition P2 causing damage to the concrete in the area A. Step 4 represents the condition which will begin to develop in case a pile B adjacent to pile A, is in process of being driven. It is assumed that the concrete in pile A has not yet set. The particles of soil forced to move, due to the displacement of the apparatus which is being driven to form pile B, must move in some direction. Some of these particles will certainly start in the direction of pile A. But the annular ring of compacted earth around A has a higher pressure-densityplasticity condition than other areas near location B, so the natural tendency for these wandering soil particles will be to go anywhere in the Pi area rather than continue movement into the P2 area. Regardless of its own density, the concrete in A would be protected by its P2 ring. This would be a safe enough deduction if only a very limited number of soil particles had to be moved, but supposing we continue to drive out particles until the high-density ring around B impinges on the similar ring around A. Then, to avoid the possible failure of A by crushing the segment of the P2 ring which will be brought under pressure by the similar B ring during driving, the core of un-

63

set concrete in A must be of greater density than that of the soil in the compressed rings. That is to say, P3 must exceed P 2 which will in turn exceed Pj. The possibility of maintaining a P2 density arch as protection for the unset concrete in pile A, even if that concrete were soft and malleable, would be dependent upon the strength and nature of the soil. As a matter of fact, this arch of soil could and in many sites did prove sufficient to meet the requirements, with the result that the foundation as a whole proved adequate. However, being "90 per cent safe" is not good enough, and under certain soil conditions the arch simply could not meet the pressures set up in the driving when backed up only by plastic concrete. With P3 of a lower order than that of the incoming P2 soil, complete shaft failure occurred. Exposed piles in such cases looked like a string of sausages. Engineers of a generation ago saw clearly that if a method could be found, as it later was, which would assure that P3 would always be greater than P2, the problem would be solved, because any pressure that could be delivered by P2 soil would be unable to disrupt P3 concrete. With P3 greater than P2, the only way failure could occur would be by forcing pile A, together with its annular ring of densified soil, to move bodily away from the location of pile B. As long as there was a field of Pi pressure open on any side of the B location, it would be impossible to move the great mass of pile A plus its annular P2 ring. The soil to create such a pressure would have to have a P factor greater than P:i, because otherwise the moving soil would bypass the A area. Our assumption has been that P3 is greater than P2, which is in turn greater than Pi. There is a condition under which this argument has, on rare occasions, proved unreliable. If a new pile is driven in an area already surrounded by other previously driven piles and their P2 rings, then the requirements of a Pi area to which soil flow may

64

FOUNDATION TYPES

take place may not be met. Pressures in soils, where the pressures will not be relieved by extensive heave, may run up to very high figures indeed. Under such conditions as these, light-shelled piles will be even more likely to fail than will the compressed types, and even pipe piles up to % 6 of an inch wall thickness have been known to collapse. The writer has gone into this theory of the crushing strength of the shafts of uncased cast-in-place concrete piles at some length because it is basic to the study of the safety of any and all types of uncased or lightshelled piles. It is the most important factor in the arguments among foundation men as to the relative merits of the cased and uncased types, and also as to the shell-thicknesses which should be required for cased types. MacArthur Pedestal Pile, Type

Two

The first successful American development in the uncased cast-in-place field came with the introduction of the Improved MacArthur Pedestal Pile (hereafter called MacArthur Type Two), which overcame several of the shortcomings of the Simplex, the earlier American, and also the European types. Note: It should be mentioned that several other types of uncased cast-in-place concrete piles have been in use from time to time. The Vibro type, still used to some extent in England, and the auger type, used quite extensively in Texas and other points where the necessary soil conditions occur, are among them. But such types of limited use and application are not discussed in this book for reasons of space. INSTALLATION O F MACARTHUB T Y P E T W O

The apparatus used for the MacArthur Type Two, which is essentially the type now in use by the MacArthur Company, required comparatively small changes in the plant and method of procedure of the earlier MacArthur types, but it produced a greatly improved product which has been employed success-

fully in many hundreds of piling jobs in the United States and Canada. The MacArthur system was also in use quite extensively in Japan before World War II. In Figure 2 the head block K is mounted on top of the leads of the driver. Hi and H 2 are the pulling blocks through which the pulling cable is reeved. The hoist and hoist drum to which the pulling cables G are led are not shown, since they may be of any standard type. A, the steam hammer, is generally a No. 1 Vulcan hammer, though almost any of the standard steam hammers might be used. B is the cushion block container enclosing a cushion block generally of wood. The forged steel core head C is attached more or less permanently to the top of the core E. F is the drive casing, generally of 14-inch outside diameter, though other dimensions may be substituted, and I is the pulling "sling" or "choker." L is a forged steel core bottom of a diameter slightly less than that of the inside diameter of the drive casing. M, the steel pan or boot, is generally used to assure the exclusion of water and sand during driving. The pan or boot is of even greater importance, however, in preventing the inflow of soil when the core is withdrawn and before the pedestal concrete can be deposited and compressed. Step 1. The sketches in Figure 2 do not purport to show the correct mechanical details, the intent being to simplify them so that operations may be easily understood without being cluttered up by a mass of details. The sketches are purely diagrammatic. Except where they are necessary to indicate the method of operation in Step 4, the mats, running beams, and the pile driver itself have been omitted. The pulling gear has been shown offset to the side of the leads and hammer, though the blocks are actually offset fore and aft in the leads. However, the eccentricity of the pulling is a feature of the plant as used and is sometimes the cause of trouble in pulling. Obviously, the resistance to pulling caused by friction on the casing will be parallel to the

MACARTHUR

AND OTHER UNCASED CONCRETE PILES

axis of the casing, and the resultant of the pulling forces should be parallel to the lines of resistance. Step I

Ü

0

in one of the intermediate lofts, possibly two thirds of the way up the leads. The advantage in this arrangement is that only the section of

A

Step 4

Step 3

Step 2

[r

65

K

io i

H

fmsz

I •• ? -. ivi

m r..

'ININMiyiMMIMiNIMiisi

A

sppii^

B

Steam Hammer Cushion Container

6

Core Head Core Casing Pulling Cables

C E F

Figure

2. Improved

H| H2 1 K L

M Mac Arthur

The upper pulling block H2 is shown as attached to the head block K, but it may be and usually is attached to a fixed pulling block

Lower Pulling Block Upper Pulling Block Choker - Pulling Sling Driver Head Block Solid Steel Core Steel Closing

Pedestal

Pile (Type

Pan

Two)

the leads below the point of attachment of the upper block need be strengthened to meet the excessively hard pulling which will

66

FOUNDATION TYPES

sometimes be required. This arrangement is upon to hold the concrete in place while the made possible by the fact that the attachment casing is raised. The hammer is now put into operation of the lower pulling block H! to the apparatus is made through a cable sling I which can while the pulling cables are held, and the be easily overhauled to any point along the concrete is forced out much as one would casing. The operation of pulling may take blow a bubble. The shape of this bubble of place in two or more short lifts. In other concrete, as with all types of pedestal piles, methods, such as the Western, where the pull- will rarely approximate the uniformity of a ing attachment is to the head block and the bubble in air or water, since the soil in which top of the casing, the entire length of the leads it is expanded will rarely be of uniform denwill have to be strong enough to meet the sity or uniform pressure. This is all to the good, maximum strain. The shortened distance since the shape will conform to the pressure made possible by this step-by-step pulling has necessary to cause a sustaining reaction in the the added advantage of permitting the use of direction opposite to the direction in which much shorter pulling cables. The advantages load will ultimately be applied, that is, in the of other methods of pulling will be pointed out direction of the axis of the shaft of the finished when discussing the types employing them. pile. This operation of forming a pedestal may The principal change in the plant between be repeated several times to increase the size the first and second MacArthur types has been and the "pack up." the elimination of the point of the core extendA weakness in this method of forming a ing below the bottom of the casing. The point pedestal has already been pointed out. If the as originally used would have eliminated the soil below the point of the apparatus is of possibility of the use of the pan M, the im- fairly uniform density and resistance, the portance of which has been stressed, and pre- slug of highly compacted concrete in the vented the compression exerted on the con- drive casing will tend to be projected downcrete, as is possible by the improved method. ward, more or less without spreading. (This Step 2. Between Step 1 and Step 2 the core theory is disputed by some engineers, who has been removed from the casing by means assert that by reason of the conditions creof the raising of the pile hammer. A charge of ated before the hammer begins to build the low slump concrete in the amount of 3 to 5 pedestal the concrete is in effect deposited cubic feet or more, depending upon soil condi- in an unlined hole.) The original method of tions, has been deposited in the casing and has forming this pile, though faulty in other ways, been compressed by lowering the core into certainly had a greater tendency to force the contact with the concrete. Here again the concrete to spread sideways, since the consubstitution of a flat-ended full-diameter core crete was deposited in an unlined hole in the bottom for the long, tapered, extension point soil before it was highly compressed by the of the earlier method has been an essential driving used to form the pedestal, which was change. The earlier point would have jammed forced to expand sideways by driving a conical wedge into the unset concrete. the stiff concrete in the casing. Step 4. Between Step 3 and Step 4, the core Step 3. Between Step 2 and Step 3, after the deposit of the concrete for the pedestal has been withdrawn and the casing has been and the replacing of the core in contact with filled with a quantity of concrete believed to it, the casing has been raised slightly to free be sufficient to form the compressed concrete the pan. The weight of the core and the steam shaft. The core has been lowered into the hammer riding it has been resting on the top casing till the core bottom rests upon the top of the concrete, and this weight is depended of the freshly deposited concrete. In this posi-

MACARTHUR

AND OTHER UNCASED CONCRETE PILES

tion the pressure on the freshly deposited concrete will be equal to the weight of the core plus the weight of the hammer. The hammer cables are left free, so that core and hammer may descend and so maintain the pressure on the concrete. The concrete used to form the shaft and the pedestal is unusually dry, generally of no more than 1-inch slump, and is deposited under pressure resulting from the weight of the core and hammer resting upon it. This method is obviously superior to that of the earlier types because the shaft when formed by the MacArthur Type Two method will be highly compacted, with a resultant concrete pressure or density which will generally be far in excess of that of the surrounding soil. The MacArthur method of forming the pedestal is very similar to that used by others, but the method of forming the pile shaft has the advantage that it requires less dependence upon the skill of the operator and is practically automatic. A great majority of the large number of pile jobs accomplished by this second MacArthur method have been entirely successful, but there are certain factors which need careful watching and good judgment. 1. Since pile shafts are generally formed through soils of widely varying density, the diameter of a shaft formed under a uniform compressive force (the weight of the core and hammer, using a No. 1 Vulcan hammer and the core of a 40-foot long apparatus, would be some 6 tons or more) would vary with the soil density. If the shaft diameter at all points exceeds the outside diameter of the forming casing, the bearing value of the pile will be increased rather than lessened by the bulges or oversize sections of the shaft. However, if the shaft passes through soft soil, nearly all of the concrete in the casing may be forced out at one or two points in the form of secondary pedestals, leaving too little concrete in the casing to form the balance of the shaft up to cutoff elevation.

67

When this excessive depositing of concrete occurs, it will be indicated by the too rapid descent of the core in comparison to the rise of the casing. The operator can stop the operation of the pulling gear, remove the core, and deposit additional concrete in the casing and then resume pulling. But this can be accomplished only if the trouble arises near the bottom of the pile, or if the combined length of the hammer, the core, and the section of the casing extending above the bottom of the leads is less than the length of the leads below the head block. If the pile is a long one, thus requiring a long apparatus, and the above conditions are therefore not met, it will be impossible to clear the core out of the casing without disassembling the apparatus, a tedious job which might result in the initial set of the concrete occurring before forming can be resumed, in which case both the pile in process of forming and the pile-forming apparatus will probably be lost. 2. Usually where the borings or the driving of the first piles have indicated the danger of striking very soft spots which would result in the excessive deposit of shaft concrete, the precaution taken will be the use of very dry concrete made with coarse aggregate —sometimes up to 3-inch diameter—and the use of an excess quantity of concrete in the forming casing, so that if, in spite of the coarse dry mix, some secondary pedestaling occurs, there will still be sufficient concrete to complete the pile to cutoff elevation. If these precautions fail to solve the problem, an attempt may be made to hold the hammer line when entering the soft pulling zone so that less pressure will be applied to the concrete at that point. This is a rather dangerous procedure, since its successful use depends entirely upon the skill of the operator. If all pressure is removed while the casing-bottom is moving upward in the soft pocket, it is possible that the soft soil will close in and cut the pile shaft in two. 3. The use of a dry concrete introduces

68

FOUNDATION TYPES

another danger, probably the most serious threat to this method of forming. When the weight of the core and hammer presses down on this sort of concrete, the friction of the concrete against the internal surface of the pipe frequently becomes so great that the concrete "locks" in the pipe, the core is lifted with the casing, and no pressure can pass through this "locked" concrete to the bottom of the casing where the shaft is being formed, so that a void is developed at this point. If the soil at this point is plastic, it will close into this void and the integrity of the shaft will be destroyed. When such a locking develops, it will be indicated by the rising of the core in the leads, a thing which should never occur at any time during the forming of the shaft. The pulling of the casing will be stopped immediately, but a blow or two of the hammer will then usually free the jam and the pulling can be resumed. Once again, the integrity of the pile depends upon the skill and speed of reaction of the operator—the human element enters in. The instantaneous eye-nerve-muscle reaction needed when pulling in soft ground is difficult to find and hard to develop, but the low incidence of failure in MacArthur work would seem to indicate that, though difficult, it is not impossible to find it. MacArthur Pedestal Pile, Type One One of the earliest rivals of the American Simplex, the MacArthur Pedestal Pile, Type One, attempted to gain the undoubted advantages of the pedestal or mushroom bottom, which was standard practice in European types. This first version is mentioned only for its historical interest, since it was long ago superseded by the improved MacArthur Type Two, already described. In one feature, this first American pedestal pile was ahead of its time. It showed a knowledge of the importance of forming the pedestal by a horizontal rather than a vertical thrust on the concrete. It would seem that a roughly spherical pedestal would be developed by forcing concrete out of a tube, much as the

soap film is blown out of a child's bubble pipe. But there is a joker in this comparison, for the soap bubble is formed of a truly plastic material, while the concrete pedestal is formed of a material which, after it has been compressed within the forming casing by the blows of the hammer, is in condition to react as a fairly strong solid. If immediately below the point at which the pedestal is to be formed there is a stratum of material of considerably greater firmness than the concrete slug, the slug will be stopped by this firm stratum and will spread out into some form of pedestal, though it will rarely be anywhere near spherical. If, however, the soil below the bottom of the forming casing is less hard than the concrete slug, then the concrete is liable to be driven down in the form of an extension to the pile shaft with little or no pedestaling effect. This was very forcibly called to the writer's attention some years ago on a foundation job for the Bahai Temple at Wilmette, near Chicago. The soil was a uniform medium-dry clay, differing considerably from the clay usually found in the Chicago area, which varies in consistency from moist-and-soft to moist-and-tough. The job was specified for uncased cast-in-place pedestaled piles. The inspector on the job was very casual, but after a considerable part of the work had been completed the owner's representative unexpectedly stated that he believed many pedestals were being omitted, particularly by the night shift. The contractor consulted with the foreman on the night shift and was solemnly assured that every pile driven by him was properly pedestaled. On the strength of this, the contractor suggested that a pier of piles should be fully excavated—an expensive process, as the piles were fairly long—and that if any of them failed to show a proper pedestal, then the contractor would pay for the work and would drive extra piles, or some other type of piles, to replace all those driven up to that time. If the pedestals were there, as of course

MACARTHUR

AND OTHER UNCASED CONCRETE PILES

he believed they would be, all expense for the excavation and delay would be paid by the owner. The offer was accepted. And it was at this point that the writer was called into the picture. The pier was excavated, and there was not the smallest sign of a pedestal on a single pile. The contractor corrected the job and took a heavy loss on it. However, the point is that later experimental work showed that the operation of "blowing out" a pedestal in that particular clay soil formation resulted only in the creating of an extra-length pile. The bottom of the pile, far from being enlarged, actually tapered off to a sort of ragged point. The hard-driven, highly compacted concrete slug acted like a projectile in a gun. The concrete should have been deposited and then spread by driving the apparatus back through it, which would have created the horizontal spreading reaction needed. I should mention again that this theory is disputed by some engineers. Simplex Pile As the earliest American cast-in-place uncased concrete pile, the Simplex is of historical interest, even though its popularity was short-lived and its disadvantages many. The point of the pile was of the same diameter as the shaft, whereas in its European counterpart on the time scale, the standard construction called for a pedestal, which added to the area at the point. In most conditions requiring the use of piling, such a pedestal would not only increase the directly loaded area but also increase the value of the bearing stratum by packing and compressing it. Moreover, in the Simplex pile no mechanical means were available to assure that the concrete would not arch within the drive casing with a resultant failure to form a continuous shaft. The means adopted in an attempt to overcome this tendency to arching was the use of very wet, highly plastic concrete, which it was thought could be depended on to pour out under its own gravity

69

head, since the weight of the concrete would be greater than that of the displaced soil. This theory simply did not work in tough plastic clays or similar soils, because it failed to take account of the soil pressures caused by the displacement of the driven piles, which in many cases far exceeded the static fluid pressures resulting from the head of wet soft concrete. Failures occurred, and the use of the Simplex method soon became restricted to certain limited areas where soil conditions were favorable to it. Compressed Concrete Piles In certain methods of forming compressed concrete piles an internal drop hammer is used, operating within the casing. Here there are certain disadvantages as against the steamoperated hammers almost universally used by the larger American pile-driving companies. The chief of these is the introduction of the human factor. Even the most skillful hoist operator cannot assure that every blow of a drop hammer will be equal and of full effect. The hammer is controlled by friction clutch and, even under the best of circumstances, it cannot be released instantly, or even approximately so. The release of the drop of a steam hammer is automatic and very fast. Also, if the drop hammer is allowed to strike at the end of the downstroke, before some braking action is applied, additional cable run-off will be the probable result. Therefore there is always the temptation on the part of the operator of a drop hammer to check the spinning of the hammer drum a fraction of a second before the hammer strikes. This may greatly reduce or even kill the impact of the blow. A realization of this fact is shown in the Engineering News Formula, so widely used in codes and specifications, by the change in the fixed factor from 0.1 for the steam hammer to 1.0 for the drop hammer. In the days when drop hammers were used widely in this country, these objections were taken into account, and many drop hammers were rigged with an automatic releasing de-

70

FOUNDATION TYPES

vice so that when the hammer reached a certain predetermined point in its upward travel, it was cut free of the cable, which was then independently overhauled down to the hammer, where a spring dog connected the two parts again. Certainly when so rigged the fall was a free one, with the exception of such friction as would of necessity be developed in the leads. This method of rigging also had the advantage of being largely free of the human factor, insofar as the effectiveness of each blow was concerned, but it was slow and cumbersome, and it reduced the frequency of the blows, which is itself an important factor in hammer effectiveness in some types of soils. Another factor to be considered in rating the comparative efficiency of drop and steam hammers is that of loss by friction on the pile driver leads—or on the inside of the casing—where an internal drop hammer is used. Against these losses, internal hammer friction and exhaust steam back pressure, which must occur in all steam hammers, should be evaluated. The comparison between the losses should certainly favor the steam hammer. However, the major fallacy which usually develops in comparing steam hammers with drop hammers arises from the baseless assumption that "impact" measured in footpounds—that is free-fall travel in feet, or its velocity equivalent, multiplied by the weight of the falling object—offers a measure of the effective energy which will be delivered at the point and time of impact. Of course this is an arbitrary and manifestly untrue assumption. To carry this comparison to the point of absurdity, imagine attempting to drive a 10-ton pile by standing on a steel cap plate at the top of the pile and firing machine gun bullets into the top of the plate at your feet. The impact and the energy release measured in foot-pounds would compare favorably with those of a pile hammer, but the driving result would be nil. Incidentally, driving by a series of explosions has been tried.

The most widely used pile-driving formula, which has at least the virtue of simplicity, is the Engineering News Formula. This formula is allowed by many building codes as a measure of pile-load capacity, under certain limited conditions. The code writers and users of this formula have realized the shortcomings of the drop hammer by handicapping it with a constant of 1.0 as against the similar constant of 0.1 for the single-acting steam hammer. This may or may not be a fair valuation of the comparative efficiencies of the two types, but it is certainly a widely accepted one. There can be no doubt in the mind of anyone with broad experience in the use of both types that any attempt to compare them on the basis of "foot-pounds of energy" is meaningless unless an efficiency factor heavily favoring the steam-operated type is introduced. But the greatest danger in the use of drophammer blows per inch as even an approximate measure of bearing value is that while a really good drop-hammer operator can get results which should, with due adjustment for the various other factors mentioned, give some sort of comparison between the value of the driving under drop and steam hammers, the prime requirement is hard to meet—the obtaining of good drop-hammer operators. These are few and far between, and a poor operator who dislikes the idea of a broken cable curling around his neck can, and almost certainly will on some occasions, use his brake or friction in such a manner that his hammer impact would scarcely qualify to crack peanuts. The principal objection to all types of uncased concrete piles is that they are not suitable for use in really soft soupy soils. SIZING UP COMPRESSED CONCRETE T Y P E S

Due to the mechanical features of the plant, these piles are likely to offer their sharpest price competition to cased types where: 1. The pile will be of short-to-medium length, say under 40 feet.

MACARTHUR

AND OTHER UNCASED CONCRETE PILES

2. The job is of such a size that the plant installation and removal cost will be a substantial part of the total bid price. Speed of installation within a matter of three or four days may be of serious importance. 3. The soil is such that the pile shaft will not pass through areas which are under high pressure, or are semifluid when the natural soil pressure is disturbed. Where there is no mechanical, positive relationship between the movement of the forming casing upward and the depositing of concrete from the lower end of it, high-pressure or semifluid soils may tend to produce a discontinuous shaft. 4. High loads per pile are permitted. Loads of 100 tons and more per pile are the rule rather than the exception in Europe under soil conditions where, in the United States, 30 to 40 tons per pile have been customary and where, even under the newer codes, loads would usually be limited to about 60 or 80 tons per pile when backed up by a test program. UNCASED

VS.

CASED

TYPES

IN

THE

UNITED

STATES

All of this argument has a very direct bearing on the question of the comparative advantages of the uncased and the cased pile, and rather incidentally answers the question as to why the United States chose the cased pile while the rest of the world voted pretty solidly for the uncased. The answer, in part at least, is that the earlier uncased pile types used in the United States lacked the advantage of the pedestal and suffered the disadvantage of a very faulty method of formation of the shaft, while the principal European type used a pedestal and had a reasonably safe method of shaft formation. On the other hand, the cased pile, while it lacked the extra carrying value given by the pedestal, offered a better guarantee of a reli-

71

able shaft, under certain conditions. It also possessed one other important feature which, particularly in this country where "time" is very much of the essence, gave it a considerable advantage—it could be driven much more rapidly than the uncased types. The development of these two major types of pile, each in its own territory, has followed a line which was perhaps inevitable and should have been obvious. The uncased pile took advantage of its inherent ability to "create" bearing values by the use of pedestals and enlarged compressed pile shafts so that now single pile loads have in some cases been stepped up to 120 tons and better. On the other hand, the cased pile has continued, until very recently, as a 30-ton pile. Under the new load-by-test programs, loads on cased piles are being stepped up considerably. However, unless the pile shafts in the better known types of light-shelled piles are considerably strengthened or the working loads on pile concrete are placed far above those allowed on concrete for other parts of the structure, there seems little chance that light-shelled piles will be accepted in this country for loads at all comparable with the loadings on compressed concrete uncased piles as allowed in other countries. Regardless of any code favors which may come to the shelled pile, the overload tests which are certain to be required will be attainable at reasonable depth in many cases only by the use of large pedestals. In Chapter 12, where light-shelled piles are discussed in detail, there is a design and a discussion of the rarely used cased pedestal pile. This pile may be the answer in many cases to the desire for a short-length, highload pile as used in other countries, which still retains the advantages in safety of shaft under bad soil conditions, and to some extent in rate of progress, now available only in cased pile types.

11 CAST-IN-PLACE UNCASED CONCRETE PILES WESTERN FOUNDATION CORPORATION TYPES

W E S T E R N Compressed Concrete piles, though they have been in use in their basic forms for over twenty-five years, still constitute the most recent of any of the widely accepted compressed concrete pile types. Western types follow the MacArthur rather than the European types in that the pile-forming apparatus consists of a core and casing rather than a casing only and require the use of a steam hammer rather than a drop hammer. In developing Western types emphasis has been placed on the elimination of the treacherous human element, which prevents uniformity in formation in other uncased concrete types, and on reducing the pile-forming procedure to positive mechanical operations. The first move in this direction is the elimination of the drop hammer, where the power of the hammer blow depends almost entirely upon the inclination and skill of the engine operator, and the adopting of a single-action steam hammer. This advance was pioneered by the MacArthur method, as pointed out in the description of that type. Western has used the single-acting and differential steam hammer exclusively, in preference to the double-acting type, because in the double-acting hammer, the power of the stroke will vary unduly with the change of steam pressure. Constant maintenance of a steady steam pressure is very difficult to attain, unless excessively large boiler capacity or steam generator capacity is used, and the

adoption of either of these reduces mobility of plant and therefore adds to the cost of the job. As pointed out elsewhere, the power impact of a single blow of a single-acting hammer is not necessarily a constant, but any failure to keep it so can be immediately detected by an observation of the front exhaust port. In the following step-by-step study of the installation of a pile by the Western method, the features of the Western plant and procedure that have been developed to meet former weaknesses are pointed out. METHOD OF

FORMING

The apparatus for forming the Western Compressed Concrete Pile and the Western Compressed Pedestaled Pile is shown diagrammatically in Figure 3. This consists of a heavy duty steel pile driver, calculated to withstand pulling strains of 100 tons or more, with head block C and bedframe O, mounted either on steel running beams P or on a caterpillar and turntable (not shown); a tubular steel drive casing Q, limited in present use to diameters of 12 to 36 inches, though there is no particular reason for these limitations; and a close-fitting, closed-ended steel driving core K. The remaining parts of the gear will be defined as their uses appear in the description of the forming of the pile. Step 1. The core K is telescoped into the casing Q and the two are driven together into the ground under the operation of the steam hammer H.

