Soils and Geotechnology in Construction [1 ed.] 1138551104, 9781138551107

This book covers the field of applied geotechnology related to all aspects of construction in ground, including compacte

494 143 12MB

English Pages 505 [507] Year 2019

Report DMCA / Copyright

DOWNLOAD FILE

Polecaj historie

Soils and Geotechnology in Construction [1 ed.]
 1138551104, 9781138551107

Table of contents :
Cover
Half Title
Title Page
Copyright Page
Table of Contents
About the Author
1: Soils—The nature of ground; Soil properties and soil behavior
1.1 The nature and composition of soils
1.1.1 Types of soils
1.1.1.1 Transported soils
1.1.1.2 Residual soils
1.1.2 Fine-grained soils vs. coarse-grained soils
1.2 Composite nature of soils—Phase relationships
1.2.1 Phase relationships based on mass
1.2.1.1 Water content
1.2.2 Phase relationships based on volume
1.2.2.1 Voids ratio
1.2.2.2 Porosity
1.2.2.3 Saturation
1.2.3 Phase relationships combining mass and volume
1.2.3.1 Wet density
1.2.3.2 Dry density
1.2.3.3 Saturated density
1.2.3.4 Buoyant density
1.2.4 Unit density of water and unit density of solids
1.2.4.1 Unit density of water
1.2.4.2 Unit density of solids
1.3 Soil composition and properties
1.3.1 Grain-size distribution
1.3.1.1 Coarse-grained soils
1.3.1.2 Fine-grained soils
1.3.2 Grain shape—Coarse-grained soils
1.3.3 Clay mineralogy
1.3.3.1 Kaolinite “AB”
1.3.3.2 Montmorillonite “ABA”
1.3.3.3 Illite “ABA”
1.3.3.4 Mixed clay minerals
1.3.4 Specific surface area
1.3.5 Cation exchange capacity
1.3.6 Carbonate content
1.3.7 Organic content
1.3.8 Atterberg limits—Consistency of fine-grained soils
1.3.9 Activity
1.4 Geostatic stresses in soils
1.4.1 Vertical stress
1.4.2 Horizontal stress
1.5 Stress history in fine-grained soils
1.5.1 Normally consolidated soils
1.5.2 Overconsolidated soils
1.5.3 Consolidation settlement
1.5.4 Underconsolidated soils?
1.5.5 Overconsolidation stress difference
1.5.6 Influence of soil stress history on KO
1.6 Shear strength of soils
1.6.1 Drained shear strength
1.6.2 Undrained shear strength
1.6.3 Sensitivity of fine-grained soils
1.6.4 Thixotropy of fine-grained soils
1.7 Hydraulic conductivity
1.8 Soil identification, descriptions, and classification
1.8.1 Soil classification
1.8.2 Soil identification and descriptions—The modified unified soil description system
References
2: Geotechnical site investigations
2.1 Introduction
2.2 Purpose of site investigations
2.2.1 Determine subsurface stratigraphy
2.2.2 Determine groundwater conditions
2.2.3 Obtain soil and rock samples
2.2.4 Perform field tests
2.3 Preliminary desk investigations
2.4 Field investigations—Boreholes and test pits
2.4.1 Hand-auger borings
2.4.2 Truck-/Track-/Skid-mounted soil borings
2.4.2.1 Solid-stem continuous flight auger borings
2.4.2.2 Hollow-stem continuous auger borings
2.4.2.3 Mud rotary wash-borings
2.4.3 Percussion drilling
2.4.4 Test pits
2.5 Soil sampling
2.5.1 Sample disturbance
2.5.2 Disturbed and undisturbed samples
2.5.3 Split-spoon sampler and the Standard Penetration Test
2.5.4 Thin-walled Shelby tube sampler for fine-grained soils
2.5.5 Stationary or fixed-piston sampler
2.5.6 Core-barrel samplers for stiff soils
2.5.7 U100 drive-tube sampler
2.5.8 Block sampling
2.6 Field boring logs
2.7 In situ testing
2.7.1 Standard Penetration Test (SPT)
2.7.2 Standard Penetration Test with Torque (SPT-T)
2.7.3 Dynamic Cone Penetrometer test (DCP)
2.7.4 Cone Penetration Test (CPT) and Piezocone Test (CPTU)
2.7.5 Field Vane Test (FVT)
2.7.6 Dilatometer Test (DMT)
2.7.7 Pressuremeter Test (PMT)
2.7.8 Borehole Shear Test (BST)
2.7.9 Plate Load Test (PLT)
2.8 Measurements While Drilling (MWD)
2.9 Site investigation strategies and minimum investigation
References
3: General construction earthwork
3.1 Introduction
3.2 Soil unit weight
3.3 Soil states related to earthwork
3.3.1 Bank to loose—Soil swell
3.3.2 Bank to compacted—Soil shrinkage
3.3.3 Loose to compacted
3.4 Determining values of swell, shrinkage, and compaction
3.5 Load and shrinkage factors
3.6 Sensitivity of earthwork calculations
3.7 Rock excavation and compaction
3.8 Spoil piles/stockpiles
Reference
4: Compaction of soils
4.1 Introduction
4.2 Laboratory compaction of fine-grained and mixed-grained soils
4.2.1 Proctor compaction
4.2.2 Degree of saturation and the line of saturation (S = 100%)
4.2.3 Maximum possible compacted dry density and water content
4.2.4 Relationship between OWC and MDD
4.2.5 Compaction energy-density growth curves
4.2.6 Relative compaction
4.2.7 Range in water content at relative compaction
4.2.8 Modified versus Standard compaction characteristics
4.3 Estimating compaction characteristics of fine-grained soils from soil characteristics
4.4 Harvard Miniature compaction
4.5 Laboratory compaction of coarse-grained soils
4.5.1 The relative density concept
4.5.2 Laboratory determination of minimum index density
4.5.3 Laboratory determination of maximum index density
4.5.4 Relation between minimum and maximum index density and grain-size
4.5.5 Compactibility of coarse-grained soils
4.5.6 Estimating relative density from one-point dry Proctor test
4.5.7 Relationship between relative density and relative compaction
4.6 Field compaction of soils
4.6.1 Field compaction of fine-grained soils
4.6.2 Field compaction of coarse-grained soils
4.6.3 Field compaction in confined spaces
4.7 Compaction specifications
4.8 Field determination of compaction characteristics
4.8.1 In-place density and water content
4.8.2 Other field tests for compacted soils
4.8.3 Large-scale field tests for compacted soils
References
5: Excavations and trenching
5.1 Introduction
5.2 Causes of soil instability in excavations and trenches
5.2.1 Increase in soil water content
5.2.2 Loss of soil strength by unloading
5.2.3 Vibrations
5.2.4 Non-uniform soil conditions
5.2.5 Surcharge loading adjacent to excavation
5.2.6 Time
5.2.7 Water seepage
5.2.8 Soil freezing
5.3 Indicators of excavation instability
5.3.1 Tension cracks
5.3.2 Subsidence or depressions
5.3.3 Toppling or falling of surface soil
5.3.4 Very wet soils
5.3.5 Surface water
5.3.6 Bulging soils
5.3.7 Sloughing/sliding
5.3.8 Lateral movement of shoring/shields
5.4 Trenches and trench safety
5.4.1 Definition of an excavation and a trench
5.5 OSHA soil types
5.5.1 Visual inspection—Identification
5.5.2 Hand-dilatency test
5.5.3 Dry strength test
5.5.4 Plasticity “ribbon” or “thread” test
5.5.5 Determining unconfined compressive strength
5.5.5.1 Laboratory unconfined compression test
5.5.5.2 Pocket Penetrometer
5.5.5.3 Torvane
5.5.5.4 Inspection Vane
5.5.5.5 Drive Cone Penetrometer (DCP)
5.6 Stability of vertical cuts
5.7 Sloping