WESTERN UNCASED CONCRETE PILES Step 2. When the required resistance or depth is attained, the core which is attached to the hammer is withdrawn from the casing. Step 3. The casing is filled with concrete R. This concrete is very stiff, usually having a slump of one or two inches at most. It is a harsh mix, the concrete aggregate never being less than 1%-inch gravel or crushed rock and, where conditions require, running up to 3inch diameter. The quantity of concrete deposited in the casing is sufficient to form a shaft from the required cutoff to the bottom of the pile casing as driven, and of the predetermined diameter. This diameter is at least % inch greater than the outside diameter of the forming casing and may exceed the casing diameter by an inch or more. The amount of concrete to be deposited for each inch of casing withdrawn from the ground is mechanically controlled in such a manner that the operation of pulling the casing will be automatically stopped the instant that a lesser amount of concrete than the predetermined quantity is being deposited. The method of withdrawing the casing and depositing the compressed concrete is the distinguishing and all-important feature of this type of pile and is as follows: Step 4. The core which is attached to the hammer is lowered into the casing till the core bottom L rests on the concrete in the casing. The pulling cables G are reeved through two sets of pulling blocks, F and M. These are usually of the 5-sheave type, giving 22 parts of line in tension when the pull takes place. A large number of cables are used to produce the powerful pull often required to simultaneously break the external friction on the casing and also the internal friction between the low-slump harsh concrete and the inside wall of the casing, and at the same time to supply the pressure needed to expand the diameter of the column of concrete being ejected from the lower end of the drive casing from its original diameter—

73

which will, of course, be that of the inside of the casing—to the predetermined diameter of the pile, which will be something in excess of the outside diameter of the casing. (The pipe forming the casing is generally of inch wall thickness, so that the diameter of the column of concrete being deposited must be expanded by inches or more.) To go through this change of shaft diameter, the entire column of concrete contained in the casing must react as a fluid. It is the necessity of maintaining this uniform and continuous expansion of the concrete during the process of being deposited which gives rise to most of the risks encountered in the previously described methods of forming uncased concrete piles. In these other methods, the concrete is caused to flow either by making it so wet that it is virtually without internal friction in the casing and therefore weak in its resistance to soil pressure. Or it is made so dry that it must be driven out in a series of small pedestals, with the multiple risk of overdriving any one of the pedestals, so that water and mud may rush in and cause separations in the shaft of the pile. Or some predetermined weight, such as that of the pile-driving hammer, is used to force out a fairly dry concrete as the casing is withdrawn. In this last method, if the weight is insufficient, because the internal friction in the casing is too great or the external pressure in the soil too high, the core and the casing will lock and rise together, allowing a separation of the concrete shaft in process of being deposited. The opposite extreme may be equally harmful, because if the weight and therefore the pressure on the depositing concrete is excessive, far too much concrete will be deposited when passing through the softer spots in the soil, leaving too little concrete in the casing to complete the shaft as planned. To meet all of these dangers, the shaft of the Western type is formed as follows (see Figure 3, Step 4): With the core in contact with the unset con-

74

FOUNDATION TYPES

crete, the pulling cables are set in operation. The lower blocks M draw the pulling collar N, which engages the casing Q toward the upper pulling blocks F. Since these upper blocks are hung from the rocker arm D, which is mounted on the hammer H, which in turn rests on the core K, it is evident that, providing the hammer

ment of the casing and concrete together could occur. The casing must move independently of the concrete if it is to move at all. If the internal friction and the external friction and the setting up of a fluid condition in the depositing concrete is too great to be broken by the power of the pulling blocks, then the

Head Block Hammer Sheaves Hammer Cables Head Block Rocker A r m Hammer Head Sheaves Upper Pulling Blocks Pulling Cables Steam Hammer Hammer Base Core Head Driving Core

Cast Steel Core Bottom M

Lower Pulling Blocks

N 0 P

Pulling Collar Bedframe Running Beams

Q R

Drive Casing Concrete

Steps 1 , 2 , 3

Step 4

Figure 3. Western Compressed Pile

cables B are slack, all of the force developed in the pulling cables G must pass through the core to the concrete on which its lower end L is resting. Under these conditions it would be utterly impossible for the concrete R in the casing Q to rise with the casing, because all of the force developed by the pulling cables would be passing through the concrete. No move-

system will become "locked" and depositing of the concrete and the movement of the casing will both cease. If the locked condition is to be broken, some outside force must be introduced and this is accomplished by striking a few blows of the steam hammer. Since this outside force reacts in a vertically downward direction through the core and into the concrete, there is still no possibility

75

WESTERN UNCASED CONCRETE PILES of the concrete rising with the casing. Until this independent movement of the casing over the concrete has been established, the system remains locked. If, during the depositing of the concrete in the casing, an arching action has taken place so that an air pocket has formed, then, since the pulling force must pass through the entire column of concrete, no extraction of the casing could happen till this "pocket" had been closed, because the pulling force could not be sustained by a pocket of air. Something further is needed. It is essential that the rate of depositing the concrete should be constant so that the diameter of the pile

Step 5

Figure 3. Western Compressed

Pile

(cont.)

should also remain constant. Under the pulling action just described, the rate of deposit could not be less than needed to form a shaft equal to or greater than the diameter of the outside of the forming casing, but unless some precaution is taken, all or a large part of the concrete might be deposited in the form of a bulb or bulbs as the bottom of the casing passed through the softer soil strata.

The first action, that of taking the pull required for the retraction of the casing, through the concrete, precludes the possibility of separation in the formed shaft. To meet the second requirement, that is, that no more than a predetermined amount of concrete shall be deposited in each increment of pile shaft, it would be necessary that the core bottom move downward at a predetermined uniform rate, which would be synchronized with the upward movement of the casing. This is accomplished in the following manner. Step 5. At the start of the pulling of the casing the pile hammer cable B is dogged off at the drum S. The other end of this same cable is attached to the pulling collar at T. When the casing Q begins to move, the T end of the cable becomes live—it pays out. The normally "live" end of the hammer cable remains dead at the dogged-off drum S. As the casing rises the T end of the hammer line B rises with the casing; slack is fed into the sheave system 1 - 2 - 3 - 4 - 5 , which permits the hammer to descend carrying the core down with it. The rate of this descent, and therefore the rate of concrete deposit, may be varied by changing the number of sheaves and the number of parts of line in the hammer head to head block system. The compensation would be at the rate of 1 to 4, that is, for each 4 feet the casing rises, 5 lineal feet of the concrete filling the casing would be deposited. Using a 14 inch by y2 inch wall casing, a shaft of 14^4 inch diameter would be formed. FORMING THE WESTERN PEDESTAL

PILE

Perhaps the most important fields of use for the uncased pile are those in which the uncased shaft is combined with a pedestal, a fact stressed by MacArthur and others, as well as Western. Where the pedestal is the important feature, it may be combined with either a cased or an uncased shaft, so that its

FOUNDATION TYPES

76

use is possible even where the pile must penetrate plastic clay soils, providing the pedestal can be formed on a hard stratum or in sandy soil. A section on the pedestal follows. But, for the moment, we will restrict ourselves to its use by Western in the uncased field. The steps required to form the pedestal and complete the pile are shown in Figure 4. Step 1. The core and casing have been assembled and driven into the ground, as shown. After the required depth or resistance has been reached, the core has been removed Step I

Step 2

Step 3

Step 4

yr tip

H

E3 Figure 4. Western Pedestal Pile from the casing and a shot of concrete has been deposited at the bottom of the casing, and the core then replaced in contact with the concrete. Note: Certain mechanical details have been omitted for the sake of simplicity. One essential detail shown is the closure plug which prevents ingress of soil or water up to the stage shown in Step 1. This plug is usually made of precast concrete, though it may be replaced with hard-packed unset concrete. Under certain soil conditions, the plug may be omitted and the same purpose served by the use of a steel pan shaped to fit snugly over the outside of the casing at the bottom. On occasion it may be made of hard-packed soil.

Prior to Step 2, the casing has been pulled up to about one half the length of the deposited concrete. During this operation the core has been in constant contact with the concrete and the pull has been taken over the top of the hammer and therefore through the core, so that there has been no possibility that separations would occur within the column of unset concrete. Step 2. After the partial extraction of the casing, the hammer has been set in operation so that the concrete still remaining within the casing has been forced out in the form of a pedestal. The apparatus—core and casing together —has been driven back into the previously deposited pedestal concrete. This enlarges the pedestal by pushing the pedestal concrete out laterally. The apparatus is stopped about a foot above the bottom of the pedestal to assure that no water or soil can enter when the core is removed. Step 3. Between Step 2 and Step 3, the core has been removed from the casing, thereby leaving an open hole in the center of the pedestal, guarded against collapse by the casing which is still in place. Step 4 shows the forming of a cased pedestal pile. A compressed concrete shaft, or an Hbeam or precast concrete shaft, might be placed in much the same manner. After the removal of the core, a corrugated metal shell has been placed in the casing with its lower end within the casing-protected hole in the pedestal, and then filled with concrete. Note: The procedure of introducing the pile shaft into the pedestal avoids the possible separation of shaft and pedestal. The core is placed in contact with the top of the column of cased concrete, but instead of being allowed to transmit the strain of the pulling of the casing to the shelled concrete, the core is "dogged off" so that it cannot move either up or down. The casing is withdrawn leaving the finished pile as shown. Note: Where it is proposed to use the pile

WESTERN UNCASED CONCRETE PILES to carry tension load, or when bending in the pile shaft may be anticipated, a reinforcing cage is wired into the shell before it is placed and concreted. T H E QUESTION OF REINFORCING

In uncased compressed concrete piles unless reinforcing is done correctly, it may largely inhibit the flow of the concrete, which must take place to fill the void left by the withdrawal of the wall of the forming casing. If an ordinary reinforcing cage made up of spiral and longitudinal bars is used, it may cause poor contact with the surrounding soil; or the cage may become greatly distorted and crushed, in which case it will fail in its purpose as a tension member and probably will leave honeycomb pockets. In some soils a triangular three-bar cage with a minimum of hooping can be used satisfactorily. If reinforcing is to be used for the purpose of transmitting tension load from the pedestal or base of the pile to the cap, it can best be placed by using a single heavy rod or a wired-together bundle of smaller rods in the center of the pile shaft where the movement of the concrete will be at a minimum during the pulling of the casing. The writer has used three bar cages of about one third the diameter of the pile shaft and placed as nearly as possible in the neutral axis with fairly good results. Pedestals While pedestals are most frequently thought of as completing uncased cast-inplace concrete shafts, their usefulness is by no means limited to this field. The shaft above a pedestal may be light-shelled cast-inplace concrete, concrete-filled steel pipe, precast concrete, structural steel, or any other type of shaft the designer wants to use. Pedestals increase the bearing value of any pile not directly bearing on an impervious stratum in two distinct and separate ways:

77

(a) they pack up and so improve the bearing value of the soil in which they are formed; ( b ) they increase the area of bearing at the base of the pile, that is, they create a "spread footing" on or in a soil stratum at considerable depth below the surface of the soil. The following are specific cases and conditions in which the pedestal has proved to be of value. PEDESTALS

AS A SUBSURFACE

"SPREAD

FOOTING"

Pedestals are indicated where the soil section is such that the weight of the proposed structure cannot be carried at the surface, but where a suitable bearing stratum can be found within economically reasonable depth, in which the load could be carried if it were possible to spread the load in the form of mat or footings on or in this stratum of "fair bearing value." It may sound as if the condition outlined would be rare and therefore scarcely worth discussing, but as a matter of fact it occurs with great frequency. A few of the territories in which the writer has met such conditions and solved the problem by the use of pedestal piles are the Jersey Meadows, Flushing Meadows, Mexico City, and New York City at half a dozen points. In addition, such conditions may be anticipated in almost any old river delta in any part of the world, and they exist in the delta of the Mississippi to a very marked extent. The use of pedestals, spreading the load on a comparatively thin stratum of material of good bearing value with strata of lesser bearing value underlying, has been demonstrated as a perfectly sound practice, but unless it has been proved by structures on such foundations in the near neighborhood, further investigation would be indicated and should include the following. 1. Complete boring data, including socalled "undisturbed samples," should be obtained to assure that area settlement caused

FOUNDATION TYPES

78

by compaction in some stratum lying below that on which the pedestals would bear will not occur. The samples should, of course, be analyzed and studied in a good soil mechanics laboratory. 2. Single pile-load tests to at least 100 per cent overload should be made. 3. Where the importance of the structure will warrant the cost, a multiple pile-load test, say on a three-pile pier, should be made. At least one of the piles of this pier test should be separately loaded, so that the behavior of a loaded "area" may be compared with that of individual piles. If the pier test shows substantially greater settlement under the same per-pile loading when compared with the single-pile test in the same or a near-by location, then it would be apparent that either the bearing stratum is "bowing" or the loaded section of the bearing stratum is partly failing by the shearing through of the area loaded by the multiplepile test. If this three-point study indicates the probability of area settlement, some other type of foundation should be considered. If no other form seems feasible, then group tests on larger groups and perhaps to greater loadings should be considered. PEDESTALS TO STEP UP

WORKING LOAD

The pedestal is most commonly and most advantageously used for the purpose of increasing the working load value of a pile, particularly in cases where the allowable working load will be based on a load test program. PEDESTALS TO DEVELOP

TENSION

A most important field of use for the pedestal, and one which is growing, arises from its ability to develop tension, that is, its resistance to uplift loads. This feature has been discussed at considerable length in Chapter 10 and need not be reviewed here.

PEDESTALS

ON

SHARPLY

SLOPING

ROCK

OR

BOULDERS

Pedestals have been used to assure a proper hold for a pile driven to seat on sharply sloping rock. This "slope" may be general, or may arise from variations where the rock surface is exceptionally jagged and rough, or where bearing may take place on large boulders. Under such conditions, the point of a pile may have a very precarious hold, which could be disturbed by future vibration or the movement of subsurface water. W e know that this easily disturbed bearing does occur, because when driving closed-ended piles to rock on some sites wc will occasionally come upon a pile which will all but meet the required resistance but will then "slip" and drive easily for many inches or even feet further before again picking up a rock resistance. Where the piles are of a type permitting internal observation during or immediately after driving, we frequently find rather sharp bends, plainly indicating that some obstruction has been struck a glancing blow. A small variation in the location or shape of such an obstruction might have caused the pile to show the required resistance, with a consequent stopping of the hammer when the pile point was only partially seated. Another blow might have caused the pile to slip, with a consequent loss of most of its bearing value. If a pedestal had been formed at the bottom of the pile, three advantages would have been obtained. 1. The pile bearing would, of necessity, have been spread over a considerable area by the pedestal and would not have been dependent on a small segment of the point's precarious bearing. 2. If the pile has brought up on a generally sloping rock surface, the pedestal will act like a "cat's paw"—the concrete of the pedestal will spread out over the rock and when set

WESTERN UNCASED CONCRETE PILES it will take advantage of minor irregularities of rock surface to prevent any possibility of future slipping. 3. The pedestal will have a considerable cross-sectional area acting in side-bearing to prevent sideslip of the pile bottom. This sidethrust will be operative to some extent even during the driving out of the pedestal, while the concrete is still plastic. If no sideslip occurs at the time of forming the pedestal, it is a virtual certainty that it cannot occur later on, when the set concrete will have a much greater holding reaction because it can no longer be moved by distortion of its mass of concrete. PEDESTALS TO PRODUCE A "BALANCED" RESISTANCE TO LOAD

The question is often asked "How can one be assured that the pedestal will be located on the projected axis of the pile shaft?" The answer is, of course, that the pedestal would lose one of its most valued characteristics if its center of resistance were of necessity uniform about the projected axis of the pile shaft. The pedestal is formed by pressure on the unset concrete, and this pressure is applied along the loading axis of the shaft. Automatically the unset concrete will flow to the points of least resistance, where it is most needed to balance the required bearing resistance. PEDESTALS OF

LONG

TO

REDUCE

SLENDER

UNSUPPORTED

COLUMNS

IN

LENGTH

BRIDGE

FOUNDATIONS

, Occasionally long slender piles or columns are required to be carried through water and to some depth through soils varying from muck immediately below the water to soils of medium bearing-value over rock. In bridge structures these foundation columns may extend for 100 feet or more above the river bottom and for a depth of perhaps 60 feet from river bottom to rock. The unsupported column length, bridge deck to rock, will therefore be

79

in the nature of 160 feet. It would be risky to assume a point of restraint in the indicated soil at any point more than 10 feet above the rock. To reduce the unsupported length of the column, it is suggested that a considerable mass of concrete be deposited around the column at a depth of about 10 feet below the river bottom. This concrete could not be deposited in the medium-firm soil by tremie or by any standard method which did not include cofferdamming to exclude the river mud which overlies the medium-value soil to a depth of 10 to 15 feet. It is not at all certain that the removal of this muck stratum would not automatically cause the mudding up of the next lower stratum. It has therefore been suggested that this mass of concrete, which will serve no structural purpose but will merely act as a supporting element to the columns, should be deposited at a depth of about 10 feet below the river bottom by the formation of a series of "pedestals." A bridge recently designed in Mexico would seem to prove the validity of this suggestion. Caisson Pile "Caisson Pile" is a trade name given to a product and method developed by the Western Foundation Corporation and used in a number of structures to meet the requirements of a foundation type suitable for loads up to about 500 tons. It is somewhat similar in load capacity and cost per ton foot to the Gow Caisson and the auger bored caisson. Like the other two, it uses the soil as its casing. It has a second limiting factor in that it is rarely economical to depths in excess of 60 feet. However, it has one substantial advantage. Because of its heavy forming casing and its churn-drilling equipment, it is at home even under extremely rough soil conditions. On a job where load concentrations required units of 450-500 ton capacity, with a fill to be penetrated consisting largely of broken masonry

80

FOUNDATION TYPES

underlain by coarse gravel and highly compacted sand, 20- to 50-foot caissons were bottomed in rock, though a core-boring outfit failed after many attempts to get 10 feet below the surface. The pile itself is something of a hybrid, partaking of the nature of the uncased concrete pile and also of the Drilled-In Caisson. The method of placing is simple. A steel casing usually 18 to 42 inches in diameter and % to 1 inch in thickness is forced into the ground by alternate driving and cleaning till it reaches rock, hardpan, or other impenetrable

stratum. The rock at the bottom is leveled by means of a heavy churn drill, and the casing is washed and cleaned. The casing is filled with water, if there is any tendency toward inflow of ground water, and is then filled with tremied concrete in sufficient volume to complete the shaft to cutoff elevation. The forming casing is then extracted by means of heavy tackle. Where pier loads, soil conditions, and depth favor its use, the Caisson Pile will give an absolutely dependable foundation and may offer the most economical solution possible.

12 LIGHT-SHELLED CAST-IN-PLACE CONCRETE PILES

B E F O R E going into detail about the various light-shelled piles, some comment is in order upon the basic difference in design and concept which constitutes a dividing line between the two principal cased pile groups. In the first group a forming apparatus carrying some type of detachable point or closure, which will remain permanently in the ground at the elevation to which it has been driven, is forced into the ground; the light shell is placed through the forming apparatus and the heavy drive casing is then withdrawn. In the second group the apparatus used for placing the light shell is in the nature of an internal mandrel on the outside of which the light shell is mounted. The mandrel and shell are driven into the ground, and when the desired depth or refusal has been reached the mandrel is withdrawn, leaving the light shell only in the ground. Certain advantages are claimed for each type and for the general classification as compared with the compressed uncased classification. It is pointed out that if permanently cased piles are used a number of pile shells may be left unfilled in any group, and after all of the piles in the specified area have been driven the shells may be reexamined to assure that crushing or distortion has not occurred as a result of the driving of adjacent piles. There is some force to this claim because the open shells simplify inspection. It is possible by keeping levels on an occasional "telltale" which may be left in one of the open piles to

check on the possibility that shells have been heaved by adjacent driving. However, visual inspection of the inside of the shells is not a complete answer because there is more than a chance that the unfilled shell, while keeping its circular shape, may have been squeezed to a smaller diameter than it originally possessed. But the loss of even an inch of diameter would not in itself be a serious matter, because the concrete shaft will in most cases have a very large safety factor. The danger arises from the fact that where the shells have been driven in intimate contact with the soil, or where the load value of the pile has been ascertained by tests on piles of full original diameter, a large percentage of friction value may have been lost by even a minor shrinkage of the cross section of the pile. To be sure of the answer, visual inspection from the top of the pile is not enough. A gauge fitting the inside of the pile shell, with not more than a %-inch clearance, would have to be run in every pile, or at least in enough piles to assure that no squeezing was taking place. Between "telltales" to indicate possible heave and gauge runs to show possible squeezing and, of course, visual inspection to prove that the pile shell contains no mud or water before filling, this shell inspection is not as simple as it may at first sound. "Light-shelled cast-in-place concrete piles" is a subclassification which includes all those cast-in-place concrete piles in which the unset concrete is deposited in a tube or shell previ-

82

FOUNDATION TYPES

ously introduced into the ground to act as a temporary form to contain the concrete—a form which is not figured as a part of the permanent bearing structure of the pile. In contradistinction, the "pipe pile" is one in which the container acts not only as a form for the unset concrete but also as a loadbearing component in the finished pile. Building codes generally draw a line between the two classes by stipulating a minimum wall thickness of the pipe or tube below which no load-bearing capacity may be figured in the container when calculating the structural strength of the shaft. For example, this minimum thickness is given as % of an inch in the New York City Code, while in the Building Officials Conference of America (B.O.C.A.) Code the minimum wall thickness for which bearing value is allowed is y 1 0 of an inch. The newest Chicago Building Code does not specifically state a thickness limit, but the rulings of the building department seem to have set a %-inch limit.

posed on a compressed concrete shaft with or without a pedestal base section, has been used on a number of jobs. Piles in which the shaft consisted entirely of light-shelled cast-in-place concrete have been driven to depths of 110 feet and more. The use of such long, light-shelled shafts depends on soil conditions. Where they can be used, they are likely to offer an exceptionally low cost for such depths. Piles consisting of light-shelled (16 gauge or less) upper sections and steel pipe lower sections can be driven to almost any depth. Since the pipe used for the lower section will usually cost two and a half to three times as much as the light shell of the upper section and since the cost of a splice between the upper and lower section will have to be added, this steel pipe and light-shelled composite pile will be substantially more expensive than an all light-shelled pile of the same length wherever soil conditions permit of a choice between the two. This type of pile is Light-shelled cast-in-place concrete piles discussed in greater detail in Chapter 13. It may be classed as a group when considered is mentioned here because it is not infrefrom an economic standpoint because in gen- quently referred to as a light-shelled (or eral they offer the cheapest type of piling— merely as a cased) cast-in-place concrete with the exception of the compressed concrete pile. pile with no permanent shell and the unThe principal makers of light-shelled casttreated wood pile—within moderate ranges of in-place concrete piles are the Raymond Conlength, say 15 to 75 feet. They are effective crete Pile Company, the Western Foundation within a very wide range of soil conditions Corporation, the Mac Arthur Concrete Pile and for loads of 30 to 60 tons or more per pile. Corporation, and the Cobi Pile Company. The above, of course, is a broad generaliza- Numerous other makes differing in mechanical tion; obviously there are many special condi- details have been on the market. tions which would require exceptions. The Light-Shelled Concrete Piles Driven higher loads are generally allowed only where with Internal Mandrels a program of field load pile testing is required. Some of the more important codes limit such piles to around 80 tons when end-bearing, and 60 tons when all or most of the load would be delivered on rock or other virtually impenetrable stratum. A combination form of pile consisting of a light-shelled section of shaft through soil not suitable for compressed concrete, superim-

Light-shelled piles of the internal mandrel driven type have a long record of successful use. The first piles of this type were introduced more than forty years ago and have been continuously on the market since that time. In number of jobs, number of piles, and total footage, Raymond leads in the lightshelled field.

LIGHT-SHELLED CAST-IN-PLACE CONCRETE PILES

83

Piles of the internal mandrel driven type include a number of variants from which to choose in meeting the special soil and site conditions of a job. While many mechanical details of installation have been changed and improved, the pile itself and the method of forming are basically those of the original installations. Perhaps the addition of the steptapered to the uniformly tapered pile should b e considered a fairly basic divergence of type.

into which the larger superimposed cylinder may be drawn down. It might appear that this plough ring would cause an opening such as would leave each section standing free from the surrounding soil. This condition could actually exist at most for only a brief time, because even the stiffest soils when forced out of their natural bed by pressure will show a degree of elasticity and would soon fill any opening which might exist.

In the uniformly tapered type the pile is driven by an internal expansible core which grips the shell and carries it into the ground when the core is driven under the blows of a hammer. After the required refusal has been reached, the core is collapsed and withdrawn from the shell. At some time after the withdrawal of the core the shell is filled with concrete.

T h e uniformly tapered pile was severely limited as to length, because a considerable degree of taper was necessary to hold the shell mounted on the expanded mandrel when driven against high external friction and to assure against the raising of the shell when withdrawing the collapsed mandrel. One of the principal advantages of the step-tapered form is that it can be used for great depths. It is offered for lengths up to 80 feet, even without the use of the steel-pipe composite lower extension section, which is offered for lengths up to or exceeding 150 feet.

T h e tapered pile does an excellent job of "packing up" where a uniform soil stratum of good or fair bearing value extends downward continuously from the surface of the soil. T h e packing up is of doubtful value, whether by this or any other pile or method, where the soil is clay or contains any large element, say 40 per cent or more, of clay. Clay soils will generally "heave" rather than "pack," and to the extent that packing occurs, consolidation may follow, with a consequent slow release of bearing value. A variant to the uniformly tapered internal mandrel driven pile is the Step-tapered pile. This pile as completed in place has the outline of an extension telescope in which the eye piece is at the lower end and the shaft consists of a series of sections each 8 or 12 feet in length. T h e diameter of each cylinder in ascending relation from the point is greater by about an inch than that of the one below it. At the top of each of the cylinders there is a "plough ring" which is greater in diameter than the outside diameter of the next superimposed cylinder, so that it may plough or expand the soil to form an opening

Another advantage in the method of placing the step-tapered pile shell is gained by the ingenious reaction developed during the withdrawal of the mandrel from the shell. T h e slight slip or play between the various sections of the core sets up a condition under which only one section of mandrel at a time is freed from the shell, while all the other shell sections above or below it exert counterfriction or anchorage to prevent the shell as a whole being raised with the stepped core. Button-Bottom

Piles

Millions of feet of this type of pile have been used on a large number of jobs since it was patented by the writer in 1925. T h e Button-Bottom type has two distinguishing features. T h e first, which it shares with the MacArthur light-shelled pile, is the placing of the permanent shell through a predriven mandrel which is later extracted. The reaction is somewhat akin to the starting up

84

FOUNDATION TYPES

of a long freight train where only one car is put in motion at a time due to the slippage allowed in the linkage between cars. Internal mandrel driven light-shelled piles may also be of cylindrical shape. The arguments for and against the placing of the shell through a protective casing rather than on the outside of a collapsible mandrel to be later withdrawn have been touched on elsewhere in this chapter. The arguments against the use of a protective casing which will be withdrawn would seem to date back to the time when engineers placed their faith and made their decisions regarding the bearing value of any type of pile primarily on some one of the dynamic pile-driving formulas. Few engineers familiar with recent studies and tests now place any faith in such formulas." The New York City Building Code states that pile-load capacity may be determined by formula only where backed by tests or local experience and even then only to a maximum working load for end-bearing piles of 40 tons or for non-end-bearing piles of 30 tons. Concerning pile-driving formulas, Terzaghi and Peck, in their Soil Mechanics in Engineering Practice,f remark: T h e bearing capacity of a friction pile can be determined only by load tests on piles in the field or less accurately on the basis of empirical values of f s (the friction f a c t o r ) . . . . However, since the reasoning which leads to this pattern involves various arbitrary assumptions with unknown practical implications, it is not surprising that even the most elaborate pile formulas are far from accurate. As a matter of fact there is no evidence that the bearing capacity computed by means of any formula is likely to be more reliable than that computed by means of equation 3 0 . 4 . [Equation 3 0 . 4 is the old, original Engineering News formula, developed in 1 8 9 8 , which meets all uncertainties by allowing, theoretically, a safety factor of 6 0 0 per cent and when checked against full-scale load tests frequently shows less than 1 0 0 per cent of working load capacity.]

When determination of bearing capacity of a driven pile is made by the use of a formula, any change in conditions occurring after the time at which the formula was applied might affect the answer previously obtained. For example, if a protective casing has been in place at the time the hammer blows per inch were computed but would not be in place when the working load would become operative, the load capacity indicated by the formula might no longer apply. Or should additional adjacent piles be driven after the pile evaluated from the formula had been driven, the true value of that pile might be either substantially increased or decreased. However, if the pile load is determined by "local experience" or by "load tests" and not by the application of a formula, the change due to the withdrawal of a mandrel, whether in or outside of the permanent shell, will be meaningless, as will also be the effect of adjacent piles. Under a "performance" specification or code, if there is any question in the designing engineer's mind as to the bearing adequacy of any pile, or of any other type of foundation from spread footings to rock-bearing caissons, it should certainly be resolved by requiring a sufficient testing program and not by the application of any theoretical driving formula. The second distinguishing feature of the Button-Bottom Pile is the button or precast pedestal used at the base of each pile. Before developing the type of button shown, extensive tests had been made of closure plugs of reinforced concrete design similar to the heads of precast concrete piles. Plugs of considerable length, say 4 feet and upwards, can be driven if well reinforced, but such plugs are cumber• See the findings in Pile Foundations and Pile Structures, a brochure published by the American Society of Civil Engineers. f Karl Terzaghi and R. B. Peck, Soil Mechanics in Engineering Practice (New York, John Wiley & Sons, 1948), pp. 177, 180.

LIGHT-SHELLED CAST-IN-PLACE CONCRETE PILES some and expensive. No matter how reinforced, the short plugs all failed in diagonal shear. The button as finally developed is entirely unreinforced; in fact the introduction of reinforcing, either vertical or circumferential, caused shatter, which no doubt should have been anticipated. Figure 5 shows the pile and method of placing. Step I

Step 2

Step

3

Figure

5. Button-Bottom

parallels the driving surface G-l. A large percentage of these buttons have been driven to high resistance under No. 0 Vulcan hammers without sign of failure. The shell C, usually 16 or 18 gauge, fully welded or plain, and corrugated, has a domed pressed steel pan D welded to its bottom. There is an aperture in the pan through which the bolt F, which is cast in the plug E, will protrude when the shell is lowered through the casing to its seat on the driven plug. The pan and shell are attached to the plug by screwing down the nut and large domed washer which is welded to it. Step 2 is an enlargement of the button E, bolt F, and nut and washer assembly J while in process of attachment of the shell to the button. Step 3 shows the assembled shell and button in the ground and ready for concreting. While the general patent on this pile has expired, certain details are still under patent. Cylindrical Internal Mandrel Type

Casing Coned Driving Shoe C Shell D Steel Pan E Button or Plug F Bolt G, Shoulder of Button G 2 Taper of Drive Shoe J Nut and Washer Assembly A B

Pile

Step 1. The button E is dug in a few inches. The driving of the button is carried out by means of a casing A. No internal core is used. The driving blows are transmitted through the casing A to the coned driving shoe B, to the sloping shoulders G-l of the button E. The driving shoe, which must take all the bursting strains set up by the hammer blows, is made of special high tensile steel. The coned shoulder of the button is cast to exactly the same taper G-l as the machine-cut taper G-2 of the drive shoe on the casing. The angle of the point of the shoe is such that it roughly

85

Piles

This pile is a comparative newcomer but a very considerable amount of footage has been installed. The pile is usually 18 to 14 gauge in thickness, lock-seamed with or without welding, corrugated, and is driven with an internal expanding mandrel. The advantages claimed for it are that, unlike other light-shelled types using an internal mandrel, it is driven in true contact with the soil without the use of an outside casing or of plough rings which may temporarily open a hole of greater diameter than that of the finished pile. If such a thing as a completely nonelastic soil exists, this driving in intimate soil contact might be a matter of importance in any situation in which the loadbearing value of the pile is determined by theory and not confirmed by full-scale loading tests on the site. Personally the writer has not yet found any soil into which a pile can be driven in which a hole opened up by driving a spud or other unit into the ground and with-

86

FOUNDATION

drawing it will not close again. Wet granular soils close almost instantly behind a withdrawn tool of any kind, while dry clays or silts may take weeks to make any substantial closure movement. However, insofar as our experience goes—and it covers thirty years and a very wide territory—all soils do return after being opened by driving. E F F E C T O F ROUGH

FILL

In rough going such as artificial fill containing broken up concrete, brick rubble, or fill produced by subway tunneling or the like, the use of an external mandrel through which the light permanent shell is placed offers a much needed protection against ripped shells and consequent entry of water or soil into the permanent light shell. It is comparatively easy to take care of a condition which allows inflow of water only, either by rapid pumping immediately before deposit of the concrete filling or by the use of bottom dump buckets or tremies, but these methods are far from positive where it is necessary to contend with an inflow of silt as well as water. One method which is not infrequently used is that of blowing out the silt and water by means of a steam syphon. The trouble with this form of pumping is that the pile shell is left filled with steam, which, because of the insulating effect of the surrounding soil, will rarely cool and condense in less than fifteen to thirty minutes. If there is any continuing inflow of soil or water, a delay of fifteen minutes to allow inspection would merely permit the building up of a new head of water, which would preclude the possibility of visual inspection. It is sometimes claimed that when the syphon shows an exhaust of steam with little or no water or foreign material, this is proof that all water and soil have been exhausted from the pile shell and concrete should be placed without delay through the steam. Of course the fact is that a syphon will frequently "choke" and blow steam while there is still water in the tube. This is par-

TYPES

ticularly likely to happen if there is soil collected at the bottom into which the syphon may have been dropped. In many cases where pile shells have been left open for a considerable time, heavy rains or slow seepage may partly fill them with water, clay, or fine silt. If this infiltration is sufficiently slow to permit a thorough clean out followed by careful inspection, there will be no reason to doubt the integrity of the concreting operation. But if the shell can be concreted only by rushing the concreting, without taking time for observation, then means should be found to seal off the inflow before concreting is attempted. Failure to take proper precautions in the matter of cleaning is particularly dangerous where the piles are of the end-bearing type. It must be realized that one or two inches of silt or fine sand at the bottom of the shell may mean an inch or more of settlement when the working load comes on. As pointed out in Chapter 41 in the discussion of the effect of pile shape on the bearing values of piles in various differing soil profiles, the cylindrical is best suited to a number of conditions. While any advantage claimed for piles "driven in contact with the soil" has been questioned, there is one very real advantage gained by the use of an internal rather than an external mandrel. This arises from the fact that the withdrawal of the protective mandrel or casing after the light shell has been placed through it frequently demands the use of very powerful pulling gear, which in turn calls for a heavy, specially built pile driver to sustain it. The internal mandrel, on the other hand, can usually be withdrawn easily from the driven shell, and therefore the necessary mechanism can be mounted on a standard crane which may be already on the job site for other purposes. A steam boiler or a steamgenerating plant complete the set up. Possibly a Diesel hammer may soon be developed which will eliminate the need of steam. The advantage gained by the use of stand-

LIGHT-SHELLED CAST-IN-PLACE CONCRETE PILES ard plant due to the elimination of heavy pulling has no special engineering importance but, as has been repeatedly pointed out, anything which will help the foundation contractor produce a first-class job at less cost should and generally will reduce costs for the owner as well, if not by direct substitution at least by the way of increased competition. The more help that the designing engineer gives the bidders, the better will be his chance of low cost by way of return. To help the bidder the engineer must know the strength and the weaknesses of the various types of foundations, not only from an engineering but from an economic angle. The cylindrical internal mandrel pile has certain mechanical advantages, as pointed out, but it suffers from a basic mechanical disadvantage when compared with the internal mandrel tapered or step-tapered type. This arises from the fact that in order to thread a long cylindrical shell on a cylindrical mandrel it is necessary either to have excessively tall leads which must carry a steam hammer at their tops when moving, or else they must have one or a number of "threading holes" generally consisting of a length of pipe of a diameter to accommodate the shell to be driven into the ground. In order to "shell up," the full-length shell is lowered into this pipe, the driver moves to the hole, lowers the mandrel into the shell, and then moves back to the point at which the next pile is to be driven. The tapered pile, on the other hand, permits the use of the last shell driven or any other previously driven shell for a shelling up hole and so avoids a lot of time wasted in moving. It is

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obvious that this disadvantage in the shelling operation becomes less important as the pile length decreases. Other things being equal, this type of pile will usually be better competition in the field of shorter piles, possibly in the length of 40 feet and under. Other factors may enter into costs, and no generalization as to efficient length can be taken too literally. The tapered internal mandrel-driven pile has another rather important mechanical advantage over the cylindrical pile of the same class. In case of a torn shell, which often occurs, when the light permanent shell is mounted on the outside of the mandrel, a second tapered shell either of part or full length can be threaded on the tapered mandrel and driven into the first shell to repair the injury. This cannot be accomplished with a cylindrical shell. Substitute piles are not only costly, but they may so unbalance the pile group that two or more additional piles will be required. This type is applicable over a large range of foundation problems. Its use has been widely licensed, which makes it probable that plant and operators will be available in any locality in which the engineer may wish to consider it. This very broad licensing policy has its dangers as well as its advantages, and it is important that the engineer or owner make sure that the company to which he plans to let the job has had sufficient experience with the plant and method to assure first-class workmanship. There is no foolproof foundation method. All must depend to some extent on the skill and experience of the operators.

13 COMPOSITE PILES

COMPOSITE or combination piles are those made up of two types, either of which type under certain conditions could be, and on occasion has been, used independently to form a complete load-bearing unit. Any of the tools and methods normally used to place piles—driving, jetting, spudding, coring, drilling, or blowing out—may be employed to form and set either or both parts of a composite pile. However, there are two general methods which apply only to composite piles. These are the follow-down method and the projectile or driven-through method. The choice between the methods hinges principally on the purpose or reason which has dictated the consideration of composite piles as the solution or one of the possible solutions to a given foundation problem. Up to about eight or ten years ago there were two reasons one or the other of which generally accounted for the use of composite piles. The first of these stemmed from the then governing lengths for light-shelled or compressed cast-in-place concrete piles which in general limited such piles to about 50 feet. While precast concrete piles ran to somewhat greater lengths, they were rarely competitive within the length range which could be reached by the cast-in-place piles. In lengths much in excess of 50 feet the per foot cost of casting, handling, and driving precast piles rises sharply. The steel pipe pile, either open- or closedended, could be used to practically any depth, but its cost was a handicap except where heavy concentrated loadings had to be met.

However, in recent years a radical change has occurred in the matter of length limits of light-shelled cast-in-place concrete piles. Where soil conditions have been favorable, light-shelled types have frequently been used in lengths of 100 feet and more and comparatively thin-walled pipe piles have been introduced for similar lengths. (The adoption of the light-shelled or the light-walled pipe types for great depths should be made only after thorough soil studies and in the light of experience. There are many soil conditions in which their use will lead to trouble.) While extreme length is no longer necessarily a bar to the use of light-shelled piles, the type of composite pile which was introduced many years ago to extend the length range of the cast-in-place pile and which has been successfully used for many millions of feet of piling is still the best bet where great depths, hard driving, and rough going may be anticipated. It is also generally highly competitive wherever long piles will be needed, even where soil conditions might be classed as "normal." The composite type, consisting of a lightshelled concrete-filled upper section and a concrete-filled steel pipe lower section, is used with minor variation by at least four of the better known pile-foundation contractors. The materials used and method of placing are as follows. A group of closed-ended steel pipe piles of predetermined length is driven so that the tops will extend about 2 feet above the ground surface. A mandrel either of the hollow, the expansible, or the step-tapered type is

COMPOSITE PILES placed in contact with the top of each of the predriven pipes and each pipe in turn is driven, using the mandrel as a follower, till the required bearing has been reached. The mandrel either carries a light shell which has been mounted on the outside before driving or the light shell is placed through the hollow mandrel and over the head of the steel pipe after the lower sections have been driven. The "come and go" in the pile length required to get bearing is usually made up in the light-shelled section so as to allow the use of a uniform length of pipe section. The pipe section—which is frequently a 10%-inch diameter pipe, with a wall thickness which may vary from % to % of an inch depending on soil conditions to be encountered—is, of course, more expensive per foot than the lightshelled section and is kept to a minimum consistent with the over-all length required and the safe limit of light-shelled section. By far the most critical factors in any type of composite pile are the design and method of placing the component parts of the splice. Numerous failures attended the earliest use of composite piles. In all or nearly all such cases the failures occurred in the splice. In later sections of this chapter several typical splices are shown and discussed in detail. Given a properly designed and properly placed splice, the steel pipe composite is one of the safest piles driven. Step-Tapered

Steel Pipe Composite

Piles

As the name indicates, this consists of a step-tapered light-shelled upper section and a steel pipe lower section. The lower section generally consists of a pipe with a 10%- or 11inch outside diameter and such wall thickness as may be considered necessary to resist bending or "sweep" during driving. This pipe section is driven almost to the ground level. A steptapered core having an 11%-inch bottom section is shelled up and set on top of the pipe and is then used as a follower to drive the lower pipe section to final bearing. The two sections

89

are concreted in a single operation. The upper and lower sections are connected by a "slip joint" which would permit the upper section to be raised somewhat by heave without disturbing the bearing value of the lower pipe section. Wherever heave of the upper section may occur, the bearing value of this followdown slip-joint type of composite pile will be largely limited to that of the pipe section. Cast-in-Place

Concrete and Wood Composite Types

Pile

The form and method of driving this pile are very similar to those just described for the steel pipe composite except that a wood pile is substituted for the concreted steel pipe lower section. The steel pipe composite is generally superior to the wood composite because it can be safely and permissibly used for far greater per pile loads than are allowed on the wood composite. Under nearly all specifications and codes, any type of composite pile is limited in load capacity to the value allotted to the weaker section, in this case the wood pile, which would rarely be allowed more than 20 or 25 tons. The steel pipe composite using a concrete upper and a concrete lower section might be so designed as to carry 60 to 80 tons. In any type of composite pile, regardless of the maker and the details of construction, the most important feature is its ability to reach greater depth than any other type of pile of anything like equal cost. Straight steel pipe piles, either open- or closed-ended, can reach equal depths, but they are considerably more costly. Wood Composite

Piles

The wood pile composite consists of a wood pile lower section, a splice section, and a castin-place concrete upper section. Note. At one time a number of wood and concrete composite piles were in use in which the upper concrete section was precast instead of cast-in-place. Theoretically such a

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FOUNDATION TYPES

pile would have certain advantages over many of the types using a cast-in-place upper section, for example, in pier and trestle work, where the driving plant would best be mounted on the completed piling. However, as with all composite piles, the key section is the splice between the wood and concrete. The splice in the case of the precast concrete to wood sections poses a particularly difficult problem because in addition to fastening the sections together against upheaval and possible bending it must also be suitable to deliver the driving impact to the head of the wood pile section. In addition to this special problem of the splice, the basic objections to the precast pile as such—primarily that of hair-cracking when handling and driving, with the resultant rusting of the reinforcing and consequent spalling and weakening of the pile—would be present in an exaggerated form because of the necessity of bucking the wood pile through the concrete section used as a follower. The wood pile composite type has one advantage over nearly all other bearing pile types—cost. Where conditions favor its use, the per foot cost of a composite pile will be lower than that of the precast and cast-in-place compressed or cased concrete types and will, of course, be far below those of the openor closed-end steel pipe or the structural steel types. The cost is frequently, though not always, below that of creosoted wood piles, and where piles of considerable length are needed it may beat the per foot cost of an untreated wood pile. For example, in the Chicago territory mixed hard wood piles up to 45 feet may at present be bought trucked to site at a low figure, but a 75-foot pile, if required in large quantities, would have to come by rail from the West Coast and then be trucked from siding to site, making the delivered per foot price above that of a composite pile having a 45-foot wood and a 30-foot concrete section.

The wood pile composite, despite its low cost, is too often neglected by the designing engineer and omitted from the specifications. Its use may be indicated or may be ruled out by the answers to the following questions: 1. Is there a water table which one may reasonably assume to be permanent? If so, can it be reached at a depth of one third or less of the anticipated over-all depth to the permanent bearing stratum to which it is anticipated that any type of pile should be taken? Note. The suggested depth of one third is, of course, meant to be used only as a rough approximation. It might be overruled by other factors. For example, if expensive wet excavation would be required to reach the depth at which plain wood piles would have to be cut off, a greater length of concreted section might prove to be warranted. Or if a creosoting plant in the neighborhood would offer an unusually low delivered price on creosoted piles, the composite pile might be out of the running even if the ratio of concrete to wood section was substantially less than 1 to 2. No hard and fast rule can be given. 2. If the load per pile efficiency study (see Chapter 30) has been made and indicated that the unit load should be substantially above 25 tons per pile for maximum efficiency, it would suggest the use of plain wood piles. Creosoted wood piles and wood and concrete piles would be unlikely to offer a proper solution. 3. Is the soil at or immediately above the point at which the splice should be made— for reasons of length efficiency and the elevation of the water table—of such a nature as to be suitable for the forming of the splice, or will it be necessary to choose a point at a considerably lower elevation? None of the standard splice designs develop 100 per cent of the strength of the wood pile immediately below the splice, and it is not generally considered that they need to do so where the soil which will surround the splice will help

COMPOSITE PILES to resist bending. However, a splice in fluid or semifluid soil should certainly be designed and tested to show 100 per cent of the strength of either wood or concrete section, whichever might be greater. Such splices have been developed, but they are expensive and probably would cancel out all or most of the savings which could have resulted from the use of this pile type if the splice had been justifiably designed on the supported splice basis and if the supported splice could be developed within an efficient depth. 4. Do the soil borings indicate that the lengths of the concrete sections will vary widely from point to point? If so, since the splice must always be below the permanent water table, the wood pile lengths to be used will necessarily be substantially shorter, and therefore the concrete sections substantially longer, than they would be on a site where the lengths of the wood sections could be approximately uniform. Since other types of piles less dependent upon uniformity of pile length would be in a better competitive position, the advantage of the wood composite might be lost. SPECIFICATIONS

Specifications for a wood composite pile should be concerned with the following: 1. Materials, that is, wood piles. Use standard specifications for untreated wood piles (Chapter 9), covering varieties to be accepted, dimensions, straightness, knots and wind shakes, points of inspection. 2. Driving of wood section, weight of hammer, protection of heads of wood piles during driving, responsibility for broomed or broken piles, responsibility for obstructions, study of "bounce." 3. Driving and forming the concrete sections. If the concrete section is to be one of the light-shelled types, the shell will either be attached to the head of the wood pile and the mandrel placed in the shell before driving of

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the wood pile is continued to final resistance, or the shell will be mounted on the mandrel and driven with it to final resistance. The concrete filling is then placed and the pile is complete. The specifications covering the upper concrete section of the composite pile should include the following: The concrete section of the pile shall be cast in a metal shell which shall remain permanently in place. The shell shall be of such gauge as to resist all pressures in the soil whether due to natural conditions or incident to the driving of adjacent piles. If obstructions in the soil, either natural or manmade, cause injury to the pile shell by ripping or distorting it, either before or after concrete has been placed, so as to reduce the area of the concrete by more than 15 per cent at any section, the pile may be condemned by the engineer in charge of the work, and one or more piles driven to take its load. The location of these replacement piles shall be given by the engineer and the piles driven by the contractor. If the injury to the pile occurs as a result of obstructions, the possible presence of which has been indicated by the borings and pointed out in the specifications, then the cost of replacement piles or other corrective measures acceptable to the engineer shall be paid by the contractor. If the obstructions causing the ripping of the shells or the distortion of the pile shaft have not been indicated by the borings and pointed out in the specifications, then all costs of replacement, including the moving to and from the site of the injured pile, shall be born by the owner. Note. In some specifications general clauses are included stating that "old foundations, old piling, large boulders and other obstructions" are anticipated and, if encountered, are to be the responsibility of the contractor. Even if these statements are correct, the net result is likely to be the inclusion by the financially responsible bidders of a large contingency. Not infrequently this contingency will either throw out the more knowledgeable and better prepared bidder, or in any case increase

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FOUNDATION TYPES

building costs in an amount greater than would have been required to protect the owner or the general contractor or both if responsibility for each obstruction had been taken by the owner. It is sometimes argued that a contingency item to cover any uncertainty should be the same whether included by the owner in his over-all costs or by a contractor or subcontractor. This is not true because any gamble must be looked at from several angles, and one of these is the percentage relationship between the amount of risk and the total amount involved in the work of which the risk forms a part. Anyone who would deliberately take a $10,000 gamble on a $10,000 job would be a candidate for the insane asylum, but a man might be entirely justified in risking the $10,000 item if it formed part of a $1,000,000 job. That is the principal reason for carrying insurance at a high premium on a property damage risk. Obviously the group of insurance companies behind the policy are justified in taking on a gamble at a figure which the contractor could not possibly risk, even though he knows that on its group of risks the insurance company makes a steady profit. All of this may seem self-evident, but the fact remains that many specifications continue to unload all sorts of longshot risks on the bidding contractors, apparently under the delusion that the owner will benefit. If there is a genuine probability, not just a possibility, that major obstructions to driving will be encountered, then it would seem that the designer should reexamine the site with additional borings, test driving, and the like and then specify methods and foundations types which could reasonably be expected to meet the bad conditions. Some of the strongest of the engineering firms do follow just this procedure, but the "smart" designs and specifications still dump the whole problem in the laps of the lump-sum bidders. The projectile or driven-through method of

placing composite piles is in general somewhat more expensive than the follow-down method because of the greater number of steps which must be taken in forming the pile, but under certain soil conditions advantages are claimed. (The method is shown in Figure 7.) Advantage is claimed: 1. Where rough fill or other tough soil will be encountered in the upper strata, the apparatus acts as a very stout spud with which to break a path through the obstructions, but unlike the usual spud this one is hollow and so protects the hole it has made till the pile has been driven through it and completed. 2. The driving takes place in two independent operations, doubling the effectiveness of the hammer. The driving of the lower section, consisting of steel pipe, H-beam, or the like, does not move the casing or hollow spud. This driving of the lower section is the same as it would have been if all fill and soil between the ground surface and the lower end of the casing had been removed prior to the start of the driving of the lower section. 3. Where the lower section of a composite pile is driven first to the surface of the ground and then followed down, any guidance given to it by the forming apparatus can be transmitted to it only by the comparatively short overlap of the apparatus over the upper end of the lower section. In the case of the driventhrough method, the lower section is controlled in much the same manner as would be the projectile during its travel down the bore of the gun, and the chance of its drifting out of alignment is correspondingly reduced. These claimed advantages of the driventhrough method—if soil conditions seem to indicate to the designing engineer that they would be advantages—must be weighed against the possible extra cost. The

Splice

Having discussed the requirements of the upper and the lower sections and the two

COMPOSITE PILES methods of placing, let us turn again to the most critical feature of any type of composite pile—the splice between the two sections. A composite pile can be no better than its splice, no matter what it consists of or how it is placed. Since the splice must be covered in the specifications, it is fortunate that there is a short cut which avoids the necessity of going into the mechanical and structural details and still reasonably assures safety. This is found in the listing of the purposes which must be served for the anticipated combination of soil profile, working loads, and kind of composite types to be permitted, and then setting up the strength and safety factors which must be proved by full-scale tests. This form of specification has been widely used, and if carefully written and supervised it results in satisfactory jobs. The following is a splice specification covering the requirements of a single site and load. It could not be described as typical because the field is so wide and the possible combinations of soil profiles and pile types and makes so numerous that many examples would be needed. The study for any single set of conditions could do no more than indicate the method of approach, which could be followed at least in a general way to find the answers to various different sets of conditions. W e make the following assumptions (see Figure 6 ) : The soil profile shows medium stiff clay to a depth of 10 feet below proposed pile cutoff, changing to plastic clay for the next 30 feet overlying a 3-foot stratum of peat, which, in turn, overlies a bed of loose wet sand for an additional 30 feet. Here a deep bed of coarse compact gravel shows 50 blows per foot and upwards on a standard 2-inch spoon under a 140-pound hammer falling 2 feet 6 inches. The blows on a 2Vi>-inch casing using the same hammer and fall vary from 150 to 250 per foot of casing penetration. An efficiency study shows that 50 tons per pile would best meet the load concentrations.