5.7.1 Simple slopes
5.7.2 Benched cuts—Single bench and multiple benches
5.7.3 Simple slope with unsupported vertical lower portion
5.7.4 Simple slope with vertical supported or shielded lower section
5.8 Shoring
5.8.1 Timber shoring
5.8.2 Sheeting as shoring
5.8.3 Mechanical shoring
5.8.4 Hydraulic shoring
5.9 Shielding
5.10 Braced excavations
5.11 Safety checklist for trench and other excavations
References
6: Foundations
6.1 Introduction
6.2 Foundation design considerations
6.3 Shallow foundations
6.3.1 Continuous-strip footings
6.3.2 Individual isolated footings
6.3.3 Combined footings
6.3.4 Mat foundations
6.3.5 Ring footings
6.3.6 Slab-on-grade as a shallow foundation
6.3.7 Additional design and construction considerations for shallow foundations
6.3.7.1 Frost depth
6.3.7.2 High groundwater table
6.4 Deep foundations
6.5 Driven deep foundations
6.5.1 Small-displacement driven piles
6.5.2 Large-displacement driven piles
6.5.3 Driven cast-in-place piles
6.5.4 Driven fiber-reinforced polymer (FRP) piles
6.5.5 Fin piles
6.5.6 Ductile iron pipe piles
6.5.7 Driven piles—Changes in soil properties from driving and aging
6.6 Drilled deep foundations
6.6.1 Bored open-hole continuous-flight auger (CFA) piles
6.6.2 Auger-cast-in-place piles (ACIP)
6.6.3 Straight-sided drilled shafts
6.6.3.1 Dry method
6.6.3.2 Casing method
6.6.3.3 Slurry method
6.6.4 Enlarged-base (Belled) drilled shafts
6.6.5 Multi-bell drilled shafts
6.6.6 Rock sockets
6.6.7 Post grouting drilled shafts
6.6.8 Pressure-Injected Footings (PIFs)
6.6.9 Drilled displacement piles
6.6.9.1 Atlas piles
6.6.9.2 Olivier piles
6.6.9.3 Fundex piles
6.6.9.4 Dewaal, Omega, Berkel, and SVV piles
6.6.9.5 SVB partial displacement piles
6.6.9.6 Helical cast-in-place displacement piles (HCIPD)
6.6.9.7 Continuous helical displacement piles
6.6.10 Installation monitoring of ACIP and ACIPD piles
6.6.11 Drilled piles—Changes in soil properties from drilling and aging
6.6.12 Grout factor
6.7 Other piles
6.7.1 Helical piles
6.7.2 Jetted piles
6.7.3 Press-in piling
6.7.4 Expander body piles
6.8 Foundations in uplift
6.9 Underpinning/upgrading existing foundations
6.9.1 Micropiles
6.9.2 Pushed (Jacked) minipiles
6.9.3 Helical piles
6.9.4 Grouted-shaft helical micropiles (GSHM)
6.9.5 Expander body piles
References
7: Earth retention and earth anchors
7.1 Introduction
7.2 Gravity and cantilever retaining walls
7.2.1 Gravity walls
7.2.2 Cantilever walls
7.3 Sheet-pile walls
7.4 Crib/bin walls
7.5 Soldier pile and lagging walls
7.6 Secant walls
7.7 Mechanically stabilized earth (MSE) walls
7.8 Shotcrete walls
7.9 Internally braced excavations
7.10 Earth anchors
7.10.1 Grouted anchors
7.10.2 Helical anchors
7.10.3 Soil nails
7.10.4 Expander body anchors
References
8: Ground improvement
8.1 Introduction
8.2 Evaluating existing ground conditions and the need for ground improvement
8.3 Ground improvement by removal and replacement
8.3.1 Removal and replacement without reinforcement
8.3.2 Removal and replacement with reinforcement
8.4 Ground improvement of loose, coarse-grained soils by densification
8.4.1 Surface rolling by vibratory rollers
8.4.2 Surface impact rollers
8.4.3 Rapid Impact Compaction (RIC)
8.4.4 Deep Dynamic Compaction (DDC)
8.4.5 Deep vibro-compaction/vibroflotation (VC)
8.4.6 Deep Explosive Compaction (DEC)