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On account of the over-all length and the somewhat tough driving indicated in the lowest level, it is decided to include a steel pipe composite pile among the permissible bid types. The general specifications have been indicated elsewhere. The splice must be designed and located to carry one or more of the following types of load: direct compression; bending; tension. 000

Cutoff

Figure 6. Sample Soil Profile for Splice Specification The correct location of the splice must be determined. The studies as to types of stresses and correct location are important not only from an engineering but also from an economic angle. A change in the splice location will include a change in the ratio between the lightshelled and the steel pipe sections. I have seen designs in which many thousands of dollars could have been saved, without any sacrifice of stability, by lowering the elevation of the splice and so reducing the expensive steel pipe section and replacing it by the much less expensive light-shelled section. In some instances the location of the splice was chosen because the soil at that elevation would permit the use of a splice of the least expensive type.

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FOUNDATION TYPES

By the use of a somewhat more costly splice the length of the pipe section could have been substantially shortened. In such a case two estimates should have been drawn up to make certain what the comparative costs would be for each combination. Let us turn once more to the engineering requirements of a composite pile and in particular a composite splice. Every case must of necessity be a special case because the problem can only be solved where the following factors are known: 1. What would be the economically correct unit load capacity? (See Chapter 30 for a study of this subject.) For the purpose of our hypothetical case we have assumed an answer of 50 tons. 2. What is shown by the soil profile? 3. Will the pile be required to develop resistance to bending or tension? If so, specifications should require full-scale tests. 4. What will be the over-all lengths of the piles, that is, what will be the elevation of the pile points in or on the "ultimate bearing stratum" and of the pile cutoffs? This should perhaps have been the number one consideration because it might eliminate the composite type altogether. At least approximate answers to these questions should be available from the engineering and general soil data. If driving and field load testing have been authorized, the suggested preliminary studies are still essential because only a small number of probings and load tests could be made and the choice of the foundation type or types to be field-tested must include the type which will finally be chosen. As to the case assumed in Figure 6, the splice specification may be determined by the following considerations: The upper light-shelled section will be much cheaper per foot than the lower steel pipe section and therefore should be as long as feasible. There would be no difficulty in driving through the medium stiff clay, the plastic clay, and the peat into the loose wet sand.

The splice should be made in the loose sand stratum at say elevation —63. The lower pipe bearing section should be about 15 feet in length to allow for a 5-foot penetration into the bearing stratum. The load on the pile and therefore on the splice will be in compression only. The upper clay strata will, quite probably, cause a tendency to soil heave in this strata. No permanent load bearing capacity should be counted upon if it is developed above the peat. The heave in the upper strata must not effect the bearing value of the steel pipe section. The principal reasons for the use of a steel pipe lower section are: 1. The necessity of eliminating the false bearing values of soil above the coarse compact sand and gravel stratum. When used in combination with a slip-joint splice, the bearing obtained in the gravel stratum can be checked from time to time by load test, or in some types of pile by retapping the pipe section independently of the light-shelled section. If a projectile type pile were used, every pile would automatically be tested for bearing in the gravel stratum, exclusive of bearing above the splice. 2. The requirement for the lowest section of the pile to be of sufficiently rugged design to permit of hard driving in the gravel in which the load must be delivered. 3. The requirement of the slip-joint type which would allow the upper thin-shelled section to be raised independently of the steel pipe lower bearing section. Since the slip-joint splice will be formed in wet loose sand, it must be so made that no soil or water can enter the splice either before or after heave has taken place to the maximum degree indicated by the soil in which the heave of the upper sections will be developed. SPLICE

SPECIFICATION

First, the splice shall be of the slip-joint type allowing for a possible maximum independent heave of the upper light-shelled section of 24

COMPOSITE PILES inches. The 24-inch lap may be reduced if after the driving of the first 50 piles it is shown that the raise of the top section will be 12 inches or less. Second, the splice shall be shown by test to be watertight under a pressure of 50 pounds. This test shall be carried out before any piling is driven. Third, though there is no theoretical bending or uplift force at the splice, a test shall be made above ground on a completed splice showing a tension resistance of not less than 5 tons and a bending resistance at the center of the splice of not less than 2 % tons. [Optional clause.] Other clauses would be required to meet different soil and load conditions. Splices can be designed and proved by test to meet any loads up to the limit of the weaker section of the composite pile, but they run into considerable cost and therefore should not be required unless clearly necessary. Other standard clauses covering materials of construction and limitations on methods have been given elsewhere and therefore are not included at this point. Precast concrete sections and H-beam sections have been used in place of wood or pipe sections to form composite piles, but they seem to offer no advantage over the concretefilled steel pipe section and would show price advantage only under rarely encountered conditions. Western Composite Piles Western composite piles consist of the following types: ( a ) a light-shelled concrete-filled upper section coupled with a wood pile lower section; ( b ) a light-shelled concrete-filled upper section coupled with a steel pipe concretefilled lower section or H-beam section; ( c ) a compressed concrete upper section combined with a wood pile, a concrete-filled steel pipe, or an H-beam section. Two distinctively different methods are offered, either of which is applicable to the installing of any of the types mentioned above. The first is the "follow-down method," in which the lower section is driven into the ground to almost its entire length. The form-

95

ing apparatus used for placing the upper lightshelled or compressed section is then mounted on the lower section and, acting as a follower, is used to force the lower section into the ground. The light-shelled section is filled with concrete, either reinforced or plain. Where a shell or pipe is used as a form or as a permanent part of the completed pile, the attachment between the two sections is made after the lower section has been driven to the ground level and before driving is resumed. The two attached sections are then together driven to their final refusal and position. If the upper section is to be of the compressed concrete type, then the apparatus mounted on the lower pile section is operated to force the lower section into its final bearing position, after which the concrete to form the compressed concrete shaft—and pedestal if one is to be used—is deposited in the apparatus and compressed by the core during the pulling of the casing of the apparatus. The second method of placing frequently used by Western is the "projectile method." This method differs from the follow-down method in that a forming apparatus, consisting of a heavy pipe casing containing a core with a base fitting snugly in the casing and shaped to fit the top of whatever form of lower section it is planned to use, is driven into the ground to a predetermined depth prior to, instead of subsequent to, the driving of the lower section. (See Figure 7.) The core is then removed from the casing; the lower section—of wood pile, steel pipe, or the like—is dropped into the casing; the core is replaced in the casing and fitting over the head of the lower section; and the hammer is operated to drive the core, which in turn drives the lower section on which it rests until the lower section has reached the required bearing. The core is then again removed, and if a cased upper section composite pile is to be formed, the shell or pipe is placed over the head of the lower section and filled with concrete. The core is then placed in contact with the top of

FOUNDATION TYPES

96

the newly formed upper section to assure that it cannot be raised, and the casing is withdrawn from the ground. Step 1 in Figure 7 shows the hammer and apparatus assembled for driving of the projectile type composite pile. Step 2. The apparatus has been driven into the ground to some predetermined depth, and the core has been removed from the casing. A section of pipe H—with a grommet ring J Step I

A B C D E F

Hammer Core Head Core Drive Casing Solid Core Base Drive Cap to Fit Projectile Section

H Pipe I Spacers J Grommet Ring K Closure Plate M Concrete N Corrugated Shell

Figure 7. Driving Steel Pipe Composite Projectile Method

by

welded to the pipe, a number of spacers I and a bottom closure plate K—has been placed in the casing. Step 3. The core C has been lowered into contact with the top of the pipe H, which fits snugly into the cap F. The pipe has been driven through the previously driven casing D till the desired resistance has been obtained. The core has then been removed and a light shell N has been placed in the casing, overlapping the steel pipe H and contacting

the grommet J. The shell and the pipe are then filled with concrete M. Step 4 (not shown). The core C is placed in contact with the top of the shell N and the casing D withdrawn over the core C. Under some conditions this method of forming has certain advantages over the followdown method. First, the heavy walled casing with the added rigidity due to the close fitting core may be driven in advance of the placing of the less ruggedly built lower section, to act as a plough through fill or other obstructions to a considerable depth, say 65 or 70 feet. The second advantage, which is more or less a corollary of the first, is that the great rigidity of the apparatus makes it possible to drive plumb and without sweep through conditions which would cause the lower section to develop a sweep or drive out of line if driven all the way from the ground line without guidance. If the lower section consists of a comparatively small diameter lightwall pipe, say a 1 0 % " x % 6 " , which is frequently used, it is difficult to retain control to anything like the degree possible when driving a 14" x y 2 " or heavier casing, stiffened by a 1 0 % " x y 2 " or heavier, core pipe. It is safe to "buck" such an apparatus to an extent which would distort a lighter pipe. Where rough going is anticipated, the apparatus is sometimes considerably longer than the finished length of the upper section of the pile. In this way the comparatively light lower section gets the full benefit of the rigid apparatus to a considerable distance below the ground surface. The light pipe is equipped with spacers which contact the inside of the casing and assure guidance throughout its driving. This advantage of "control" is of great importance when the "projectile" section is a light pipe, but it is of still greater importance when the projectile is a wood pile because even the best grades of wood piles are bound to have some "sweep." Nature just doesn't produce wood sticks as straight as steel pipes,

COMPOSITE PILES and, due to the limberness of a wood pile, the tip section can b e only imperfectly controlled from the butt section. Joints for Composite

Piles

The key point of any composite pile is the joint. This is true because the "heave" of the soil surrounding the upper section of a composite pile will tend to raise this upper section. If such a raise and consequent separation take place, the only usable bearing value remaining to the pile will be that which may be developed by soil friction on the heaved upper section, and the pile will b e useless. While a number of different joint designs have been used, there are two basic classes of joints, the slip-joint (Figure 8 ) and the locked-on joint (Figure 9 ) . In Figure 8, B is the lower steel pipe section which, it is assumed, has shown no tendency to rise with the heave of the soil since it is anchored by firm soil at depth. A is the upper light-shelled section which tends to move up with the heave caused by driving adjacent piles. (Heave of a pile is caused by upward movement of displaced soil, which is a cumulative action greatest at the top and zero at the bottom of a pile.) A rope grommet C is attached to the pipe near its upper end. Cemented to the inside of the light shell of the upper section A is a rubber gasket D. T h e inside of the gasket fits tightly over the pipe. If the heave of the soil, indicated by h, is sufficiently stiong to move the top section of the pile, that section will slide up over the pipe section while a seal is maintained by the rubber gasket D which moves up with the light shell to which it is cemented. Tests of this gasket have shown that it will exclude water under a pressure upwards of 130 pounds per square inch. Other structural details may b e substituted to develop the same basic requirements. A spirally corrugated shell A is the splice shown in Figure 9. The welded to the pipe B, are shaped to troughs of the shell. T h e shell is

used in nubs E , fit in the screwed

97

down over the head of the steel pipe till it contacts and bites into the gasket D, just above the rope grommet C. Here again other structural details are frequently substituted to obtain similar results. If a slip-joint such as is shown in Figure 8 has been used, a rise of the upper section without movement of the lower section will do no harm providing that no piles are filled A B C D E H

Upper Light-Shelled Section Lower Steel Pipe Section Rope Grommet Rubber Gasket Nubs or Spacers Heave of Soil

X / X

V

u/

0 c

H

»

«H c :

H /

/

7

V •D

•c

B-

Figure 8. Steel Pipe Composite with Slip-Joint

Figure 9. Steel Pipe Composite with Locked-on Joint

with concrete within the utmost range of pile heave and separation till all piles in the area of heave have been driven, and that sufficient "lap" between the two sections of the pile has been allowed so that no separation and no leakage of soil or water can take place during the period between the driving and the concreting of the pile. One further precaution must be taken—the

98

FOUNDATION TYPES

testing of a sufficient number of lower sections to make certain that the only heave occurring is that in the upper sections. This can be checked by placing a section of pipe of 1-inch diameter or more through a number of the upper sections and extending down to the tip of the pile or to the top of the lower section, and taking level readings on the top of these telltale pipes till all driving has been completed within the indicated radius of heave, say to a distance of 10 or 12 feet or more from the piles in which the telltales have been placed. This check for lower heave should be carried out on a number of piles at the start of the placing of composite piles on any job, but if no heave is found on the first dozen or so checked, it would seem that only an occasional pile need be checked throughout the remainder of the job. As a matter of fact this movement of the lower section occurs only rarely and is generally confined to cases where the upper section of the composite pile is very short in comparison to the lower section. Pipe Pile Composite

Specifications

Because, as has been said, the critical point in all composite piles, regardless of type or maker, is the splice joining the two sections, this matter should be covered in the design engineer's specifications. Below is given a sample which should meet all contingencies: The pile shall consist of an upper and a lower section and a splice joining these two sections. The upper section shall consist of a light metal shell, corrugated or plain, but of such thickness and form as to resist external pressure without distortion causing a loss of cross-sectional area at any point in excess of 10 per cent of the area within the shell, as measured before driving. The shell shall be filled with controlled concrete having a twenty-eight day test strength of — (or of such twenty-eight day strength as will carry the full working load of — tons, without exceeding 3 3 % per cent of this test strength of the concrete). The lower section shall consist of: A concrete-

filled — Grade B Petroleum Institute or equal — steel pipe of such diameter and wall thickness as together with the concrete filling will develop the full design working load of — tons per pile without exceeding a stress of 9,000 pounds per square inch on the steel of the pipe wall, plus 3 3 % per cent of the twenty-eight day test strength of the concrete filling. The splice shall be composed of such materials and shall be so constructed as to resist separation of the upper and lower sections under a vertical pull test of 10 tons made after the concrete has reached its twenty-eight day strength (or equivalent lesser time as shown by test concrete specimens if High Early strength concrete is used for the tests), and to resist a bending load at the splice equal to 50 per cent of the bending strength of the upper or lower section of the pile immediately above or below the splice whichever is the lesser. SPECIAL

CLAUSES

If the pile is required to take a tension load, then separation tests should be required to show at least 150 per cent of the working tension load. If the joint is of the "fixed" type, that is, if the splice is not so designed as to permit slippage between the two sections prior to concreting, then the following clause should be included in the specifications: In addition to the test demonstrating that the splice will develop a 10-ton resistance against separation of the sections after the concrete has set, it shall be demonstrated by test that the splice will develop at least a 5-ton resistance to separation of the two sections before the joint has been concreted. If the joint is of the "slip" type, the following two clauses should be included in the specifications: During the driving of the first 50 piles telltale pipes of 1-inch diameter shall be placed in at least 10 per cent of the piles (to be selected by the engineer or architect in charge of the work). These pipes shall be in contact with the bottom of the lower section, where this section consists of a tube, or at the top of the lower section, when

COMPOSITE PILES the section is of solid form. Levels shall be taken at the tops of these telltale pipes and shall be referenced to points beyond the possible range of any soil disturbance developing from the driving of the piles or from other causes. Levels shall also be taken at the tops of the upper sections of these check piles. Both sets of levels, that is, to tops of upper section and tops or bottoms of lower section, shall be rerun at least once every twenty-four hours till further pile driving at increasing distances causes no further increases in the readings. These readings are made to guard against two dangers. The first is the possibility that the original bearing value of the pile, as indicated by the resistance to movement under the final blows of the hammer, may have been injured or destroyed by the heaving of the pile. This condition will be indicated by the upward movement of the bottom of the pile as indicated by a rise of the top of the telltale pipe. Where there is evidence that the point of any pile has been raised above the initial elevation to which it was driven, the pile shall be redriven till the original resistance has been reestablished. The heave at the point may not indicate any loss of bearing of the lower section because the heave of the lower section is not infrequently the result of pack up of good bearing soil below the point of the lower section. On the other hand, serious loss of bearing may have resulted from external friction on the lower section having pulled that section up out of the socket in the bottom bearing soil in which it developed much or all of its load bearing capacity. The retapping of the pile will clearly indicate from which of the two possible causes—pack up below the point or frictional uplift on the shaft—the heave has resulted. Note. In some specifications the requirement is that all heaved piles must be redriven to the elevation to which they originally penetrated. Where the heave occurs as a result of the tightening-up effect of the driving of adjacent piles having so highly compacted the

99

material immediately below the point of a driven pile as to force it up, it may be almost or quite impossible to drive the heaved pile back to its original position. All that is needed, and this should be required, is that the original pile resistance and therefore bearing value shall be reestablished. The second possibility against which a guard must be set up is that while the bottom or bearing section of a pile may not have been moved, the upper section may have been raised independently of the lower section. This condition is particularly likely to occur where a "slip" rather than a "fixed" type joint has been used between the two sections. It is evident that if, due to driving of adjacent piles, this separation should occur during the period of time intervening between the completion of the driving and concreting of a given pile and the final setting up of the concrete in that pile, on which setting up the tension value of the joint is based, then the two sections of the pile will certainly be pulled apart and the continuity of the pile shaft will be destroyed. The only sure way to avoid this risk is to prohibit the pouring of concrete in any pile till all pile driving within a range which causes heave measured at the top of the upper section has been completed. To accomplish this purpose there should be a clause in the specifications as follows: After sufficient pile top levels have been checked to indicate the area of top heave to the satisfaction of the engineer, he shall establish a "radius of heave." No pile shall be filled with concrete till all piles within this radius have been driven. If this cannot be done, then pile driving shall be stopped within this area of heave till the concrete in the poured piles has reached a sufficient set to develop the required test strength of the splice. This condition shall be proved by the breaking of concrete samples buried near the concreted piles at the time of pouring. The practice of taking twenty-four-hour check levels shall be continued even after the concrete in the splice has set to assure that the entire pile has not been lifted, and

100

FOUNDATION TYPES

shall not be discontinued till all driving within the radius of top heave has been completed.

installation of piling for any load capacity on any site.

M E T H O D O F PLACING

The use of any formula as a basis of pile loading is a matter of doubtful wisdom and certainly is inadvisable unless numerous other structures in the same neighborhood, founded on piles driven through similar soils and to penetrations based on the formula to be used, are standing up satisfactorily. Where the magnitude or importance of the structure will warrant the expense, pile-load tests should always be made. The opinion of the writer, as repeatedly stated, regarding the use of any pile-driving formula is "don't." Where the expense of standard load tests is too far out of line with the cost, size, and importance of the proposed structure, it would be worth while to check on the cost of a "quick test" to 150 per cent only of the working load, which may prove to be within the money. Where the working load per pile does not exceed say 45 tons if a crane of 60 tons weight is available at the site, a test may be made for a surprisingly small amount, using the weight of the crane as the principal load. Such a test may also be made cheaply by using the pile-driving plant for weight. Certainly the cost of a bad foundation is about the most extravagant expenditure that can be made on any job.

The piles shall be placed either by hammer driving or by a combination of driving with spudding or jetting. [See Chapter 26 for specifications covering jetting and spudding.] The hammer shall be of either the single- or double-acting steam type or any other type in which the length of stroke and freedom of drop are automatically controlled and the speed of striking is not less than 30 blows per minute. The minimum impact per blow shall be not less than 15,000 foot pounds and the weight of the falling part not less than 75 per cent of the weight of the pile or apparatus to be driven. If the working load per pile does not exceed 30 tons, the bearing value of the pile at the finish of driving may be deduced either by application 2 WH of the Engineering News Formula, L = ^ ^ (in which L is the working load in pounds, W, the weight of the falling part in pounds, H, the height of the falling part in feet, S, the set under the final blows in inches); or by the results of not less than two load tests performed and judged as given below. Note. A set of core borings—a minimum of five on any site and not less than one for each 2,500 square feet of built-over area— should, in any case, be a prerequisite to the

14 PRECAST CONCRETE PILES

P R E C A S T concrete piles have been used for more than forty years and in practically every country in which modem building operations have been carried out. As one travels round the world one sees precast piles in almost every little entry port, even where the jungle and the wilderness crowd down to the water's edge. Until fairly recently the precast pile was in active competition in nearly all fields where any type of bearing pile was required, but cost and certain engineering considerations are now largely limiting precast piles to their rightful field, that is, to the class of work where the piles must extend through water or air for a considerable distance as freestanding columns. Such structures as trestles, piers, viaducts, and wharves are frequently supported on precast piles. In most cases where piles are to be driven on dry land a less costly and more permanent structure will be possible, if some form of cast-in-place concrete pile, H-beam pile, or wood pile—when the loads permit—is used. Large diameter precast piles using prestressed reinforcing have proved satisfactory both in stability and cost under certain conditions. Where precast piles might formerly have been used above dry land to form the supports of the deck of a trestle or the like, it is now frequent practice to drive cast-in-place piles cut off at or below the ground line and capped. The structure is then completed by installing cast-in-place bents and cast-in-place deck. While the decking is most frequently castin-place, it is occasionally built up of precast

members, which may or may not b e formed of prestressed reinforced concrete. This change from precast piles with a castin-place deck to cast-in-place piles with a precast deck is perhaps more reasonable than would at first appear. It eliminates precast members where they would have to suffer under the violence of the pile-driving operations, while using precast members of fairly massive cross section at points where their use will eliminate very expensive form work. The choice and design of precast, prestressed trestle or bridge superstructures is outside the scope of this book. But it may b e in line to suggest the considerations which would indicate under what conditions a full study might be economically worth making. A number of factors enter into the comparative costs of precast and cast-in-place decking for bridges or trestles. These include the height of the bridge, the availability of room for casting beds, and the possibility of the use of cheaper labor rates in casting yards at some distance from the site of the work. The problem is too complicated to permit of any generalization as to the economic advantage of either type of decking and can usually be solved only by making and pricing fairly complete alternate designs. Precast concrete railings and balustrades were used on some of the earlier concrete bridges, and in many cases they disintegrated in a comparatively short time. Method

of Making and

Driving

Precast concrete piles have been built by many methods and in dozens of shapes and

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FOUNDATION TYPES

forms. Many of these were patented at the time of their introduction but few types are now controlled by patent. The following items form a skeleton specification for the making and driving of precast piles. They are not offered as being the correct specification. They could and should be widely varied to meet special requirements of use. The items noted cover the salient points but not by any means in the only way in which they could be covered. When used by a contractor reasonably experienced in reinforced concrete construction and handled and driven by a pile driving operator with some experience and with the proper equipment, specifications written along the lines indicated would produce a good job of precast concrete piling. MATERIALS O F CONSTRUCTION

The concrete shall be controlled concrete showing test strength at twenty-eight days of not less than 4,000 pounds per square inch. The reinforcing steel shall be Grade — having a yield point of not less than —. The ratio 'n' of steel in the longitudinal bars to the cross section of the concrete for the above stresses shall be —. Note. The blanks should be filled in to meet the requirements for reinforced concrete as set up in the code or standard specifications governing at the site of the work. S H A P E AND D I A M E T E R

Piles shall be cylindrical or tapered. The cross section shall be square with chamfered comers or octagonal or circular. The area of any section shall be equal to that of a circle of 13-inch diameter for piles of a length of 30 feet or less and increase one inch in diameter or equivalent in area, when of other than circular cross section, for each 5 feet of added length up to a maximum length of 55 ft. or 60 feet. Note. Precast concrete piles have been used in lengths well over 100 feet, but specialized designs to meet site, handling, and driving conditions should be used beyond 60 feet.