8.4.7 Compaction grouting (CG)
8.5 Ground improvement of soft, fine-grained soils by consolidation
8.5.1 Consolidation by surcharge preloading
8.5.2 Consolidation by surcharge preloading with wick drains
8.5.3 Consolidation by vacuum preloading
8.6 Ground improvement of fine-grained soils by admixing
8.6.1 Dry mixing of chemical admixtures
8.6.1.1 Admixing with lime
8.6.1.2 Lime piles
8.6.1.3 Admixing with cement
8.6.2 Deep mixing method: Soil-cement columns and soil-lime columns
8.6.3 Wet placement—Lime slurry injection
8.7 Ground improvement by reinforcement
8.7.1 Stone columns
8.7.2 Compacted sand piles
8.7.3 Rammed Aggregate Piers™ (RAPs)
8.7.4 Controlled Modulus Columns™
8.7.5 Vibro-Concrete Columns
References
9: Problematic soils
9.1 Introduction
9.2 Highly compressible organic soils
9.2.1 Occurrence of organic soils
9.2.2 Identification of organic soils
9.2.3 Mitigation practices for organic soils
9.3 Liquefiable coarse-grained soils
9.3.1 Occurrence of liquefiable coarse-grained soils
9.3.2 Identification of liquefiable coarse-grained soils
9.3.3 Mitigation practices for liquefiable coarse-grained soils
9.4 Collapsible soils
9.4.1 Occurrence of collapsible soils
9.4.2 Identification of collapsible soils
9.4.2.1 Indirect methods
9.4.2.2 Direct methods
9.4.3 Collapse “Sensitivity”
9.4.4 Mitigation practices for collapsible soils
9.5 Expansive soils
9.5.1 Occurrence of expansive soils
9.5.2 Identification of expansive fine-grained soils
9.5.2.1 Indirect methods
9.5.2.2 Free Swell Test
9.5.2.3 Direct methods
9.5.3 Swell “Sensitivity”
9.5.4 Shrinkage behavior
9.5.4.1 Linear shrinkage test to describe shrinkage behavior
9.5.4.2 Shrink Test
9.5.4.3 Shrink “Sensitivity”
9.5.5 Mitigation practices for expansive soils
9.6 Highly sensitive “quick” clays
9.6.1 Occurrence of “quick” clays
9.6.2 Identification of “quick” clays
9.6.3 Mitigation practices for “quick” clays
9.7 Dispersive soils
9.7.1 Occurrence of dispersive soils
9.7.2 Identification of dispersive soils
9.7.2.1 Double Hydrometer Test
9.7.2.2 Pinhole Dispersion Test
9.7.3 Mitigation practices for dispersive soils
9.8 Corrosive soils
9.8.1 Introduction
9.8.2 Laboratory tests for corrosion potential
9.8.2.1 pH tests
9.8.2.2 Electrical resistivity
9.8.3 Corrosivity rating
9.9 High-sulfate soils
9.10 Unusual or special soil conditions
References
10: Inspection of geoconstruction
10.1 Introduction
10.2 Inspection of shallow foundation construction
10.2.1 Verify footing locations, dimensions, and elevations
10.2.2 Evaluate subgrade
10.2.3 Observation of placement of reinforcement and concrete
10.2.4 Quality assurance of concrete
10.2.5 Observation of backfill placement
10.2.6 After construction
10.3 Inspection of driven piles
10.3.1 Before installation
10.3.2 During installation
10.3.3 After installation
10.4 Inspection of drilled shafts, bored piles, and micropiles
10.4.1 Contractor setup
10.4.2 Shaft excavation
10.4.3 Reinforcement and concrete placement
10.4.4 Post-installation record
10.5 Inspection of helical piles
10.5.1 Before installation
10.5.2 During installation
10.5.3 After installation
10.6 Inspection of ground anchors
10.6.1 Contractor setup
10.6.2 Anchor-hole excavation
10.6.3 Reinforcement and concrete placement
10.6.4 Post-installation record
10.7 Inspection of ground improvement
10.8 Inspector’s toolkit
Index

Citation preview

Soils and Geotechnology in Construction

Soils and Geotechnology in Construction

Alan J. Lutenegger

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2019 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-1-138-55110-7 (Hardback) 978-1-4987-4101-9 (Paperback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www. copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging‑in‑Publication Data Names: Lutenegger, A. J., author. Title: Soils and geotechnology in construction / Alan J. Lutenegger. Description: Boca Raton : Taylor & Francis, a CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa, plc, [2019] | Includes bibliographical references and index. | Identifiers: LCCN 2018050303 (print) | LCCN 2018052316 (ebook) | ISBN 9781315320526 (Mobipocket) | ISBN 9781498741026 (Adobe PDF) | ISBN 9781498741040 (ePub) | ISBN 9781498741019 (paperback : acid-free paper) | ISBN 9781138551107 (hardback : acid-free paper) | ISBN 9781315380643 (ebook) Subjects: LCSH: Geotechnical engineering. | Soil mechanics. Classification: LCC TA705 (ebook) | LCC TA705 .L86 2019 (print) | DDC 624.1/51--dc23 LC record available at https://lccn.loc.gov/2018050303 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents

About the Author 1 Soils—The nature of ground; Soil properties and soil behavior 1.1

1.2

1.3

xvii 1

The nature and composition of soils 1 1.1.1 Types of soils 1 1.1.1.1 Transported soils 1 1.1.1.2 Residual soils 3 1.1.2 Fine-grained soils vs. coarse-grained soils 4 Composite nature of soils—Phase relationships 6 1.2.1 Phase relationships based on mass 6 1.2.1.1 Water content 6 1.2.2 Phase relationships based on volume 8 1.2.2.1 Voids ratio 8 1.2.2.2 Porosity 8 1.2.2.3 Saturation 9 1.2.3 Phase relationships combining mass and volume 9 1.2.3.1 Wet density 9 1.2.3.2 Dry density 10 1.2.3.3 Saturated density 10 1.2.3.4 Buoyant density 10 1.2.4 Unit density of water and unit density of solids 10 1.2.4.1 Unit density of water 10 1.2.4.2 Unit density of solids 12 Soil composition and properties 14 1.3.1 Grain-size distribution 15 1.3.1.1 Coarse-grained soils 15 1.3.1.2 Fine-grained soils 17 1.3.2 Grain shape—Coarse-grained soils 19 v

vi

Contents

1.3.3 Clay mineralogy 19 1.3.3.1 Kaolinite “AB” 20 1.3.3.2 Montmorillonite “ABA” 21 1.3.3.3 Illite “ABA” 23 1.3.3.4 Mixed clay minerals 23 1.3.4 Specific surface area 24 1.3.5 Cation exchange capacity 25 1.3.6 Carbonate content 26 1.3.7 Organic content 27 1.3.8 Atterberg limits—Consistency of fine-grained soils 28 1.3.9 Activity 31 1.4 Geostatic stresses in soils 32 1.4.1 Vertical stress 32 1.4.2 Horizontal stress 34 1.5 Stress history in fine-grained soils 35 1.5.1 Normally consolidated soils 37 1.5.2 Overconsolidated soils 37 1.5.3 Consolidation settlement 38 1.5.4 Underconsolidated soils? 38 1.5.5 Overconsolidation stress difference 39 1.5.6 Influence of soil stress history on KO 39 1.6 Shear strength of soils 40 1.6.1 Drained shear strength 41 1.6.2 Undrained shear strength 43 1.6.3 Sensitivity of fine-grained soils 44 1.6.4 Thixotropy of fine-grained soils 45 1.7 Hydraulic conductivity 45 1.8 Soil identification, descriptions, and classification 47 1.8.1 Soil classification 47 1.8.2 Soil identification and descriptions—The modified unified soil description system 48 References 50