Piles shall be marked at two points or more by a band of paint to indicate the points of attachment of the handling slings. These points shall be those which theoretically produce uniform stress, as nearly as feasible, throughout the length of the pile. Regardless of the form or construction of the pile, at least five specimens from the earliest batch cast shall be tested for hair cracking when lifted by slings attached as above. If cracking is shown, a change may be made in the number of sling points or in the reinforcement on the next piles tested. If the piles will be exposed to alternate wetting and drying, the concrete mix or the reinforcing or the slinging or all three should be made to develop greater resistance to bending and so to reduce handling cracks. Note. If the job warrants the cost, one or more piles should be driven to the required refusal for the work, the pile or piles pulled, the tests for cracking repeated, and changes in pile design made if necessary. HAIR

CRACKING

Hair cracking due to handling and driving is presumably the cause of a large percentage of the destructive deterioration of piles exposed in water. The more corrosive nature of salt water will probably cause more rapid failure than occurs in fresh water, but the sequence of events is the same and is as follows. A limited amount of water enters the hair cracks and on reaching the reinforcing causes the formation of rust. The rust, being about 2y 2 times the volume of the steel which it replaces, causes an internal expansion which opens the cracks slightly. Additional water then enters, causing still further rust and expansion. After the cycle has been repeated a sufficient number of times, the concrete covering the reinforcing spalls off, leaving the reinforcing fully exposed and subject to still more rapid rusting till in many cases the bearing value of the pile is completely destroyed. This spalling action occurs slowly, if at all, in piles completely buried in soils.

PRECAST CONCRETE PILES Hair cracking is not unusual but almost universal in precast concrete piles. Some time ago the Harbor Commission of Los Angeles invited a number of contractors who made and drove precast piles (the writer's company among them) to design, cast, and deliver one or more precast piles made to their own designs and by their own methods. These would be tested by the Commission with a view to use in future work. The findings of the investigating committee were published. They stated that not a single pile when tested before driving was free from hair cracking, which was presumably caused by the handling. Conditions were aggravated by the subsequent driving and pulling. At another time the writer was asked to supervise tests on a precast pile type under consideration by a large firm in Pittsburgh. This pile was basically the same as the electric power and light poles made by the same company. The idea was to reverse the pole and drive it as a pile. In the light of experiments elsewhere it seemed almost certain that the reversed poles would show hair cracking after if not before driving, but to my surprise the day after the pulling of the first of these piles I received a wire saying that the pile showed no cracks under examination. The answer to the apparently contradictory test results was quite evident after I reached the site and observed the method which had been followed. The piles had been driven in a soil of high frictional value where the superintendent in charge of the work had rightly anticipated a rather heavy pulling job. This type of pole or pile is made by a spinning process which creates an extremely dense and strong concrete. The finished pole has a hole several inches in diameter through its longitudinal axis. The superintendent had had a 2-inch bolt set through this central hole. The bolt carried a nut and a large washer on its lower end and a pulling eye on the upper end. When pulling gear was attached to this

103

eye and operated to withdraw the pile, the shaft of the pile was subjected to a heavy compressive reaction which closed the circumferential cracks almost completely. When the pulling process was changed by pulling on a carefully equalized choker at the top of the pile, the shaft was put in tension instead of compression and complete circumferential cracks appeared at about 2-foot intervals throughout the full length of the pile. The manufacturer abandoned the idea of reversing his poles to change them into piles. As previously suggested, it would seem advantageous in all cases where the piles would be freestanding through water to include a requirement for the driving, pulling, and testing for hair cracks of at least one or two piles. However, we have never seen this requirement included in any standard specification for precast piles. The test for hair cracks consists merely of cleaning the pile and washing or immersing it in a tinted fluid—any water solvent dye will do. The tinted fluid will penetrate the fine cracks, and since the fluid at these points will be in greater depth than on the uncracked surfaces, the cracks will show up like pencil marks. The test water should be only slightly tinted. A strong dye would act like paint to obscure the entire surface. JETTING

Some or all of the following points should be included in a precast pile specification: The piles shall be placed by jetting as nearly as possible to their final position. Unless the piles come to an end-bearing on an impenetrable stratum the final 12 inches shall be driven under the hammer after the jets have been cut off.

The most effective method of jetting precast piles is by means of jet pipes cast in the pile. The specification may read: A 2-inch pipe shall be cast in the pile along its central axis. The lower end of the cast-in-jet pipe shall carry a threaded sleeve into which nozzles

104

FOUNDATION TYPES

of varying sizes can be screwed so that experiments may be run on various different discharges and pressures to determine the most efficient nozzle size and pump capacity. Change of nozzles may also be important to meet changes in the soils to be penetrated. The same advantages can be developed where independent jet pipes are used. As a generalization the less the volume of the jet water and the greater the pressure (within the limits necessary, to erode the material ahead of the pile point) the better. Great volumes of water tend to muck up the site and add to the cost by delaying the progress. Also the greater the volume of water, the greater will be the chance that previously driven adjacent piles may be robbed of their support. It has been found that in sandy soils piles may be jetted when located on centers as close as 2 feet 6 inches, without disturbance to adjacent piles. To accomplish this, however, high pressure and comparatively small volumes of water should be used. The combination of compressed air with the jet water is sometimes effective. The best and cheapest type of jet nozzle which the writer's company has used is made by taking short sections (three inches) of ordinary cold rolled shafting, of a diameter slightly greater than the inside diameter of the sleeve at the end of the jet pipe, boring holes of various diameters through the centers of these shaft sections, and threading the outsides of the shaft sections so that the jet nozzle may be screwed into the bottom sleeve on the jet pipe. With four or five different nozzle diameters it is very simple to arrive at the best pressure and volume for any given soil condition simply by changing nozzles. One of the most important features, often a difficult one to attain, is the maintaining of a smooth nozzle and a consequent smooth discharge flow. Since the nozzle may be bounced and battered on boulders or other obstructions, the ordinary nozzle soon loses its shape. The suggested solid section of shaft-

ing will stand almost any punishment without injury to the lip of the nozzle. There are two soil conditions under which jetting should rarely if ever be used. The first is in the presence of a clay soil. It is all but impossible to jet a hole in clay which will be much bigger than the jet pipe itself, and such a small-diameter hole will rarely serve much purpose. Of course by using thousands of gallons per minute and perhaps a 1,000pound pressure, it is possible to tear out great hunks of even the stiffest clay, as is done in placer mining, but the resultant hole would be unfit for the placing of a pile. The second soil in which jetting should rarely if ever be attempted is coarse gravel or even sand with any large percentage of its volume consisting of boulders with diameters of 2 or 3 inches or more. The writer attempted at one time to place piles through a soil made up of sand and about 25 per cent by volume of small boulders up to 3 or 4 inches in diameter. The pumps delivered a good volume at good pressure, but the weird result was that after being driven about 6 feet into the ground without jetting, the pile, acting under the influence of hammer and jet combined, actually drove out of the ground instead of into it. The answer was simple enough. The pile, which had little or no side friction, bounced at every hammer blow, the fine sand was washed out of the soil by the jetting, and with each bounce the small boulders rolled or were washed into the space below the point of the pile. The final answer was the abandonment of piling in favor of a mat foundation. If jetting is to be employed in the setting of battered piles, it is essential that the jet either be cast in the pile or otherwise fastened securely to the pile so that it will operate directly ahead of it. Here again high pressure and low volume should be used if possible. Also, if the pile can be driven to some depth without turning on the jets, it

PRECAST CONCRETE PILES will lessen the tendency of the jet water to boil up vertically, which makes it difficult to hold the pile to its true location and batter. FURTHER

PRECAUTIONS

Let us return to possible precautionary requirements. To avoid shattering the pile tops, it is essential that the top of the pile on which the hammer will strike be truly at right angles to the major axis of the pile. It may be possible to compensate for % inch or so of eccentricity between the striking plate and the pile head by inserting a sheet of tough plastic or plywood on the head of the pile. But the better "cure" is to avoid the prime cause of the trouble by a rigid requirement for accuracy in the right-angled relationship of the pile head to the long axis of the pile and for inspection before driving to make sure the specified tolerance has not been exceeded. The thin pad of tough plastic does seem to lessen pile head fracture even where the axial relationship is not correct. It may at times be necessary to sacrifice driving power by the use of heavy cushion blocks in steel containers in order to decrease shatter. Water-cement ratio is even more important in the making of precast piles than in most other classes of concrete structures; 2-inch slump or less is desirable. The minimum concrete cover over the reinforcement should be no less than 2 inches, and where exposed to salt water 3 inches. Longitudinal reinforcing steel should have an area of 2 to 4 per cent of the gross cross section of the pile. The area of the spiral steel should be about .005 per cent of the cross-sectional area and the spacing or pitch equal to the outside diameter of the pile. It is generally required that the spacing of the ties or the pitch of the spiral be reduced to about one third of that specified for the balance of the pile shaft at each end for a distance of 2 to 3 pile diameters. A ruling sometimes found in specifications

105

is: "The concrete cover over the steel shall be i y 2 to 3 inches depending on the degree and nature of the exposure. The maximum cover should be used where piles are in salt water at high tide and subject to drying out at low tide." Frost may also be a serious factor, particularly after slight cracking has occurred from other causes. The longitudinal reinforcement of a precast pile should be dictated almost entirely, by the anticipated handling stresses. Where the piles are to be driven rather than jetted into place, the circumferential reinforcing is the prime factor in their resistance to shatter. This brings the designer up against the same troublesome factor which is the roadblock in so many foundation problems—the lack of any mathematically provable relationship between pressure and impact. The usual assumption that foot pounds of energy can offer a measure of force which can be transformed into static pressure may be a convenient tool on occasion, but unfortunately it is unprovable, and if pushed beyond a very limited field it will lead to absurd conclusions. In several codes and specifications there will be found a stipulation that "the length of a precast pile shall not exceed 35 (or less) times the average over-all diameter of the pile." Another clause found in some specifications says: "The joint committee or American Concrete Institute formulas for long spirally and vertically reinforced concrete columns may be used for designing the free standing section of a precast pile." This would seem to be a very misleading statement because these formulas do not even touch the real design problems governing the construction of a precast pile, which are those of handling, driving, and the concrete cover needed to protect the reinforcing against corrosion. The theoretical stresses resulting from handling can be determined with a fair de-

106

FOUNDATION TYPES

gree of accuracy. But anyone who has Note. Many important docks on the New watched even a first-class crane operator York waterfront and elsewhere have been pick up heavy loads knows that the stresses built on treated wood piles or on plain wood which may occur in, say, a 50-foot precast piles cut off at elevations at which they were pile while being picked up from the casting expected to remain permanently wet. This bed and handled into the leads of a pile piling carries a heavy reinforced concrete driver may be far greater than the maximum deck which could be expected to last for a stresses which would occur to a column of hundred years. Unfortunately, there are two the same dimension which had been built serious threats to the permanence of the wood into a structure. piles and therefore of the dock. First, the life of If there is uncertainty about the disruptive the section of a creosoted wood pile above the forces engendered by the handling of a pile, permanent mud line is an unknown quantity, it is obvious that those set up by driving with particularly where future infestment by maa heavy steam or drop hammer are at present rine borers is an unknown quantity. Second, beyond the reach of engineering analysis. Pos- fire in the section of the pile which may dry out sibly the studies of the wave action in the travel and which lies below the concrete deck may of impact impulses through a long pile or other destroy the dock. This has already resulted in unit struck by a hammer may lead to a usable millions of dollars of damage in New York harpile-driving formula and a more logical basis bor alone. of structural design of the driven unit. Plain wood piles are subject to attack and The greatest obstacle to the development quick destruction at any time that marine of accurate empirical formulas to cover the borers move in, as they have done in many design of precast piles lies in the nature of the Atlantic ports as far north as Boston and beinjury to be prevented. The danger is not yond. Plain wood piles are also a fire hazard. that of immediate collapse or even obvious Precast concrete piles crack and ultimately rapid deterioration. Cracked piles may carry disintegrate. Wood piles burn or are detheir loads for fifteen or twenty years or more stroyed by marine borer attack. Two other before they reach the point of failure. Full- solutions have been tried with moderate sucscale loading and driving of test specimens, cess; one is the use of creosote impregnation followed by careful checking for cracks which of precast concrete piles. This type of pile is might lead to future corrosion, would give effective against the infiltration of water some indication of the probable course of through hair cracks and the resultant rusting failures, but the testing would be expensive of the reinforcing. In order to get any depth and at best could only approximate the effects of impregnation the concrete of the pile must of long periods of exposure. be porous, and a really strong porous concrete is almost a contradiction in terms. Uniformity in the creosoted "jacket" is a must since failure Other Problems in the Precast Pile Field needs only to be at one point to destroy the Where a comparatively short-lived struc- integrity of the pile. Piles have been made ture is required, precast piles may be designed with a core section of dense strong concrete to give a reasonable probability of a satis- and a creosoted jacket of porous and therefore factory answer, but in harbor developments— weaker concrete. This method is expensive. piers, docks, and the like—which may be in use Jacketing with cast-in-place reinforced confor fifty or a hundred years or more, some crete will do the job, but the jacket must assurance of greater permanence would cer- extend well below any possible scour line, tainly be desirable. and this is expensive and difficult to assure.

PRECAST CONCRETE PILES As a method of reinforcement and rehabilitation of weakened piles, this jacketing serves a purpose and has been used to a very considerable extent in many different forms. As a method to be employed for all piles of a new structure, it would be very expensive. There would seem to be only one simple, cheap answer to this problem of permanent,

107

fireproof, insectproof, rustproof pile support for piers and docks. This would be by the use of composite piles where neither creosoted wood nor precast concrete would enter into the structure at any point. Such piles have already been developed and used, though only in a minor way for dock purposes.

15 MEDIUM-WALL STEEL PIPE PILES

T H E medium-wall steel pipe pile is a halfway stop between the light-shelled pile and the standard closed-end steel pipe pile. The two most widely used members of this group are the Union Metal Monotube Pile and the 3/IQ-inch wall steel pipe pile, the shells for which are made by at least four of the steel companies. These piles are usually 10 to 12% inches in outside diameter. Union Metal

Monotube

Pile

The Union Metal Monotube Pile, first used in 1928, was tested about two years earlier, in an experimental way only, by two or three pile-driving companies. The writer's company was one of these, and its first tests resulted in some wrong conclusions which caused considerable loss before the true facts were understood. The piles were then modified and applied under the conditions which fitted their use. The leading characteristic of the mediumwall steel pipe pile group is the carrying of a part of the working load on a permanent casing made of a special high-grade steel or a steel specially processed to give it added toughness. On occasion the shell may be formed or supported so as to permit of driving, even though the casing has been reduced to one half or even to one third the thickness formerly considered the minimum required for a closed-end steel pipe pile. The Union Metal Monotube Pile was the pioneer in the group. The original pile was a fully tapered one, and this is still frequently true. However, most Monotubes installed to-

day are of the two-section type, which is a combination of tapered section and nominally constant diameter extension section (see Figure 10). The writer's company had been frequently asked by the makers (the Union Metal Manufacturing Company of Canton, Ohio) to carry out a thoroughgoing test on their pile. When it was learned that the pile they wanted tested

O

Figure 10. Union Metal TwoSection Monotube Pile This fluted, tubular, steel pile is driven directly without internal mandrel, then filled with concrete. Standard Monotubes have an 8-inch diameter tip; butt diameters of 12, 14,16, and 18 inches; and are manufactured with casing thicknesses of 3, 5, 7, 9, and 11 gauge steel. consisted of an 11-gauge concrete-filled fluted steel shell which had been originally designed and extensively used for power or light poles, and that they proposed to drive it without any supporting mandrel, under a No. 1 Vulcan hammer, we refused—as we figured it—to waste the time on tests, even though they were to supply the test piles, delivered on any job on which we might be op-

MEDIUM-WALL STEEL PIPE PILES

109

erating, without cost to us. However, they type and gauge of shells we had used on would not let the matter rest and kept in- the Cincinnati test, but struck one very consisting that even if only as a matter of courtesy tradictory result. we should spend the few hours necessary for For the making of the depth tests, on the a test. basis of which the bulk of the piling for the Nothing came of it for a time, but later we job would be ordered, the maker had 9-gauge ran into an unusual and annoying condition poles immediately available, and these were on a piling job we were doing for the Cin- used on the depth test work. Since the weight cinnati Union Terminal. The terminal site, and therefore the inertia of the 9-gauge test including the assembling yard, covered a large piles would be somewhat greater than that acreage, and in the process of leveling out of the 11-gauge which we proposed to use, for the yard, a 10 to 20 foot fill had been and since we still had some faith remaining placed over a considerable section of the site. —since entirely lost—in the numerous pileMany buildings and streets had formerly been driving formulas in which inertia of the on the site, and these were supposed to have driven pile was considered as a prime factor been thoroughly removed wherever new struc- in the increase of resistance to driving, we tures were to be located, before the placing of overdrove the depth test piles slightly to the fill. When we ran into what appeared to compensate for the difference in inertias. be unusually tough soil conditions and sunk When it came to the driving of the job a pit to investigate, we found that a well- piles which had been ordered at the lengths paved city street had been left undisturbed indicated by the test piles, we found them under the fill. to be without exception too long by 10 or 12 The writer had become a little weary of feet, even though we drove them to a greater the insistence of friends of the Union Metal resistance than had been used when driving Company that their "light pole pile" would the test piles. The pile formulas seemed to be drive through anything anywhere, so we de- working in reverse. Where the lighter job cided that the time had come to humor them piles should have gone substantially deeper and wired for half a dozen of their wonder than the heavier test piles, they actually piles. The piles came and the Union Company reached refusal at a much shallower depth. engineers with them. We got set to make a We were beginning to learn a fact which few humorous remarks, but the fun didn't we have since seen demonstrated on other even get started, for we proceeded to slam sites and with other types of soil and different half a dozen %-inch wall 8-inch tip-tapered types of piles—the rigidity of a pile against fluted metal piles down through a pavement bending is, under many soil conditions, a which had come close to stopping %-inch matter of far greater importance then its wall steel pipes! inertia. For example, on another occasion in We did a lot of figuring, and thanks princi- Washington, under soil conditions totally dispally to the help of the Union Metal Com- similar to those at Cleveland, steel piles, pany engineers we came up with some of the which apparently could not be made to peneanswers—and they still hold good, though trate through a very tough soil stratum when they don't paint quite so rosy a picture as we empty, passed through the stratum readily after being prefilled with concrete. The inthen supposed. Naturally on the strength of the Cincinnati ertia was greatly increased, yet the same tests, we had no hesitation in biting off a hammer which failed to drive the empty tube large piling job for an incinerator in Cleve- proved amply powerful to drive the full tube. land. We finished the job all right on the same We concluded that the surprising results

110

FOUNDATION TYPES

of the Cincinnati tests, while explained in part by the extra toughness of the metal in Union Metal tubes resulting from the cold working of the metal, were probably due to a far greater extent to the column strength imparted to the pile by the fluting. This has been borne out by the successful driving of hundreds of jobs using these fluted piles and of a thickness as low as 11 gauge. We know of no comparable record on unfluted pipes. Our next experience with the Union Metal pile was at Binghamton, New York, where we undertook a piling job on a basis of 11gauge Union Metal piles. The per pile load was not high and the driving was not carried to excess, yet to everyone's surprise, when the first pier of piles was examined before concreting, it was found that more than half the tubes were closed up like an accordion for at least several feet from the top. After the successful driving of several jobs of similar piles to much higher penetrations without the loss of a pile, this reversal of form seemed very puzzling. Considerable testing and soil studies followed and we came up with a fairly obvious answer. The soil on the Binghamton site is a glacial deposit containing a considerable number of medium-sized boulders (1 foot and upwards in diameter). When a pile glanced off a boulder it was deflected, though probably not enough to prevent its acceptance under normal specifications for plumb. However, the driving plate of the hammer held in rigid leads and contacting 100 per cent of the top of the tube at the start of driving would remain in its initial position after the tip of the pile hit a boulder. This caused the pile to deflect or tend to deflect from its originally plumb position. The driving plate would then be in driving contact with only a part of the head of the pile, say, for argument, 50 per cent of the pile head. The hammer blow on the pile head of a perfectly plumb pile would spread its impact over the full cross-sectional area of the metal of the tube, but the same impact on the head of the pile tube when

even slightly out of plumb would have to be resisted by only some fraction of the full cross-sectional area. Where the thickness of the pile wall was about yg of an inch, as in the cases in question, the stress in the steel of a plumb pile under a blow of a No. 1 Vulcan hammer, such as was used on the Cincinnati tests and the Cleveland and Binghamton jobs, would evidently remain below the yield point of the unusually tough steel used in these tubes. But as soon as the pile deflected from the plumb without a compensating change in the angle of the driving plate, the same hammer impact would have to be absorbed by only some fraction of the cross-sectional area of the tube, and at some point of eccentricity the stress in the steel would exceed its yield point and failure would follow. It might seem that once the first buckling had taken place the impact would, from that point on, be redistributed to develop the resistance of the full cross section and would so remain till the deflection from the plumb again increased. However, once the top of a steel pipe or tube begins to buckle under hammer blows, the load can no longer pass down through the vertical axis of the wall of the pipe but must be transmitted through and around the curve of the buckled section of the pipe, which is far less resistant to further distortion than the normal vertical section. The crumpled section continues to spread under impact or pressure further and further down the pipe. This reaction has been observed innumerable times in the driving of steel pipe piles, where the wall thickness is insufficient or the quality of the pipe is low, say Grade A lapweld pipe instead of the Grade B tubing or electric weld pipe, which should invariably be used. The solution to a situation such as that at Binghamton is obvious. It simply requires the use of a heavier gauge tube, or the use of an internal mandrel to stiffen the tube and prevent the initial buckling. The conclusion which a designing engineer

MEDIUM-WALL STEEL PIPE PILES should reach is certainly not that all Union Metal or thin-walled pipe piles should be of certain specified gauges that could not be buckled under any driving conditions, but rather that where the use of such piles seems desirable but might encounter major obstructions, an ample set of borings should be made available. From these borings the makers of these types and the engineer himself should be able to approximate the gauge which might be required. The maker will rarely guarantee the sufficiency of any assumed gauge, but the maker's opinion together with the engineer's should give grounds for a decision as to whether the type should or should not prove competitive, and therefore whether it belonged in the final specification. If so, the minimum gauge required should be stated. If no minimum gauge is stated, there is a strong possibility that some pile-driving outfit entirely unacquainted with these piles may take the work on the basis of a wall thickness which cannot possibly meet the conditions shown by the soil studies. No matter how financially sound such a contractor may be, the job will probably suffer a considerable and expensive delay. It is not suggested that the owner should accept responsibility for the need of heavier wall tubes than the minimum stated, in case such heavier wall should prove necessary. That gamble should be left strictly up to the bidders and the specifications should say so. The Union Metal Monotube Pile has been used in a wide variety of soils and in the supporting of many types of structures. One of its advantages stems from the simplicity and flexibility of the plant required to drive it. Where the choice of gauge has been correctly made, there is little more reason to anticipate difficulties than there would be in the placing of wood piles. Concrete-Filled.

Pipe

The second most widely used pile in this thin-walled pipe group is the concrete-filled pipe, usually using wall thicknesses of .188

111

to .25 inches. Pipes for this pile are supplied by a number of mills. To date, however, the Armco Spiral Butt-Welded Foundation Pipe (see Figure 11) has seen the widest use.

Figure 11. Armco Spiral Butt-Welded Foundation Pipe This pipe pile is driven using equipment similar to that used to drive a wood pile or Union Metal Monotube Pile. The pipe, whose wall thickness is generally between .141 and .250 of an inch, is filled with concrete after the driving operation. The remarks regarding necessary wall thicknesses, offered in connection with the Union Metal pile, are in considerable degree applicable to the thin-wall pipe piles. The mere fact that some certain make of pipe of a certain wall thickness has been successfully installed under a given set of conditions is by no means proof that the same pipe will be equally successful everywhere it may be tried. Where conditions were right, we have successfully driven .188 wall pipe using No. 0 Vulcan hammers driving in excess of 6 blows per inch, but we have also known such pipes to fail. We have had them fail under driving by lighter hammers to less resistance; .25 inch wall pipe has generally been the solution, but we have gone to .279 wall on occasion. As in the case of the Union Metal pile, the biggest hazard occurs where boulders will be encountered. This situation is aggravated where high per pile working loads, 60 tons and upwards, are required because of the

112

FOUNDATION TYPES

excess driving needed to meet the 120-ton test loads usually specified. The concrete-filled pipe pile has only come into wide use comparatively recently. Its first use on a large job, so far as the writer knows, was for the foundations of the Onondaga County War Memorial in Syracuse, New York, under which 752 piles of an average length of 48 feet for a per pile working load of 40 tons were used. The piles consisted of 10%"* concrete-filled pipes. They were driven with a No. 1 Vulcan hammer. The soil was clean and no serious difficulties developed. The thin-walled pipe, though considerably heavier than the tubes customarily used in Union Metal piles, seems at present to be competitive in price. Where lengths and loads favor their use, piles of this type are often competitive with light-shelled or compressed concrete piles. Here, as in the case of the Union Metal pile, ample borings and some judgment is called for in selecting a minimum wall thickness. An important feature in the choice among piles of the light-walled tube types is that of shape. The pipe piles are of necessity cylindrical. The Union Metal pile uses various different tapers and combinations of tapers. The upper sections above a tapered lower section are sometimes cylindrical in form. Choice between cylindrical and tapered shapes and between the various tapers available should be made to fit the soil profile on the site. Swage Bottom and Other Pipe Piles

Thin-Wall

One member of this "medium-thin-wall bearing group" is the Swage Bottom Pile driven by the Western Foundation Corporation. Another variant of the same idea is the pile driven with an internal drop hammer or core. Neither of these has been used extensively in the United States. The basic value of these piles is that they offer a method of driving pipes having a wall

thickness as low as y8 of an inch, even in rough soils. Note. Most of the new codes permit permanent bearing value for the full thickness of a shell, tube, or pipe of %-inch wall thickness or more when used as a container for a concrete fill and left permanently in the ground. However, the driving by hammer blows delivered on the top of a tubular container of ^-inch wall thickness will cause pipe failure in many soils. While plain ^-inch wall tubes may crumple under compressive hammer blows, the same tubes will stand a very high tensile load. The idea employed in some of these pile types is to place the tube in the ground by tension or by a combination of tension applied at the bottom of the tube and compression applied at the top. The Western Foundation Corporation lightwall steel pipe pile, used under the trade name Swage Bottom Pile, is made in the following manner (see Figure 12): Step 1. The tapered plug F is set on the ground, and the pipe is set over it so that it will extend down over the plug to some extent. Step 2. The core C, attached to the hammer A and carrying a heavy drive plate B which is welded to the core C, is lowered into the pipe and the hammer is set in operation. Step 3. At first the hammer blows will affect the pipe D only, driving and swaging the pipe. As soon as the point has been driven up into the pipe D till contact is made between the bottom of the core E and the plug F, the driving force will be taken largely by the plug, and the pipe will be driven and pulled into the ground to the desired depth. The pipe then will be filled with concrete. The bending forces caused by obstruction in the soil will be met by the heavy-walled closefitting steel core. The core is of such a length that it will contact the top of the long-tapered precast

MEDIUM-WALL STEEL PIPE PILES concrete plug only after the pipe has been driven over the tapered plug to such an extent that the bottom 6 or 8 inches or more of the pipe has been "swaged" or belled out to the maximum degree possible without causing splitting of the pipe.

A

Hammer

D

B

Drive Plate

E

Pipe Core Bottom

C

Core

F

Tapered Plug

Figure 12. Swage Bottom Pile

The bottom of the pipe which is in swagedout high-pressure contact with the coned upper section of the concrete plug develops a very strong friction bond between the plug and the pipe, so that when a hammer blow is struck causing the point to descend it will drag the pipe down with it. However, if the power of the hammer blows is such that the swaged point tends to descend more rapidly than the pipe will follow it, the top of the pipe will come in contact with the drive head, and the combined tension and compression will prevent any separation of pipe and point. It might seem that the simplest way of ac-

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complishing the transmission of the driving force in the form of tension in the pipe would be to weld a plate to the bottom of the pipe, introduce a core of a length greater than the inside length of the pipe, and then drive on the plate bottom, thus dragging the pipe into the ground. This arrangement was the first tried, but unfortunately the direct blows delivered through the core and to the plate invariably tore the plate free of the light pipe or ripped the pipe above the plate. Even with comparatively thick-walled pipe, % of an inch and thicker, no welding was developed which would withstand the impact necessary to overcome the combination of the inertia of the pipe plus the outside friction on the pipe. On the other hand, the swage point, allowing a differential movement between the pipe and the plug, even though very slight, has proved satisfactory on the jobs of swage point piles driven to date. The piles of the four types just discussed place dependence on the bearing value of the tube or shell, which acts also as a container for the concrete which forms the second factor of the load-bearing capacity of the pile. The wall thicknesses of the tube or pipe commonly used in these piles will not exceed Ys to y4 of an inch. This leads to the first warning. Where there is any reason to suspect that corrosive soil or soil water may contact the tubes, a careful chemical soil analysis should be made by a dependable laboratory. Fortunately this is not an expensive requirement. There are several types of piles—not those immediately under discussion—which will give full protection even in highly corrosive soils. The second precaution involves the sampling of the steel tubes or pipes in each shipment to obtain a record of carbon content, yield point, compression, and tensile strength. There seems to be a popular idea that the manufacture of steel and steel products has

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FOUNDATION TYPES

reached such a high degree of perfection that frequent sampling is no longer needed. Recently the writer's company drove 1,750 piles of approximately %-inch wall thickness. We ran into unexpected trouble on account of the crumpling of the pipes under driving. A check was instituted, and it was found that the carbon content varied between 0.3 and 0.7 per cent, and the variation in the yield, compression, and tension values in pipes tested varied almost as widely as the carbon. The third precaution which should apply to these and all types of piles would be a careful study, as outlined elsewhere, to determine which of the possible pile shapes will best and most efficiently meet the requirements in the light of the soil profiles on the site and the per pile working load best adapted to the load concentration of the structure to be supported. As argued elsewhere, the aim of the specification writer should be to cover all properly competitive pile types, insofar as possible, by a single broad performance specification and to avoid details, particularly when these details apply to only one method or maker. Where two or more basic foundation groupings such as caissons, piles, or mat foundations are to be included in the bid requests, it would be difficult or impossible to write a single blanket specification sufficiently broad to cover the situation. The best that could be done would be to write a specification for each basic group, for example, one specification for caissons, a second for piles, and a third for mats and floats. The steps in the formulating of a blanket performance type specification to include several groups of piles are as outlined below. (A similar blanket specification might be written for other major groups, but for simplicity I limit this particular study to piles.) The engineer will have studied the loads inherent in the nature and proposed uses of the structure to be supported. Since a performance specification is his aim, the first

section in the specifications should contain a listing of the loads to be carried, covering class, tension, compression, bending, torque, and the range of their magnitudes; the use to which the structure will be put; and any special conditions such as vibration, impact, high wind, or seismic loads. No performance specification can be written and no clear picture of conditions will be obtainable by the prospective bidders unless they have these basic facts. The second group of facts needed by the engineer and the foundation contractor alike will consist of detailed reports of the soil borings and soil studies which will have been made under separate specifications and under separate contract placed by the engineer. As repeatedly stated, there is great advantage to be gained and much money to be saved on any substantial piling job if a third group of facts contains a detailed report of the results of a program of full-scale pile load tests, the contract for which has been let under separate bids and specifications. If this information has not been developed before foundation piling loads and layouts have been frozen, most of the value will be lost and the writing of a performance specification will be difficult, sometimes impossible. With all of the above required information in hand, the engineer would make a study of the most efficient working load per pile. (See Chapter 30.) If the working load is not decisively indicated, it may be necessary to make two or even three layouts based on different pile load assumptions. At this point the engineer would be ready to write the technical specifications for the foundation piling. The following items should be included: 1. Piles shall be of one of the following types [for example, creosoted wood piles, cast-in-place concrete piles, or other types], in accordance with the layouts shown in the drawings. 2. Pile load tests made in accordance with the requirements of Code [or as specified in Chapter 30 of this book] shall be carried out before

MEDIUM-WALL STEEL PIPE PILES driving of any piling. Tests will be considered satisfactory if the net settlement under 200 per cent of the working load, applied and removed as required by the above code (or specification), is less than % 0 0 of an inch per ton of test load. Note. As suggested in Chapter 30, practically all test procedures are unnecessarily expensive, and the information made available in them is less complete and less accurate than it should be. The above paragraphs are written around present standard practice. If the preliminary load test program included satisfactory tests on one or more of the permitted types, no added tests would be required. 3. The working loads per pile shall be as follows: For cased, cast-in-place concrete piles tons. For creosoted piles tons. This load capacity shall be demonstrated by the making of load tests at pile locations selected by the engineer.

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If the preliminary test program included satisfactorily tested piles of the exact type proposed by the bidder, then the number of tests required under this proposal may be reduced by the number already carried out. In calculating the load-carrying capacity of any pile, the following unit loadings shall not be exceeded: the concrete shall be controlled concrete as required and for the loads shown on the drawing No. The steel shall be structural grade or if in the form of pipe shall be Grade B or its equivalent. Steel in the form of reinforcing bars shall not be stressed in excess of 14,000 pounds per square inch. Steel in the form of pipe shall not be stressed in excess of 9,000 pounds per square inch. However, steel pipe used as permanent casing for the concrete of the pile shall be considered of no structural value unless the thickness of the wall of the pipe is % of an inch or more.

16 STRUCTURAL STEEL PILES

IN practice the structural steel pile is limited to the H-beam sections and fabricated rail sections. Occasionally fabricated sections are used as master piles in heavy cofferdam construction, but the wide flange beams are favored for this use. The most popular H-beam sections for use as bearing piles are those between 10 and 14 inches, varying in weight from 42 pounds for the 10-inch section to 117 pounds for the 14-inch. H-beams as now supplied are generally structural grade. It used to be possible to get H-beams for piling in a somewhat tougher steel. On a job recently completed, 73-pound 14inch H-beams were driven under a singleacting steam hammer, having an impact of over 30,000 foot pounds, to refusals of 50 blows and upward to the inch. The tops of a few of these piles crumpled badly, though most of them took the same punishment without any sign of failure. Upon checking it was found that all of the piles qualified as structural grade, but those that failed showed close to the minimum permissible carbon content. The bulk of the piling far exceeded minimum requirements. Where really rough going is anticipated, it would be well worth while to make an effort to get steel with a minimum carbon content well above the usual minimum structural requirement. Failure of the steel under severe driving usually occurs at a point immediately below the hammer anvil, but where the pile consists of more than one section the failure may occur at a splice, if the lower section is soft. We have driven millions of feet of H-piles and

the indications of failure below the ground surface have been few, but such failures occur. This question of quality of steel is doubly important because H-piles are so frequently chosen to penetrate soils which would defy any other type of foundation unit except those in the caisson group or the large diameter open-end steel pipe piles, both of which would be more costly than the H-beams. The structural steel pile has been steadily growing in favor and use since its introduction fifty years ago. The rate of this growth in popularity would have been much faster if it had not been for the doubt of its resistance to rust or chemical attack. For many years nearly all H-pile specifications contained a stipulation that the effective cross section of the pile must be reduced by the deduction of a n inch, sometimes even % of an inch, Vie to allow for rust loss. While the same rust factor was at that time applied to steel pipe piles, the exposed surface of the H-beam was so much greater in proportion to its weight than that of the pipe that the rust factor constituted a severe handicap to the H-pile. The steel companies and others have made through the years many tests and checks and have demonstrated, at least to their own satisfaction and to that of many, probably most, engineers, that the facts are these: 1. Steel buried in any natural soil will suffer little or no loss by rust even where the soil is alternately wet and dry. 2. Electrolysis rarely attacks a pile. I personally have never seen a pile which would seem to have suffered from such an attack.

STRUCTURAL STEEL PILES 3. In artificial fill containing chemical wastes even in ordinary cinder fill, attack does occur. Under such conditions concrete jacketing of the piles, at least through the fill, would be indicated. Considerable use has also been made of the bitumastic enamels for the same purpose. The enamels are probably permanently effective when applied hot and where proper care is taken against scratching or otherwise injuring them in handling and driving the piles. It is obvious that driving a coated pile through rough, compact fill is asking for trouble. The concrete jacket is safer but also, of course, more expensive. 4. H-beam piles should normally be considered only when bearing on rock or gravel hardpan. Many attempts have been made to develop a non-end-bearing H-pile by various

117

methods of lagging applied near the lower end of the pile. This may sometimes be effective, but where a pack-up of soil is needed to develop the required bearing, other types of piles, such as closed-end steel pipe, are more positive in their reaction and will generally prove more economical. 5. It has been claimed that the H-pile having a much greater surface in contact with the soil than any other pile of comparable weight will develop a greater friction. In general this claim is of doubtful validity because many H-beams which have been extracted show the channels of the beams packed with soil. Failure has occurred as a shearing of the soil along a line circumscribing the four points of the flanges—not along a trace following the contour of the steel section.

17 OPEN-END PIPE PILES TO ROCK

T H E closed-end steel pipe pile may be considered as a special form of the cast-in-place cased concrete pile, though there is in fact a basic difference between this type and the light-shelled pile. The light-shelled pile is perhaps more closely related to the uncased castin-place pile in that the shell is not figured as a permanent part of the structure of the pile, being merely a temporary form for the concrete—a form which instead of being withdrawn by mechanical means is presumed to be disposed of by the slower process of rusting. The steel pipe pile should be, but generally is not, figured for shaft strength, as a reinforced concrete column in which the steel of the pipe acts in two ways, that is, as both vertical and circumferential reinforcing. Under most codes and specifications the shaft strength of the pile when acting as a short column is figured by calculating the area of the steel (or the gross area of the steel less some assumed rust allowance) at an arbitrary unit value, for example, 9,000 pounds per square inch, to which there is added an allowance for the contained concrete, arrived at by multiplying its cross-sectional area by an assumed unit load value of perhaps 1.000 pounds per square inch. This might result in a reasonable if somewhat arbitrary load allowance, if the restraining value of the steel, acting as hooping, received proper consideration, but such consideration is rarely given, the concrete unit load allowance being set at or below that accepted for unreinforced concrete of the same test strength.

Very substantial savings would follow if due allowance were made for the high load capacities demonstrated in tests throughout the past forty years or more for concrete-filled cast iron and steel pipe when used as building columns. Where the designer is not hampered by controlling codes or specifications, the savings which could be obtained by using concrete-filled pipe piles on a load-by-test basis to cover the strength of the shaft as well as that of the pile in place would frequently bring this class of pile into the picture from which it is now excluded, with resultant substantial savings for the owner. The use of arbitrarily chosen values for concrete and steel without regard to their comparative areas and unit load capacities, and frequently without any requirement as to the class of steel in the pipe or load tests on the finished pile shaft, leaves a good deal to be desired and can scarcely be considered as engineering design. Turn now from the question of the strength of the concrete-filled steel pipe pile shaft in place to the special considerations which should enter into the choice and design of the open-end steel pipe pile. In point of numbers, depth reached, and general all-round safety record, the openend concrete-filled steel pipe pile occupies an enviable position among foundation types. In the matter of cost its situation is far less favorable, for under average conditions where its use would be possible, its cost will very generally be far above that of other usable

OPEN-END PIPE PILES TO ROCK foundation types of caissons or piles. Where conditions are suitable for the use of the open-end pipe pile, the H-beam pile will very frequently underbid it. The 14-inch 73-pound or larger H-sections are often accepted for loads comparable to those of the open-end steel pipe pile. Where no requirements other than a maximum working stress in the steel are made and where this stress is closely comparable to that which is allowed for the steel in the wall of the pipe, the pipe pile will rarely be competitive with the H-pile. Both the H and the open-end pipe or rail pile are excellent foundation bearing units, but as an engineered product the quality and safety factor of the pipe pile must be admitted to be greater than those of the H-beam. The openend steel pipe pile is in fact a quality product, and where used is generally chosen on that basis rather than on a competitive price. When to Consider Open-End Steel Pipe Piles The first reason then for giving consideration to the open-end steel pipe pile when setting up the bid group for a job arises from the importance or special load requirements of the superstructure rather than from soil conditions. When this factor is determined, certain soil characteristics must be studied and certain bearing requirements met. 1. There must be bedrock of fair to good quality to which the loads in any case would be taken. If bearing strata of such quality as would satisfy the building loads would be reached substantially above the rock, there would be but small possibility that the openend pipe pile would be competitive. The question of the quality of rock needed as a base for this pile type has been much discussed. The testing of diamond core samples gives very little information regarding the bearing value of the same rock in its natural bed, where only a minor fraction of its surface will be loaded. The method very gen-

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erally used to determine the sufficiency of the rock is covered by the requirement usually found in the specifications that when all material has been removed from the inside of the pipe, the pipe must completely refuse further penetration under driving with a specified type and weight of hammer. Since absolute refusal is almost impossible, a movement under a certain number of blows to some small stipulated extent may be stated as representing "total refusal." For example, the requirement may be that movement under 10 blows of a hammer developing 15,000 foot pounds of impact shall not exceed y4 of an inch when the inside of the pipe is cleaned down to the cutting edge. 2. After it is determined that rock of sufficient bearing can be reached, the water condition to be encountered should be checked. Water-bearing soil will have little effect on the feasibility or cost of this type of pile except where flow under high pressure must be expected immediately over the rock. The driving of the pipe will cut off the flow from all upper water-bearing strata. If the water immediately over the rock is at such pressure as to result in artesian flow, it will still cause but little trouble, providing the placing of concrete by tremie methods is permitted. Where the operator is skilled in the placing of concrete under water, the densest and highest strength concrete can be obtained, but a certain amount of experience and care is needed. Where the head is such that flow will continue to occur even above the cutoff level of the pipe, the placing of undisturbed concrete as a seal at the base of the pipe can always be managed in one of three ways. The simplest and frequently successful method where it can be used without interference with other work is to allow the flow to continue for a few days, during which it may silt itself off. The second is to add a length of pipe, possibly 20 or 30 feet or more—if the hydraulic head does not exceed the cutoff level by too great an amount—and

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then place the concrete seal by means of a special bottom-dumping bucket lowered to the bottom of the pipe. The last and most costly method is to fit a gasketed sealing head to the top of the pipe at ground level, and using a grout pump attached to a pipe or hose which will reach to the bottom of the pipe to be sealed, pump grout down to form the seal under any required pressure. 3. The next factor affecting the probable usability of this type is that of the depth to the bearing stratum. As previously indicated, the cost of sealing the pipe bottom and stopping possible washout of cement when concreting may be an important factor in the over-all cost of the open-end pipe pile. This operation will occur once for each pile, regardless of its length, and consequently it will have a far greater effect on the per foot cost of, say, a 30-foot pile than on one of 100 feet. Also the competitive foundation types will be more numerous and usually lower priced for the short pile than for the long one. Where superstructure demands, nature of soil, and water contingency are favorable, a depth to bearing of 60 feet may warrant the inclusion of the open-end pipe type, but a depth of 100 feet would be much more likely to do so. 4. The cost of clearing out the pipe is an important factor in the over-all cost and of course must be studied in the light of the information disclosed by the core borings. Blowing with compressed air, the method most commonly used, is very effective where the soil is preponderantly sand silt or small gravel. Jetting, coring, auger boring, and rotary wet drilling are also frequently effective. If the core borings are sufficient in number and have been carefully taken and recorded, an engineer or operator familiar with this sort of work can make an estimate as to the method and time required. But unless similar work has been done in or near the site he is studying, there will need to be qualifying clauses affecting both progress and bid price.

At one time I supervised a job of over a thousand piles in which nearly half had to be diamond-drilled and shot in order to reach bedrock through boulders. 5. The last factor to be considered is that of load magnitude and concentration. When a job requires only a small number of piles and a small total footage, the cost of installing the fastest, and therefore normally cheapest, methods of cleaning may eliminate open-end steel pipe piles, in which case the per foot cost will be high. But this factor has probably little more effect on the open-end piles than it has on other types with which it generally competes. As to load concentrations: the efficiency range might be roughly placed somewhere between the various types of heavy load caissons and the large diameter cast-in-place piles. It would possibly be right where piles of 100 tons working load would show top efficiency, but, as suggested, this would not be decisive. Other equally important factors would have to be given weight. Close spacing is generally permissible for open-end pipe piles, so the possibility of saving in the capping should be given consideration. Many jobs such as bridge piers have been and are being constructed by placing a sheeted open cofferdam through the more stable upper soils under comparatively low hydraulic head, and then driving bearing piles or small diameter caissons to rock within this coffer. If the smaller units are properly chosen to meet the requirements for their use, the combination of large diameter upper caisson and smaller diameter piles or caissons will very frequently offer an excellent engineering solution at minimum cost. To recapitulate, as a competitor in the foundation field, the open-end steel pipe pile may be worth considering where: 1. Rock bearing will be required. 2. Soil study indicates few boulders over 10-inch diameter or over % of the diameter of the pipes to be used.

OPEN-END PIPE PILES TO ROCK 3. Soil borings do not indicate tough clay in considerable depth. Note. A soil profile showing sands, small gravel, organic or inorganic silts, or "mud"— any or all of these—indicates that conditions for driving and cleaning will be favorable. The lack of such a profile is not conclusive evidence that this type should be ruled out. 4. Depth to the rock-bearing stratum will probably range between 30 and 150 feet. Soft or partially weathered rock, which would be tough to penetrate and clean out of the pipe, will not exceed, say, 10 to 20 per cent of the over-all length of the pile. 5. The distribution of the building load is such that a per pile load around 80 to 100 tons or more will show maximum economy. The comparative cost of the mats or caps to deliver the working loads to the bearing units must be taken into calculation. No one of the considerations named above (except the requirement for an ultimate sound rock-bearing stratum) could in itself and singly eliminate the open-end pipe pile. All should be weighed before making a decision. Specifications In addition to the general requirements and considerations for specifications discussed in Part Four, some or all of the following clauses should be used where applicable when openend steel pipe piles are to be included. MATERIALS OF CONSTRUCTION

The design of the pile shafts shall meet the requirements of the Code of or (where no Code governs) shall be in accordance with the requirements of Manual for short fully supported columns.

Regardless of the code or authority used, it is frequently necessary to add a requirement concerning the grade or characteristics of the pipe to be used, such as "the pipe shall meet the requirements of the American Petroleum Institute for Grade B pipe or its equal

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or such other authority as may be chosen." Sometimes a stipulation as to the carbon content and the minimum yield point will be sufficient and will assure maximum competition among the pipe makers. It is unwise to specify extremely high final driving refusal except when pipes of a quality to stand such punishment are also specified. If the pipe is too soft it may start to crumple before the required refusal has been reached. In such case a series of cutoffs may be required. Apparent refusal may in considerable degree be the result of driving on a slightly crumpled or an irregularly burned pipe top. The outside diameters of the pipes shall be inches and the wall thickness inches or as shown in the drawings.

On the other hand, a specification requiring too high a carbon and yield point may result in steel so brittle that shattering may occur. This is particularly likely when driving will proceed in really cold weather. I remember one job where Grade C pipe had been ordered. In zero weather a number of pipes shattered like glass. The work had to be shut down or the pipes preheated when the temperature approached zero. Where any standard code governs or where the specifications of any particular society or department are to control the design, paragraphs covering the concrete should be included at this point. Specifications for tremie concrete where used and the permission to use high early strength concrete may need to be added. No top plates will be required. Drive shoes (where required) shall consist of close-fitting steel rings not less than inches in thickness and extending one pipe diameter above the bottom of the pipe. They shall be fully welded to the pipe at the top of the ring. Splice welds and ring welds shall be as shown on the plans. The weld at the bottom of the drive shoe shall be V'd out and the V filled with a hard rod weld such as "brazo."

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PLACING T H E P I P E

The first sections of the pipe shall be not less than feet. The top section may be of such lesser length as will be required to make up the pile length from rock seat to cutoff. TOLERANCES

The location of the top of any pile as driven shall not vary from its design location by more than 3 inches except by special permission of the engineers. The pile shall not deviate from the plumb or the indicated batter by more than 2 per cent of its length. Any bowing of the pile shall be within such limits that from the top of the pile at least some part of the bottom of the pile can be seen. Where a group of piles are driven, the center of gravity of the group shall not vary more than 1 % inches from the center of the load to be carried. When any of these tolerances are exceeded, full information shall at once be conveyed to the engineer in charge, who shall decide on the corrective measures required. In case obstructions not shown in the plans or boring data are encountered within 3 feet below pile cutoffs and do not extend more than 4 feet below cutoff, they shall be removed at the contractor's expense. In case obstructions are encountered more than 4 feet below pile cutoff or, though encountered at a higher elevation, they extend to a depth more than 4 feet below cutoff, that portion of the obstruction extending 4 feet below cutoff will be removed at the owner's expense.

In case obstructions are encountered the engineer may call for jetting, spudding, or drilling. [Descriptions of these special tools and methods are given in Chapter 26.] Such special methods shall be performed in accordance with the following specification. Where special methods and tools are required, they will be paid for on an hourly or cost plus percentage or fee basis as set up in the bid sheet.

PLACING T H E CONCRETE FILLING

Immediately before concrete is deposited in the pile shaft, all foreign material including water shall be shown to be removed, if possible. A sounding rod shall also be used to make sure that rock has been exposed. If it is found impossible to cut off the flow of water into the pile, to such a degree that not more than 6 inches of water will enter the pile in a period of 3 minutes, then the pile may be filled with clean fresh water to a height above the natural head of incoming water, and after all inflow of water has ceased a tremie seal poured. After the tremie concrete has set, the water in this shaft shall be pumped out and the balance of the shaft concreted in the dry. Immediately before the tremie seal is deposited, a small sand pump shall be lowered to the bottom of the pipe and a suction sample taken to assure that no sand silt or any other substance except water remains over the rock. The pipe and concrete shall be cut off within y 2 inch of the cutoff elevation shown on the drawings.

18 CAISSONS

T H E line of demarcation between piles, piers, and caissons is rather arbitrary. Some codes have set the dividing line at the 22-inch diameter, but many precast piles of this and greater diameters have been used, and since they were in every sense similar to units of lesser diameter they have generally been considered as piles. Other codes have classed as piles all foundation units driven or placed with closed ends, or of such a size or shape that manual inspection of the soil at the bottom could not be made. There are also many cases where large-diameter units are not formed in such a way as to permit of visual inspection. Dredged caissons of great size are usually sealed by the depositing of tremie concrete. While Drilled-In Caissons may frequently be visually inspected, the standard and by far the more accurate method of evaluating their bearing is by the use of the drilling log of the socket. Visual inspection of rock or the breaking of small diameter core samples, even by the expert, is a very second-class method of assessing its bearing value. The New York City Code draws the line between piles and caissons in this sentence defining piles: A pile is a structural unit introduced into the ground to transmit loads to lower strata or to alter the physical properties of the ground, and is of such shape, size and length that the supporting material immediately underlying the base of the unit cannot be physically inspected.