2 Geotechnical site investigations 2.1 2.2

Introduction 53 Purpose of site investigations 54 2.2.1 Determine subsurface stratigraphy 55 2.2.2 Determine groundwater conditions 55 2.2.3 Obtain soil and rock samples 55 2.2.4 Perform field tests 55

53

Contents

vii

2.3 2.4

Preliminary desk investigations 56 Field investigations—Boreholes and test pits 56 2.4.1 Hand-auger borings 57 2.4.2 Truck-/Track-/Skid-mounted soil borings 60 2.4.2.1 Solid-stem continuous flight auger borings 63 2.4.2.2 Hollow-stem continuous auger borings 64 2.4.2.3 Mud rotary wash-borings 66 2.4.3 Percussion drilling 68 2.4.4 Test pits 69 2.5 Soil sampling 71 2.5.1 Sample disturbance 71 2.5.2 Disturbed and undisturbed samples 74 2.5.3 Split-spoon sampler and the Standard Penetration Test 74 2.5.4 Thin-walled Shelby tube sampler for fine-grained soils 77 2.5.5 Stationary or fixed-piston sampler 80 2.5.6 Core-barrel samplers for stiff soils 82 2.5.7 U100 drive-tube sampler 85 2.5.8 Block sampling 86 2.6 Field boring logs 89 2.7 In situ testing 90 2.7.1 Standard Penetration Test (SPT) 90 2.7.2 Standard Penetration Test with Torque (SPT-T) 98 2.7.3 Dynamic Cone Penetrometer test (DCP) 99 2.7.4 Cone Penetration Test (CPT) and Piezocone Test (CPTU) 100 2.7.5 Field Vane Test (FVT) 101 2.7.6 Dilatometer Test (DMT) 102 2.7.7 Pressuremeter Test (PMT) 103 2.7.8 Borehole Shear Test (BST) 103 2.7.9 Plate Load Test (PLT) 104 2.8 Measurements While Drilling (MWD) 104 2.9 Site investigation strategies and minimum investigation 108 References 109

3 General construction earthwork 3.1 3.2

Introduction 111 Soil unit weight 111

111

viii

Contents

3.3

Soil states related to earthwork 114 3.3.1 Bank to loose—Soil swell 115 3.3.2 Bank to compacted—Soil shrinkage 116 3.3.3 Loose to compacted 119 3.4 Determining values of swell, shrinkage, and compaction 121 3.5 Load and shrinkage factors 123 3.6 Sensitivity of earthwork calculations 127 3.7 Rock excavation and compaction 130 3.8 Spoil piles/stockpiles 132 Reference 133

4 Compaction of soils 4.1 4.2

4.3 4.4 4.5

Introduction 135 Laboratory compaction of fine-grained and mixed-grained soils 137 4.2.1 Proctor compaction 137 4.2.2 Degree of saturation and the line of saturation (S = 100%) 142 4.2.3 Maximum possible compacted dry density and water content 143 4.2.4 Relationship between OWC and MDD 145 4.2.5 Compaction energy-density growth curves 145 4.2.6 Relative compaction 147 4.2.7 Range in water content at relative compaction 148 4.2.8 Modified versus Standard compaction characteristics 148 Estimating compaction characteristics of fine-grained soils from soil characteristics 149 Harvard Miniature compaction 153 Laboratory compaction of coarse-grained soils 156 4.5.1 The relative density concept 156 4.5.2 Laboratory determination of minimum index density 159 4.5.3 Laboratory determination of maximum index density 160 4.5.4 Relation between minimum and maximum index density and grain-size 160 4.5.5 Compactibility of coarse-grained soils 163 4.5.6 Estimating relative density from one-point dry Proctor test 163