This is probably as good as any method of differentiating between the two types of founda-

tion, but in this day of high pile loads it might be well to forget the dividing lines between piers, caissons, and piles and call them all foundation units. Recently the writer's firm installed some 500 piles for Stone and Webster Engineering Corporation and the Westinghouse Electric Company. These foundation units carried up to 500 tons each, but for purposes of the contract they were called "piles." Many Chicago Wells generally classed as caissons have been installed for less unit loadings than these so-called piles. It may seem that the question of name and definition is of little importance. However, the fight which, at the time of writing, is being carried on by one of the largest piling companies in an attempt to classify its product in United States building codes under the title "Pressure Injected Footings"—though it is generally known throughout the world as a pile—shows that mere nomenclature is taking on a new importance in the field of foundations. The move is being made in order to qualify a product for higher loads than would be allowed by building codes for similarily installed units which come within the definition of piles. Various types of caissons are discussed in this and the next two chapters. With the exception of Drilled-In Caissons (Chapter 2 0 ) , the so-called types are classified not on the basis of the finished product but on the method used in forming them. For this reason they have been treated rather differently than other types of foundations—spread footings, mats, and piling of all types and makes.

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The designing and specifying engineer generally has an easy task in writing a specification for caissons. He need only require that the caissons shall be carried down to some certain stratum and shall deliver their loads without exceeding a per square foot intensity of loading on the chosen bearing stratum. He may leave the choice between belling out and enlarging the caisson shaft to the low bidders on the work. If, however, the caissons may be belled, the specifications should include the vertical thickness of the base section below the bottom of the bell and the angle of spread of the coned section. This is often arbitrarily specified to be 30 degrees from the vertical. Some study should be given to this angle—it should be related to the nature of the soil in which the bell will be formed. Load testing of the soil at the depth at which the caisson load will be delivered to it should always be required where load will be transferred to any soil except rock, or even where rock is badly shattered or weathered. If the borings and the inspection of the caissons before concreting indicate that the hardpan or the bearing soil is of irregular character, at least 20 per cent of the caissons should be tested by means of quick loading of a plate not less than 1 square foot in area. If the assumed bearing value is not shown by the earlier tests, the caisson would be carried to a greater depth or added area should be developed. Where the caisson loads will be delivered directly to rock which the core borings on the site have shown to contain areas or pockets of shattered or weathered rock, or to include clay or other soft material below the depth required as a minimum for bottoming out, a short rock core, say 5 or 6 feet, should be required in every caisson before concrete is placed. If any core shows material of poor bearing value, the caisson in which such faulty bearing material shows up should be carried to such increased depth as needed to

get a satisfactorily 5-foot test core. Unless an inclusion of clay or the like shows in the core, a rock recovery of 60 per cent or better should generally be considered "satisfactory." There could be no fixed rule in this matter because the class of the rock would have to be taken into consideration. In coring some of the softer rocks there will be a considerable loss of core bv chewing up of the rock under the action of a diamond core drill. These precautionary load tests and drillings are very frequently omitted from the specifications, but for any important foundation they are worth the extra cost. PRECAUTIONS

AGAINST

CAS

The presence of natural gas in the soil to be penetrated has, at times, vastly increased the expense in numerous caisson jobs of standard types. Worse still, it has frequently caused loss of life. At times gas is encountered in such volume that men can work in caissons only when equipped with oxygen masks, an expensive performance if hand digging is required. Rather surprisingly the interference from gas may be greatest when the flow is least. At one site in Cleveland the gas was not in pockets at or near the contact line between the rock and the overburden, nor immediately below the hardpan, as we had found it on numerous other occasions, but was actually contained in the moist semiplastic clay. This condition was called to our attention by the fact that we could not drive closed-end steel pipe piles through what proved to be the gas-bearing stratum, using standard tools and methods. While the penetration under a blow of the pile hammer would set the pile about two inches, the rebound was practically 100 per cent. Result: progress was impossibly slow. When driving through tough clay soils, the rebound will frequently reach 75 per cent of the penetration, but 100 per cent rebound was new to us. W e had samples of this rebound stratum

CAISSONS brought up and then discovered why we could not drive through. The soil was a moist clay "sponge," shot through and through with tiny bubbles of gas. This resulted in an amazingly resilient soil structure. We finally found means to penetrate and seal off this stratum. Hand digging would have been almost impossible in such soil. Ordinarily where the gas is in pockets, it will, if left open, exhaust itself in a matter of three or four days at most, but when the gas is contained in the soil itself, merely leaving the top of the soil exposed to atmospheric pressure will not cause the drawing off of the gas. Each time a fresh shovelful of soil is taken a small additional amount of gas will be released. Even running pilot holes ahead will not release all of the gas, unless the holes are practically touching. A knowledge of precautionary measures in case gas is encountered is important, and such measures should be stipulated in the specifications. First, a strict "no smoking" rule should be enforced within a radius of 100 feet of the caisson top. Under average conditions a 50foot radius would probably be perfectly safe, but it is our observation that, like speed limits on the highway, whatever the specified safety zone may be, some smart fellow will cut it to a half or a quarter, and that could result in an explosion—and has on occasion. Second, a miner's safety gas-indicating lamp should be kept on the work. Frequent compulsory use should be called for. An indicating lamp lowered to the bottom of a caisson will show gas in such small quantities as would not otherwise be noticed. The gas most frequently encountered in Chicago, Cleveland, and many other points is highly explosive when the mixture of gas and air is right. These explosions may be violent enough to blow down a drill rig or pile driver and can, of course, cause death to those in close proximity. The danger of death due to inhaling unex-

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ploded gas is, of course, very real, though for the most part these gases are not highly toxic, and illness would generally give warning before a fatal amount of gas had been taken into the system. However, this warning period does not occur with all types of gas. At the River Rouge plant of the Ford Motor Company, we were ready to inspect the bottom of a Drilled-In Caisson. No one had observed any indication of gas, and this incident took place before we had made it a rule never to put any man down in a caisson until the safety lamp had been first run to the bottom with no showing of gas. The man who was to inspect the bottom of the hole was in the cage about to be lowered, when for no particular reason we told him, "if anything should go wrong, signal by pointing your flashlight upward and we will haul you out at once." The cage was scarcely 6 feet below the ground when the light flashed up in our faces as we leaned over to watch the descent. We called to the man in the cage but got no answer, so we snapped him up to ground surface again. To all appearances the man, slumped down in the cage, was stone dead. Happily appearance did lie, for after hours in an oxygen tent he showed signs of life, though he was a sick man for a week or so. When he was able to tell us his end of the story, we learned that he had not voluntarily signalled us at all. It was pure chance that when he slumped unconscious in the cage the light got slanted up. The man had been totally out from the moment his head went below the top of the caisson. The doctor told us just a few seconds more of immersion in the heavy deadly gas would have meant certain death. It was only by God's mercy in the accident of the lamp that he came through to life. I say again that specifications for all types of caisson work should carry a requirement that safety lamps be available on the work at all times and be used whenever men are to

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enter the caissons. Where men are required to work in caissons, whether of the open or the compressed air type, all the safety rules which apply to mining should be enforced. Even with the best of care, there is the possibility of a sudden blow-in of gas between one shovelful and the next. This is most apt to happen in excavating the last few feet of the overburden just above bedrock. Another hazard to be met in caisson work, though in this case largely a financial one, is the danger of encountering a high-pressure artesian water flow. We have observed an artesian head of 200 feet above the highest point in the surrounding terrain and, where mountains are not too far distant, it would seem probable that heads far in excess of 200 feet might occur. But most of our foundation work in this country is in comparatively flat terrain, and heads of more than 50 feet above the surrounding ground level need not be anticipated. However, where work in a caisson is carried on under compressed air, an added 50 feet of pressure might well run the necessary working pressure above the legal limits. Open caisson would scarcely be attempted if heavy flow of water even at general ground level had to be anticipated. The Drilled-In Caisson, more fully discussed in Chapter 20, can meet an excess head of 50 feet or somewhat more with little added cost and small delay, simply by carrying on the operation through an extended casing in which the necessary counter-waterhead will be maintained. About 25 feet is the greatest excess head with which we have had occasion to deal to date, and the Drilled-In Caisson method handles that head easily. Chicago

Wells

The Chicago Well is a special type of open caisson. It was first tried out about forty years ago in an attempt to meet soil conditions which occur in and around Chicago but rarely elsewhere. It met the Chicago need so well

that it has remained, through all these years, the principal type in this territory wherever the load concentrations are of sufficient magnitude to warrant its use. The Chicago Well has been used to a limited extent in Detroit, Philadelphia, Cleveland, Cincinnati, and a few other cities. In discussing the field of use and the economic efficiency of the Chicago Well type, it should be remembered that there are two classes: the "rock-bearing class" and the "belled-out in hardpan" class. The rock-bearing class has not been uniformly satisfactory, even in its native Chicago, because when the hardpan stratum has been punctured, water under such pressure as to require completion of the well by comprcsscd air methods has frequently been encountered. There is no doubt that a first-class job can be accomplished where air is used, providing that the water condition has been anticipated and preparation has been made in advance for the switch over to air. Unfortunately, it is all but impossible to know where the water will be encountered, because this "black water" seems to be in comparatively narrow streams or in rock fissures and seams. A very wet caisson with a water flow so great that it precludes the possibility of maintaining a dry hole in the open during the operation of concreting may lie between dry holes 25 feet or less from it. The hazard inherent in these rock-bearing Chicago Wells is not limited to that of water. Marsh gas in killing quantity has been struck in a number of cases, with the death of workers resulting. This gas is practically odorless, and where it is accompanied by sulphur water its presence is hard to detect. Comparatively few rock-bearing Chicago Wells have been used in recent years. With the advent of newer mechanically operated digging devices, which eliminate the danger from gas and offer positive means of controlling the flow of water, eliminating the danger of washout of the cement during the

CAISSONS concreting operations, their use will probably continue to decrease. The case for the hardpan type of Chicago Well is much stronger. In downtown Chicago, wherever the load concentrations are high, say 400 tons and up, the hardpan type Chicago Well is most frequently the choice. For lesser load concentrations closed-end steel pipe and various other types of cast-in-place concrete and composite piles generally are chosen. Where load concentrations are really high, running to several thousand tons, rock caissons may still be indicated. A number of abortive attempts have been made to introduce in Chicago other caisson types and methods which have been highly successful elsewhere. For example, jobs have been undertaken, and in some cases they have been successfully completed from an engineering standpoint, using the auger boring methods so extensively used in Texas, Detroit, and at many other points, particularly throughout the Southern states. The reason for the success of the auger boring in some areas and its lack of success under Chicago Loop conditions should be obvious enough to anyone who spends a few hours watching the operation of each in its natural habitat. While auger bored caissons have, under special conditions, been installed in soils of but little cohesive value and even through obstruction-filled soils and soils carrying free water which had to be contained by means of removable driven casings, such conditions would normally rule them out of competition. On the other hand, auger bored caissons are well nigh unbeatable price-wise, providing the loads are of such magnitude as to require any type of caissons and providing the soil is a slightly moist clay—such as is widely encountered in Texas—of such a nature that an auger bored hole will remain open without casing sheeting or lagging for sufficient time to permit of the concreting of the hole from cutoff to bearing stratum. The soil must also be of such quality as will permit main-

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taining a roof without shoring after the automatic belling-out tools have cut the bell. Note. Where hand work is needed in bottoming out or in forming the bells, it is customary to lower drums or other temporary casings into the hole, leaving them in place till the diggers have completed their work, and then removing them prior to concreting. In Chicago the soil almost meets the requirements for the efficient use of the auger boring method, but that "almost" leaves a fatal gap, as a number of hopeful foundation contractors have paid a high price to learn. A very brief study of the standard operations used in the forming of a Chicago Well should make the difference clear. With minor variations the forming of a typical Chicago Well includes the following steps: 1. Where there is loose sandy soil or artificial fill at or near the ground surface, a sheet pile cofferdam, or a horizontally sheeted boxing, having a clear diameter somewhat greater than that of the well shaft to be formed, is placed so that it extends from the ground surface to a short distance below the top of the natural clay bed. 2. A shallow circular excavation is then made in the clay and a "set" of staves consisting of tongue and groove vertical lagging is set in this circular hole. Note. Under present practice this circle of lagging is from 4 feet upwards in diameter. Until about ten years ago many 3-foot diameter caissons were used, but hand digging in so constricted a space was found to be inefficient to such a degree as to make a 4-foot caisson cheaper than a 3-foot caisson even when the greater volume of concrete per foot of depth was taken into account. 3. As soon as the circle of lagging has been set, two or more expandable steel rings are introduced and wedged or jacked out so as to force the lagging into pressure contact with the outside clay wall. Where soil conditions permit, the lagging will be in lengths of about

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4 feet, but where the socket must be dug in soft clay, a ring of lagging may be no more than 18 inches in depth. The key to the economic feasibility of the Chicago Well caisson type rests in the "time lag" existing between the removal of any given soil and the first substantial return movement of that soil. All those who work extensively in soils, whether as excavators, pile drivers, or caisson diggers, recognize that all soils from hardpan to mud will return in some degree into a hole dug into them and left open under atmospheric pressure. Movement begins immediately after the hole has been dug, but it may be microscopic in amount and so slight and so slow that for practical purposes it will be negligible, or it may be complete and virtually instantaneous. Numerous factors contribute to this important time factor when applied to soils in natural bed. These factors include grain size and shape, soil pressure, soil structure, moisture content, and permeability, in fact all of the index factors. Even the most careful sampling and analysis of specimens will not permit of a theoretical solution which may be stated in terms of units of movement and

units of time. Actual measurements within excavated shafts seems to be the only way to reach a practical and reasonably dependable answer. The variation in the extent of the advance excavation and the speed which can be developed in placing the lagging and wedging offers a flexibility in the Chicago Well which is totally lacking in the auger bored caissons. In the auger bored type it is necessary to complete the hole in one operation if any substantial cost advantage is to be realized. Where the time lag of the soil to be penetrated is small, Chicago Wells are driven on a three-shift continuous schedule. The details of the handling of the soil, the setting of the lagging, the digging of the bell in the hardpan, and finally the concreting of shaft and bell are more or less simple, though they leave plenty of room for individual skill and ingenuity. One final word of warning. Unless a sufficient amount of Chicago Well work has been done in a given territory, so that a reasonable knowledge has been acquired as to the varying time lag to be anticipated in the soil to be penetrated, the use of Chicago Wells is a pretty hazardous procedure.

19 DROP-SHAFT OR FREE-AIR METHOD OF FORMING CAISSONS

T H E drop-shaft or free-air method of forming caissons has been widely used for both large and small diameter caissons where conditions are suitable. Not infrequently caissons are started under free air by the drop-shaft method, but with working chambers so formed that compressed air may be used if water or obstructions cause trouble at depth. The procedure followed is simple. As applied to the placing of individual caissons to carry the pier loads of a building, it consists of the construction of a reinforced concrete working chamber of such outside diameter as may be required to provide the needed bearing area on the rock. If the caisson is to be belled out on hardpan the shaft diameter will be governed by the room required to accommodate the digger or diggers and their spoil buckets. For convenience the shaft may be 5 or 6 feet in diameter. The working chamber will be equipped with some sort of cutting shoe, usually a steel plate ring, which will be forced into the soil under the weight of the caisson as the work proceeds. After the working chamber is set a section of shaft is formed and poured above it. The weight of the chamber and shaft together with the excavation taking place inside the working chamber causes the caisson to sink. When the top of the shaft nears the ground line, another section is poured and this process is continued till rock is reached. The rock is stepped if necessary, or the bell is dug if greater bearing area is required, and the shaft

is filled with concrete up to the cutoff line. All this sounds simple, and it is just so long as all goes well. But any type of hand-dug caisson job can turn into a nightmare if soil conditions have been misjudged. I recall a job many years ago which I had the misfortune to handle. Fifty-odd drop shaft caissons 6 to 8 feet in diameter were to be sunk to rock at a depth of about 60 feet. The first half dozen caissons, well scattered over the site, went down about half way to rock just as they had on the drafting board and in the specifications. Then they all balked like so many army mules. We built shaft sections above the tops till we did not dare go any higher and still only an occasional short drop occurred. We resorted to caisson weights. Then we buried dead men around the outside of the caisson and rigged in the heaviest pulling jacks obtainable by undercutting the shoe till the soil slipped down a bit and brought the shaft down with it. Some depth was gained, but the men in the working chamber were operating 6 feet below the cutting shoe, and if by evil chance the friction on the shaft let go they might well have been buried alive, so that method was abandoned. Finally we reverted to dynamiting, which I at least had only met by hearsay and never want to hear of again. The first overloaded shaft around which we shot plunged 15 feet. Everybody thought we had the job by the tail till we went back to digging, only to find that the soil was working like a pan of grandmoth-

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er's bread dough and we could not control the caisson we had shot. The next move was highly sensible but rather humiliating— we abandoned the drop-shaft caissons and put the structures on piles. Free-air handdug caissons are wonderfully simple—in theory! The large diameter drop-shaft caissons as applied to bridge piers and the like may be a hundred feet and more in their greatest

TYPES

dimension and generally will have numerous compartments and shafts through which the work may be carried on by power tools, sometimes by dredging operations where unwatering may be difficult or impossible. The designing and placing of such structures may require years of work, and each is a study by itself, not subject to generalizations and not applicable to building foundations to which I have limited this book.

20 DRILLED-IN CAISSONS

THE DriUed-In Caisson, which the writer developed and patented, together with certain methods and apparatus adapted to the placing of such caisson, was planned to meet one broad need—the need for a foundation type which could be guaranteed to develop bearing of sufficient value to carry any structure load to rock at any depth no matter what the nature of the overburden. This was not intended to be a cheap foundation and certainly by no means a general utility tool to be used in all conditions and under all structures. While it would be physically possible to install Drilled-In Caisson foundations to rock through a thousand feet or more of Mississippi Delta mud, it would be highly improbable that it would ever be economically sound to do so. Most of the jobs installed to-date— and there are a substantial and growing number of them—have been in the range of 50 to 150 feet from soil surface to rock. The maximum depth so far has been 250 feet. There should be no serious trouble in extending this by at least another 150 feet. The feature of this caisson which is new is not so much its ability to reach rock at any financially feasible depth but rather the facility with which it can be carried down through soil which no other type of caisson could penetrate. One of the earlier Drilled-In Caisson jobs was for the foundation of one of this country's largest warehouses, the thirteen-story Brooklyn Navy Yard warehouse, where 180 caissons carrying individual loads up to 1,270 tons were installed to depths

of 150 feet with 10-foot sockets drilled in the bedrock (one of the hardest of the igneous rocks). The caissons had to pass through a terminal moraine where hardhead boulders floating in a sandy clay which became "live" when disturbed were encountered in great numbers and in diameters of 1 to 8 feet. YVater pressure at the rock line was sufficient in some caissons to cause artesian flow, which had to be checked by high pressure grouting to prevent the inflow of the silt which was frequently encountered close above the rock. If there is any caisson method in use, other than the Drilled-In Caisson, which would have made a rock-bearing job possible, none of those consulted before and during the work had ever heard of it. Admiral Harris Associates under the Admiral's personal guidance designed this foundation, and the Western Foundation Corporation, in association with the Spencer, White and Prentis Company, carried it through. The Drilled-In Caisson is, as has been stated, primarily intended to meet the really tough conditions where loads are high, rock bearing is desirable, and the overburden difficult to penetrate. It may under some conditions be economically feasible to install this class of caisson with a belled-out bearing on hardpan rather than the typical socket in rock, but in doing so one of the main advantages, the delivery of load by bond, is lost. Figure 13 shows in three steps the usual procedure in the forming of the caisson. Step 1. The steel casing A, which forms the

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permanent part of the finished caisson, is usually 24 or 30 inches in diameter and % to % of an inch in wall thickness, though of course other sizes and thicknesses may be used. Grade B pipe should be standard. Grade A has too low a yield point, and Grade C, while of high yield point and structurally desirable, will not infrequently shatter like a piece of glass if driven when very cold.

Figure 13. Drilled-In Caisson The drive shoe B plays an important part in shutting off the silt or fine water-bearing sand which so frequently lies immediately over bedrock. These shoes may be made of nickel, tungsten, or other alloy steels, but high carbon tool steels—when an expert in tempering is available to do the tempering after the shoe has been welded on—seem to best meet the requirements. Step 2 indicates the most generally used method of advancing the tube through the soil and obstructions and drilling the all-

important socket in the bedrock. For oil-well drilling in the softer rocks the high speed rotary drilling machines have almost entirely replaced the c h u m drills, but the problem of oil-well drilling differs greatly from the problem of setting large-diameter pipes, sealing them off at the rock line, and then drilling a short large-diameter socket at the bottom of the pipe. Setting the soil pipe for a deep well-drilling operation is an almost negligibly small part of the whole operation. If a large boulder or other obstruction is in the way of the soil well, the driller need merely pull his soil pipe a foot or so and start over, providing he is working with a mobile drilling rig. Even if he is planning to go to great depth and is using a fixed drilling tower he can well afford to spend an extra day or two to worry his way past a few boulders. The Drilled-In Caisson contractor is in quite a different situation. In the first place, he must frequently do his drilling in tough igneous rocks. Also, he must be certain he will be able to maintain a predetermined location for his pipe, probably within an inch or two of a definitely predetermined location, since column loads cannot be shuffled around to meet his convenience. Finally, he must be in shape to move quickly from one location to the next. The load-carrying capacity of the caisson shaft is that of a combination column in which the large-diameter outer pipe, the heavy structural core extending into the socket, and the concrete filling of pipe and socket combine to carry the load. Step 3. The delivery of the load to the rock is by a combination of direct bearing of the cutting shoe and the concrete and by bond transferring the load carried by the structural core through the concrete to the rock wall of the socket.

21 GROUPING IN GENERAL

IN the study of certain subjects such as chemistry, the various members which make up the objects to be studied are gathered together in "families" or "groups" in which all the individual members of a given group share certain characteristics and depend upon certain similar factors though they may differ otherwise. This system of study by groups is not widely used in the design of buildings because the principal elements and factors which must be considered are generally basically simple. There are only a few materials commonly considered in the design of superstructures—stone, brick, wood, concrete, plastics, steel, nickel, and copper. The forces to be considered in foundations are also limited and simple—tension, compression, bending, vibration, and shock acting vertically or laterally or in combination. These qualities and applications of the various materials and forces are subject to clear-cut definition and only limited variations. When we study foundations we find that the same basic materials and forces will enter into the design, but they must serve in combination with an almost infinite variety of soils and soil reactions which must develop the ultimate load resistance—the final carrying force. In this tangle it becomes imperative to use the timesaving method of study by groups. There is a second point at which it becomes necessary to form correct and usable groupings. This is in the development of the final group of types and methods which will form the basis of the specifications for any par-

ticular job. It is easy under almost any combination of load and soil to develop and specify several safe engineering solutions. But to get the right economic answer there should be specified a group, any of whose members would be permitted by sound engineering, from which the competitive bidding will indicate the solution which can be obtained at the least cost— the correct engineering and economic answer. Some of the following discussions of grouping as a tool of design or specification writing may seem to be extraneous and also repetitious. However, it is hoped that they may serve a purpose by clarifying the general idea of grouping as a short cut on the road to a final design and a competitive specification. Pile and general foundation type groupings are used, consciously or subconsciously, by any designing engineer studying a new foundation. His choice, if intelligently made, must contain at least a decision as to materials of construction, method of forming and placing, and shape. It is important that the possibles in each of these choices should be clear in his mind, but whether the approach is by analysis—the route of the Elimination Table—or by synthesis—the route of the Graphic Method—progress will be at a snail's pace if every type, subtype, method, and make must be individually considered and some sort of comparative unit value set upon each characteristic. The problem may be simple or complex, but in either case the road, if one is ever to find time to reach the end of it, should be traveled with the seven league boots of grouping. The first or general grouping of foundation

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types is given in Table 2, the Elimination study, or, where they do meet the engineering Table, in Chapter 7. In this table types are requirements, because other groups further grouped according to their outstanding char- down in the scale of costs will also meet those acteristics of design and accepted fields of requirements. usefulness, and listed in order of their probaObviously, before attempting to use the ble comparative cost. Elimination Table a study must have been Having studied the Elimination Table, the made of the load requirements of the prodesigning engineer, who is not dealing with posed structure and the results must have an abstract problem but with a specific job for been clearly listed. Also a study must have the success of which he is primarily responsi- been made of the load-carrying characterisble, may speedily discard the types which tics of the soils underlying the site (as outobviously do not fit into his particular pic- lined in Chapter 41 on the Graphic Method), ture. Presumably he is left with two or more and these also must have been clearlv listed. types which would seem to qualify, not only The proper evaluation of this information from the angle of engineering sufficiency but as to loads and soils, as modified by many also from that of reasonable cost. It is then engineering and economic factors, constitutes his province to make up his own grouping, the subject matter of the largest part of this the specific grouping on which he will ask book. for bids. However, the subject of this chapter is not This grouping will not be based on similar- primarily the use of the Elimination Table or ity of appearance or materials used, nor will the Graphic Method but rather the use—and, it be restricted to similar methods of installa- incidentally, at times the abuse—of grouping tion. It will focus on the result he wants to as at tool. get for the owner at the most reasonable figure consistent with complete safety. Time may GROUPING BY COST, MATERIAL, METHOD OF INalso be of the essence. Making up this bid- STALLATION, OR SHAPE EFFICIENCY ding group may mean crossing the boundary The final bidding group, as the writer is using lines of types as listed in the Elimination the term, is based primarily on interchangeTable. It may mean opening the bid to types ability. For example, a group on the basis of which the less experienced engineer would cost should contain only foundation types or exclude without even considering them. But methods which would at least frequently and if the engineer can in this way come up with under many conditions produce a foundation a specification which will insure keen com- in a similar range of costs. This sort of grouppetitive bidding, he is almost sure to get a ing is important. satisfactory job and a satisfied client. Another basis of grouping which is widely The procedure in the use of the Elimina- used, and has been used to some extent in tion Table consists in the listing of all com- this text, but which may frequently prove mercially used types of foundations and meth- deceptive rather than helpful, is that based ods of foundation construction; dividing these on the use of various materials of construcinto groups which have at least some degree of tion, primarily concrete, steel, or wood. Still similarity as between the members of the another grouping is based on similarity of group; arranging the groups approximately in method, formation, or placing. the order of their usual cost; and then eliminatHaving studied the soil profile, the water ing entire groups instead of individual types or table, the physical characteristics of the soils methods, either because they do not meet the to be penetrated, and possibly their chemical engineering requirements of the job under characteristics, the designing engineer might

GROUPING IN GENERAL profitably turn to a grouping table listing the principal methods of forming and placing. Such a table might be used positively by taking from it the limited number of methods he would be willing to accept, or negatively by eliminating the methods he would consider to be unsafe or at least undesirable. For example, a rough fill containing many "jagged stones" or lumps of broken concrete might eliminate the method of forming by the driving of light shells in contact with the soil and subsequently filling them with concrete, as also the use of creosoted piles where the creosote wood "jacket" would be disrupted. A surface stratum of soft clay might write off the method of forming by placing unset concrete through a heavy casing and withdrawing the casing while maintaining pressure against the unset concrete. Other considerations might further reduce the list of possibly usable types and methods of placing or forming. The advantage gained by the study of such a list of methods is that by its use the subject may be reviewed only once instead of having to be reintroduced in each study of each individual type. Also by getting this one important factor settled the necessary restrictions may be at once written into the tentative specifications. The grouping by methods of forming and placing will have proved a useful tool both as a timesaver and as a precautionary measure against the possibility of overlooking a needed safety requirement. The following constitute the elements of a grouping on the basis of method: 1. The method consisting in precasting concrete units and, after the concrete has set, introducing these units into the ground by driving or jetting or by placing in preexcavated holes. 2. The method consisting in casting in a pile location unset concrete under pressure, using the ground as the form. 3. The method consisting in placing a permanent form in the ground by driving or jetting and filling it with unset concrete.