135

Contents

ix

4.5.7 Relationship between relative density and relative compaction 165 4.6 Field compaction of soils 166 4.6.1 Field compaction of fine-grained soils 166 4.6.2 Field compaction of coarse-grained soils 168 4.6.3 Field compaction in confined spaces 169 4.7 Compaction specifications 170 4.8 Field determination of compaction characteristics 172 4.8.1 In-place density and water content 172 4.8.2 Other field tests for compacted soils 174 4.8.3 Large-scale field tests for compacted soils 175 References 177

5 Excavations and trenching 5.1 5.2

5.3

5.4 5.5

Introduction 179 Causes of soil instability in excavations and trenches 179 5.2.1 Increase in soil water content 180 5.2.2 Loss of soil strength by unloading 180 5.2.3 Vibrations 181 5.2.4 Non-uniform soil conditions 181 5.2.5 Surcharge loading adjacent to excavation 181 5.2.6 Time 181 5.2.7 Water seepage 182 5.2.8 Soil freezing 182 Indicators of excavation instability 182 5.3.1 Tension cracks 182 5.3.2 Subsidence or depressions 183 5.3.3 Toppling or falling of surface soil 183 5.3.4 Very wet soils 183 5.3.5 Surface water 183 5.3.6 Bulging soils 183 5.3.7 Sloughing/sliding 184 5.3.8 Lateral movement of shoring/shields 184 Trenches and trench safety 184 5.4.1 Definition of an excavation and a trench 184 OSHA soil types 186 5.5.1 Visual inspection—Identification 188 5.5.2 Hand-dilatency test 188 5.5.3 Dry strength test 189 5.5.4 Plasticity “ribbon” or “thread” test 189

179

x

Contents

5.5.5 Determining unconfined compressive strength 189 5.5.5.1 Laboratory unconfined compression test 189 5.5.5.2 Pocket Penetrometer 190 5.5.5.3 Torvane 191 5.5.5.4 Inspection Vane 193 5.5.5.5 Drive Cone Penetrometer (DCP) 194 5.6 Stability of vertical cuts 194 5.7 Sloping 196 5.7.1 Simple slopes 197 5.7.2 Benched cuts—Single bench and multiple benches 199 5.7.3 Simple slope with unsupported vertical lower portion 199 5.7.4 Simple slope with vertical supported or shielded lower section 199 5.8 Shoring 201 5.8.1 Timber shoring 201 5.8.2 Sheeting as shoring 201 5.8.3 Mechanical shoring 203 5.8.4 Hydraulic shoring 203 5.9 Shielding 204 5.10 Braced excavations 205 5.11 Safety checklist for trench and other excavations 207 References 209

6 Foundations 6.1 6.2 6.3

6.4

Introduction 211 Foundation design considerations 211 Shallow foundations 213 6.3.1 Continuous-strip footings 214 6.3.2 Individual isolated footings 218 6.3.3 Combined footings 221 6.3.4 Mat foundations 222 6.3.5 Ring footings 223 6.3.6 Slab-on-grade as a shallow foundation 223 6.3.7 Additional design and construction considerations for shallow foundations 225 6.3.7.1 Frost depth 225 6.3.7.2 High groundwater table 225 Deep foundations 227