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4. The method consisting in driving or jetting a forming apparatus into the ground, placing a container within this apparatus, filling the container with unset concrete, and removing the forming apparatus. 5. The method consisting in driving openend steel pipe to rock, cleaning out the soil contained in the pipe, and filling the pipe with unset concrete. 6. The method consisting in driving or jetting into place units consisting of structural steel, wood, or any other material capable of withstanding the stresses of handling and placing. A form of grouping which may be used in much the same manner as the method grouping has been studied in some detail in Chapter 41. It is the "shape efficiency" grouping. The grouping based on the shape of the pile would contain the following: 1. Pile shapes intended to develop bearing capacity primarily by direct bearing on firm material, including cylindrical piles, pedestal piles. 2. Pile shapes intended to develop bearing capacity principally by means of friction between pile and soil: tapered piles with large surface area, cylindrical piles of large diameter through the frictional bearing stratum, piles with shaft concrete placed under high compression against the soil. 3. Pile shapes intended to develop bearing by a combination of direct bearing and frictional bearing. There are many combinations of shapes, developed by a study of the soil profile: composite piles, combinations of cylindrical and pedestal shapes, combinations of cylindrical and tapered shapes. Frequently, but often inaccurately, the delivery of load is defined as being by one or the other of two forms—by "end-bearing" or by "friction-bearing." Conditions may permit a choice of either method, but where, as often happens, the borings eliminate one method or the other entirely, then a considerable num-

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ber of entries may be scratched in the elim- an upward movement. The amount of this ination handicap and time saved. For ex- tension was assumed to be the friction value ample, if no end-bearing can be reached of the pile, and the differences between the within a thousand feet, as is the case in the total downward load at failure and the tenMississippi delta, there is no use wasting time sion pull necessary to start the pile was asstudying any of the many strictly end-bearing sumed to be the end-bearing value. foundation types. However, it is just here Such tests are expensive and they would that it becomes important to set up clear only be justified where gross settlements or definitions of "end-bearing" and to substitute the possibility of even minor differential setfor the very inadequate term "friction-bear- tlements would be of vital importance. Such ing" some other description which will offer conditions might be anticipated, for example, a better line of demarcation. where the piling would be required to supThe New York City Building Code has at port long-span concrete arches of bridges or least improved on the usual loose use of the hangars or the like, or under structures interms "end-bearing" and "friction-bearing" tended to house very delicately adjusted by limiting the term "end-bearing" as applied gauges and instruments. to piling to those which deliver their loads There are two possible errors in the deduc"principally in direct bearing on rock or hard- tions made from the tests described. The first pan or boulder gravel formation directly over- is that downward friction and upward friclying rock." It has entirely eliminated the tion on a driven pile are not always and term "friction-bearing" and substituted the necessarily the same. This is particularly true description "all bearing piles which do not where the pile is driven through a varved develop their loads principally by direct bear- clay, the strata of which may bend slightly ing on rock, hardpan or boulder gravel for- with the passing of the pile shaft but may act mations directly overlying rock." It is impos- as "toggles" clamping the pile against upward sible for any driven pile to avoid elements of movement when pulling is attempted. The both friction and direct bearing, and it is dif- second error is the assumption that both fricficult in many cases to state the proportion of tion-bearing and end-bearing can always opthe load-carrying capacity which is developed erate simultaneously. The soil in which the in usable form by end-bearing and that which friction bearing is developed will generally is developed by friction-bearing. have a low "modulus of elasticity," and conThis analysis of the amount of end-bearing siderable movement of the pile shaft may be and of friction-bearing developed by any par- necessary to develop the maximum frictionticular pile foundation, where both friction- bearing value as shown by a pull test. If the and end-bearing elements would each be lower end of the pile is bearing on or in a capable of developing considerable values, is material which is virtually impenetrable and a difficult one to make. To measure the ratio possessed of a high modulus of elasticity, only between the two, field load tests have occa- a very slight movement of the pile will be sionally been made. It would seem worth- possible under load without partial failure of while to repeat here what has been reported the shaft or of the stratum in which the end bearing is developed. Little or no friction previously about such tests. The method used consisted in the driving may have entered into the downward load of a pile to, or slightly into, the end-bearing capacity indicated by the test load. The value stratum, load-testing the pile to incipient shown by the load test may of necessity be failure, and then pulling the pile while meas- practically all end bearing. uring the maximum stress needed to produce This combination of friction- and end-

GROUPING IN GENERAL bearing is frequently assumed by designers, but where a pile presumably could develop both end-bearing and friction values the only safe assumption, regardless of test, would seem to be that all of the working load bearing value will be in the form of end bearing and that the pile shaft must be designed accordingly. There are conditions under which the choice of a non-end-bearing pile would be preferable even though bearing on rock could be obtained by the use of a somewhat longer end-bearing pile. One such case was observed in the design of a foundation for a building intended to house very delicately adjusted gauges and machinery. Tests were made on both steel H-beams carried to rock at a depth of about 100 feet and on piles which were driven into, and pedestaled to compact, a fairly resistant stratum at a depth of 40 to 50 feet. Tests were run to ascertain the settlements under anticipated loads and the possible effects of vibration. The net settlements of the two types of piles were approximately the same. But due to elastic deformation the gross settlements were considerably higher on the long rock-bearing piles. As would be expected, the effect of the piles upon the deadening of vibration was greater with the short piles driven in well-compacted soil than it was where the long piles subject to relatively great gross settlement were used. One might generalize that the effect of piles upon vibration is a function of gross rather than net settlement. Grouping based on method of delivery of load should be used with extreme caution. A subdivision line between major groups which should be defined is that which divides piers, caissons, and piles. There are certain bearing units which might be classified as

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large diameter piles or perhaps equally well as small diameter caissons. Some books and codes base the distinction between piles and caissons solely on the diameter of the shaft; for example, if the shaft diameter exceeds 22 inches the unit is called a "caisson." Others draw the line on the basis of method used in forming—if the unit is placed primarily by machine and includes the operation of pile-driving hammers, it is classed as a "pile." If the principal method of placing is a digging operation, whether by mechanical means or by hand power, it is classed either as a "pier" or a "caisson." W e lack a universally accepted dividing line even between "pier" and "caisson." The line between piers and caissons is generally drawn on the basis of the ratio between average diameter and depth but no agreement exists as to the ratio which is to govern. Technical terminology is certainly of importance, but lacking a generally accepted rule the writer has here, more or less arbitrarily, used a combination of the maximum working load and the minimum diameters as the deciding factors and come up with the following suggested rules. Any load-bearing unit belonging to a class or type of units customarily carrying working loads of 300 tons or more and having a minimum diameter of 22 inches may be classed as a "caisson," if placed by a method which does not compact or otherwise change the nature of the supporting soil. Any load-bearing unit of lesser load capacity or of smaller diameter is designated as a pile. Any load bearing unit placed in a preexcavated opening and having a ratio of height to average diameter not exceeding 4 to 1 will be classed as a "pier."

22 SELECTING THE BID GROUP

W H E N making up a group of types to be used to promote competitive bidding, weight must be given to efficiency of type. The two factors which have been repeatedly stressed in this book as essential to the proper design and construction of foundations are: safety (good engineering) and economy (low cost). In the Elimination Table the various types of foundations have been grouped in a general way, in order of their usual relative cost. But this may prove to be an oversimplification and may result in unnecessarily high costs, because in order to get competitive figures on which to base the letting of work, comparisons are usually made on a unit basis. If, say, three types or makes of piles are included in the specifications, the request for bids treats them as equal alternates, and the lowest per foot price or the lowest lump sum based on an assumed length and an adjustment for addand-deduct footage is the basis of the award, the result may be the awarding of the work far above the true low bid. The reason for this is that where comparison is made on a purely unit price basis, too little weight is given to the comparative efficiency of the types grouped for bid purposes. Thought is usually given to safety and precautions are taken to assure delivery of the loads at points below dangerously compressible strata, but frequently the effect on cost— the comparative efficiency of the types—is given small consideration. It is realized that any attempt on the part of the designing engineer to set up differing as-

sumed lengths for various members of the group of foundation makes and types which he has decided to include in his specifications is a difficult and hazardous undertaking which will involve him in endless arguments. Varying the working loads may be equally resented, no matter how obviously justified it may be. There are two ways in which this matter of efficiency can be handled. The first, which is generally applicable only to fairly large jobs, is the carrying out of a prequalifying test program including two or more types. If this program precedes the foundation design, the savings, which may run into a very large percentage of the total cost of the foundation, will accrue to the owner. If intelligently used by the designing engineer, it should assure a correct design from both angles— safety and cost—and there will be no room for argument regarding the unit loads and the lengths to be allowed for each type. On smaller jobs, or where time precludes the use of a prequalifying test program, the emphasis must fall on the intelligent selection of the members of the group to be specified. The first requisite to this end is a clear understanding of the elements entering into the bearing values of all types of foundations. There is fairly wide agreement as to what constitutes these bearing elements. Where a single clearly defined element governs the load-carrying capacity, such as the compressive bearing value of the soil immediately below a spread footing, a mat, or a float, or where the bearing stratum may be

SELECTING THE BID GROUP assumed to be impenetrable under the compressive forces in the foundation unit, the problem becomes comparatively simple. Soil sampling followed by laboratory and field testing carried out by a first-class soil mechanics laboratory should give the answer for area loading under spread footings. Sampling and penetration tests should suffice to define the bearing value of soils on or into which end-bearing piles will be driven. In most of these cases the only problems to be solved are: How can the foundation units be placed through the overburden? And what is the lowest priced safe design of the pier, caisson, or end-bearing pile when treated as a fully supported column? In spite of this apparent simplicity many end-bearing piles seem to have been designed without benefit of any column formula, if, indeed, direct end-bearing on a fully restrained column can be said to require a formula. Tapered end-bearing concrete piles to be driven through soils of negligible friction value to reach rock are frequently included in specifications, with permitted diameter and working load values under which the concrete will be stressed to 2,000 to 2,500 pounds per square inch. On occasion I have seen such loadings allowed by specifications which provided that the piles were to be driven through new pumped-in sandy fill which would certainly settle in time, with a resultant reverse drag on the pile shaft. The whole problem of the choice of a group of competitive types which will meet the engineering requirements equally well becomes much more complicated where the piles are non-end-bearing. Three principal elements enter into the capacity of a non-end-bearing pile, and they need to be understood and carefully applied before setting up the group specification for any project. These elements are: 1. The direct end-bearing value. Obviously no pile having substance can be without some end-bearing value.

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2. The value of the friction between the wall of the pile and the soil penetrated. This will be governed by two factors, the roughness factor and the pressure factor. When these factors are high, failures not infrequently occur by "shear" in the surrounding soil rather than by adhesion or friction of soil in contact with the pile shaft. 3. The volume of soil displaced by the pile. This will react in two ways and is not infrequently the principal governing factor in the load-bearing capacity. First, the packingup of the soil will have an effect on the friction value because it will add to the pressure of the soil against the pile. Second, the displacement of the soil will increase the direct bearing value by increasing the density. It is obvious that these three elements must be balanced against the soil profile before they can be given even approximate values. Now to the question of shape. One argument which crops up frequently is whether "tapered surfaces" will or will not produce more friction per square foot of surface in contact than would be developed by cylindrical surfaces. As noted, two factors govern friction: the roughness factor of the surfaces in contact and the degree of pressure between these surfaces. Neither of these would be affected by the angle existing between the plane of contact and the surface of the earth. The pressure between the pile surface and the surrounding soil will equal the natural soil pressure at any given depth and location, plus the pressure increase resulting from the compaction caused by the driving of the pile into the soil. If the surface of a cone may be considered as consisting of an infinite number of annular steps of increasing diameter, each having a vertical and a horizontal surface, then the friction surface of a tapered pile could be determined by integrating the vertical faces of the rings and the direct-compression bear-

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FOUNDATION TYPES

ing area would equal the summation of the horizontal faces of the rings, plus the area of the pile tip. For example, if in a pile of circular cross section the bottom diameter is 8 inches and the diameter increases by steps of 1 inch at 8-foot intervals, then between the 8-to-9-inch step the area delivering load from the pile by friction would be the surface of an 8-inchdiameter cylinder 8 feet long and the direct bearing values would be concentrated at two points: the bottom of the pile, with an area equal to that of a circle of 8-inch diameter, and the annular ring or step, with an area equal to the difference between the area of a circle of 9-inch diameter and that of a circle of 8-inch diameter. Certainly where the steps are of finite proportions the basis of the reactions of a stepped pile and of a cylindrical pile are identical. The friction reaction will take place over an area equal to the summation of the vertical surfaces, and the horizontal reaction will take place against an area equal to the summation of all horizontal surfaces in contact. The difference in the total reaction of two piles, each of which has the same friction area and the same total direct-bearing areas, will depend upon the distribution of these areas with relation to the profile of the soil into which the piles will be driven. If the soil is of approximately uniform character from the surface of the ground to the bottom of the pile, the probable answer would be spread footings or mats and no deep foundation would be considered. Since the load will start to spread with depth below any point at which it is delivered to the soil (and this is assumed to be a uniform soil), the higher the point of transfer of load the better. However, there are two modifying factors which may enter. 1. If the soil is physically similar at all depths, it will nevertheless be of increasing bearing value with increasing depths because of increasing pressures. 2. Even in uniform soils, piles may be re-

quired if the direct bearing or shear values of the soil are not adequate to prevent the footings settling into the soil. In such a case piles are frequently used in order to deliver the load by means of friction over a greater contact area than would be obtained by spread footings. Piles used in this way are sometimes described as "vertical spread footings." Such piles almost always serve in two ways: in addition to increasing the area of contact, and by so doing decreasing the intensity of the load delivery, they also serve to improve the bearing value of the soil contacted, by reason of the compaction of the soil. Under the conditions assumed there would be an argument in favor of the use of tapered piles of large volume. Such piles would deliver their loads as near the surface of the ground as possible and would also develop their maximum compacting effect near the soil surface where their maximum diameter would be found. Another advantage which the short, heavily tapered pile may offer as against a cylindrical pile is that the taper tends to inhibit ground swell. This will tend to increase the heave, if any, of the pile itself while diminishing the heave of the soil. In the case where the pile develops very little end-bearing, the heaving of the pile would be of small importance since it would occur because of soil compaction, and any direct bearing value of the point might be expected to increase rather than decrease. Where a stratum of low, but still of some, bearing value, is underlain by soil of lesser value than that of the top stratum, the heavily tapered pile may still serve well, providing the spread of the load will be certain to reduce the intensity of loading at the lower stratum to well below the bearing value of that stratum. It is admitted that this passing of load through a fairly good stratum to one of less value needs study. It should be preceded by thorough laboratory analysis of undisturbed samples and should be checked

SELECTING T H E BID GROUP by full-scale load-testing on the site. Whenever it is feasible to carry structural loads by means of piles or caissons through all strata of doubtful bearing values and deliver them to a well-defined stratum—underlain by no strata of poor bearing value—of sufficient strength to carry the loads as delivered, it is certainly good practice to do so. Unfortunately, in many river deltas, such as that of the Mississippi, and in deep marshes or volcanic lakes, it is often impossible to get direct end-bearing at any depth.

141

In a large majority of cases where pile foundations are indicated, the fundamental function of the pile is and should be to carry loads through strata of poor material and to deliver them to a stratum of good bearing quality. Where true end-bearing on rock, hardpan, heavy gravel overlying rock, or the like can be reached with reasonable expenditure, an end-bearing type of pile or caisson is indicated, and the cylindrical shape will nearly always be the proper answer.

PART THREE • LABORATORY AND FIELD TESTING

23 FIELD TESTING: SOIL-LOAD TESTS

BORINGS can be, and often are, either meaningless, resulting in nothing, or expensive to the contractor who mistakenly trusts them when they have not been properly taken. This chapter will offer suggestions as to how, where, and how many borings should be made and how samples should be taken. Many books have been written covering every phase of soil mechanics. Any attempt even to summarize the subject here would at best be a rehash of material which has been far more fully treated elsewhere. The purpose of this chapter then is to point out the extent of the information which should be obtained under varying circumstances of soil and site, and to show where and how this information may be obtained, that is, the technique of field sampling and testing. Again it is the economic angle which is stressed. It would always be interesting to have the most complete soil studies possible to obtain, but the expense of such complete studies would only rarely be warranted. The designing engineer should certainly know the nature and the extent of the information which he should require under any given set of conditions, although it is not to be expected that he should know in detail the field procedures in sampling or the laboratory procedures in testing or the methods and tools required in full-scale field testing. At present it would seem that the former popularity of field testing of soils in their natural beds has waned almost to the vanishing point. Fifteen or twenty years ago soilload tests were accepted as common practice among careful engineers and construction

men. Today, most of the same men would depend largely on laboratory testing on a greatly scaled-down basis. The writer feels that both methods have their place and both should be used. While it may be claimed that only rarely will fieldtest results be given the same meticulous check-and-analysis as those conducted by unhurried scientists who have given their lives to similar problems, it can also be stated that rarely will any ivory-tower setup cover all factors to be met with on the site. REQUEST FOR BID ON SOIL-LOAD

TESTING

One of the well-known material testing laboratories might advantageously undertake the making of full-scale field soil tests, or the work might be undertaken by a soil mechanics laboratory or by a contractor specializing in core borings. However, the writer does not happen to know of any instance where either of the first two types of organization has been so employed. Perhaps this is because at least a limited amount of heavy equipment—crane, bulldozer, hydraulic jacks, and the like—would be needed for any heavy testing operation. Of the many soil tests observed, by far the greater proportion have been made by the building contractors themselves. A limited number have been made by drilling and core-boring companies, and they would seem to be the most logical people to undertake the work since they are field rather than laboratory operators and would normally have some of the equipment and also the personnel needed. A good deal of care and some skill and

LABORATORY AND FIELD TESTING

146

knowledge are called for in the making and interpretation of soil tests. T h e engineer requiring such services should start out with a careful specification, which would include at least a general description of the plant and method to b e used, because, unlike pile-load testing, no standard procedure has been established. TYPICAL SPECIFICATION FOR SURFACE OR NEAR SURFACE SOIL-LOAD

TEST

Figures 14, 15, and 16 show the loading and gauging assemblies to be used in making a soil-load test. Steel Plate To Be Safely Flatter Than Natural Slope Drainage Ditch 2-12"* 12"* 24" Loading Blocks Figure 14. Loading and Gauging Assemblies Be Used in Making a Soil-Load Test: I

to

1. The pit shall be rough-excavated to within 1 foot of the elevation of the top of the soil stratum which is to be tested. [The locations and elevations at which tests are to be made are indicated on Figure 14.] 2. The excavation shall be as indicated [in Figure 15]. The clear space at the bottom of the excavation around the test block shall be not less than 5 feet wide at all points. 3. The entire bottom of the pit shall be handexcavated to an elevation 6 inches above the level at which the test will be applied. 4. A 1-foot deep drainage ditch shall then be dug at the toe of the excavation and a sump box 18 inches deep placed in one corner. The suction hose of the drainage pump will be introduced into the sump and the discharge carried to such a distance as will prevent any back seepage of the drained water. If there is any tendency of the soil to slough into the drainage ditch, the ditch shall be in the form of a rubble drain.

It is important that the opportunity for free water flow be maintained in the ditch and sump, but equally important that the natural waterhead at the point of application of the test load be disturbed as little as possible. A too-deep ditch and sump may cause a drying out at the point of test which may entirely change the natural bearing value of the soil tested. Careful check should b e maintained on the drainage throughout the duration of the test. T h e intent is to remove only the surplus water due to rain or other abnormal conditions. Note. If the natural water table is substantially above the elevation of the soil to be tested—say 4 feet or more, in a loose soil—it may be impossible by the method outlined to get a dependable valuation of the bearing of the soil under working load conditions. A fairly accurate answer can usually be obtained by forcing an open-end thin-wall pipe of 30 inches or greater diameter into the ground to such a depth as will retard the upward flow of the water through the pipe to such a degree that it will not "float" the sand. The necessary depth of penetration of the pipe below the level of test would of course depend on the looseness or porosity of the soil and the natural head of the seepage water, but something around 4-foot penetration of the pipe would usually serve. If necessary the top of the pipe through which the test will be made will be placed at or near the natural waterhead. If water conditions are very severe, it may b e necessary to fall back on soil sampling—as nearly undisturbed or compensated as possible—and laboratory testing. The results would be less certain, but the cost would be substantially lower. 5. After hand-trimming the bottom of the pit, a depression 2 4 " x 24" x 6 " deep shall be excavated by trowel, filled to a depth of 4 inches with dried medium-grain sand, and troweled off to an accurate level with a carpenter's hand level to assure a truly horizontal and uniform finish.

F I E L D TESTING: SOIL-LOAD TESTS

147

/

m

X Section

Figure

A

Loading Post - 12" I.D. 1 / 2 " Closure P l a t e s

B

Gauge

Frames of

3/8"

Welded

C

Loading

Plate

D

Loading

Block

E

Extensometer - O.OOl'

F

Loading

G

3 " I.D. Standard

Y-Y Wall

Pipe

with

H-Beams

Rating

Jack Pipe

15. Loading and Gauging Assemblies to Be Used in Making a Soil-Load Test: II

Note. After the hand-trimming of the bottom of the work pit has been completed, set planks so that workmen handling the bearing blocks will not disturb the soil to be tested. It should be remembered at all times that while engineers and laboratory assistants are trained to work with some degree of delicacy and care, laborers are not, and the accuracy and value of field tests is frequently destroyed

by the clumsiness of those who must be depended upon for the work. 6. After placing and leveling the sand, a bearing block—consisting of two 12" x 12" x 2 4 " carefully sized timbers, bolted together and capped with a 1 " thick 2 0 " x 2 0 " steel plate—shall be placed firmly on the sand [see Figure 14]. Levels shall be taken with a transit or engineer's level on the four comers of the bearing block as

148

LABORATORY AND FIELD TESTING

soon as it has been placed, as a means of checking that it has not been disturbed during the erection of the loading mechanisms. The levels will be rechecked just before starting of the application of load. Note. The setup shown in Figures 14, 15, and 16 would permit of test loads u p to 60 tons. The test load is shown as applied on a 4-square-foot bearing plate, b u t bearing blocks of other sizes could b e substituted.

T h e larger the area tested u p to the size of the largest building footing, the more accurate will t h e test be. 7. The loading and gauging assemblies shall be installed in the following order after the initial placement of the loading block as previously specified: a. The four 3" ID pipe gauge supports shall be sledged to refusal at the locations shown on Figure 15.

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