211

Contents

6.5

6.6

6.7

6.8

xi

Driven deep foundations 230 6.5.1 Small-displacement driven piles 230 6.5.2 Large-displacement driven piles 231 6.5.3 Driven cast-in-place piles 233 6.5.4 Driven fiber-reinforced polymer (FRP) piles 235 6.5.5 Fin piles 237 6.5.6 Ductile iron pipe piles 239 6.5.7 Driven piles—Changes in soil properties from driving and aging 240 Drilled deep foundations 241 6.6.1 Bored open-hole continuous-flight auger (CFA) piles 242 6.6.2 Auger-cast-in-place piles (ACIP) 244 6.6.3 Straight-sided drilled shafts 244 6.6.3.1 Dry method 247 6.6.3.2 Casing method 247 6.6.3.3 Slurry method 250 6.6.4 Enlarged-base (Belled) drilled shafts 250 6.6.5 Multi-bell drilled shafts 254 6.6.6 Rock sockets 255 6.6.7 Post grouting drilled shafts 256 6.6.8 Pressure-Injected Footings (PIFs) 256 6.6.9 Drilled displacement piles 258 6.6.9.1 Atlas piles 260 6.6.9.2 Olivier piles 261 6.6.9.3 Fundex piles 261 6.6.9.4 Dewaal, Omega, Berkel, and SVV piles 263 6.6.9.5 SVB partial displacement piles 263 6.6.9.6 Helical cast-in-place displacement piles (HCIPD) 266 6.6.9.7 Continuous helical displacement piles 270 6.6.10 Installation monitoring of ACIP and ACIPD piles 271 6.6.11 Drilled piles—Changes in soil properties from drilling and aging 272 6.6.12 Grout factor 273 Other piles 274 6.7.1 Helical piles 274 6.7.2 Jetted piles 277 6.7.3 Press-in piling 278 6.7.4 Expander body piles 279 Foundations in uplift 281

xii

Contents

6.9

Underpinning/upgrading existing foundations 283 6.9.1 Micropiles 285 6.9.2 Pushed (Jacked) minipiles 288 6.9.3 Helical piles 289 6.9.4 Grouted-shaft helical micropiles (GSHMP) 290 6.9.5 Expander body piles 290 References 292

7 Earth retention and earth anchors

301

7.1 7.2

Introduction 301 Gravity and cantilever retaining walls 301 7.2.1 Gravity walls 302 7.2.2 Cantilever walls 305 7.3 Sheet-pile walls 308 7.4 Crib/bin walls 309 7.5 Soldier pile and lagging walls 310 7.6 Secant walls 313 7.7 Mechanically stabilized earth (MSE) walls 314 7.8 Shotcrete walls 316 7.9 Internally braced excavations 317 7.10 Earth anchors 320 7.10.1 Grouted anchors 321 7.10.2 Helical anchors 324 7.10.3 Soil nails 327 7.10.4 Expander body anchors 329 References 331

8 Ground improvement 8.1 8.2 8.3

8.4

333

Introduction 333 Evaluating existing ground conditions and the need for ground improvement 337 Ground improvement by removal and replacement 337 8.3.1 Removal and replacement without reinforcement 338 8.3.2 Removal and replacement with reinforcement 339 Ground improvement of loose, coarse-grained soils by densification 340 8.4.1 Surface rolling by vibratory rollers 340 8.4.2 Surface impact rollers 344 8.4.3 Rapid Impact Compaction (RIC) 348 8.4.4 Deep Dynamic Compaction (DDC) 352 8.4.5 Deep vibro-compaction/vibroflotation (VC) 360

Contents

xiii

8.4.6 Deep Explosive Compaction (DEC) 365 8.4.7 Compaction grouting (CG) 369 8.5 Ground improvement of soft, fine-grained soils by consolidation 370 8.5.1 Consolidation by surcharge preloading 370 8.5.2 Consolidation by surcharge preloading with wick drains 371 8.5.3 Consolidation by vacuum preloading 373 8.6 Ground improvement of fine-grained soils by admixing 374 8.6.1 Dry mixing of chemical admixtures 374 8.6.1.1 Admixing with lime 375 8.6.1.2 Lime piles 377 8.6.1.3 Admixing with cement 378 8.6.2 Deep mixing method: Soil-cement columns and soil-lime columns 379 8.6.3 Wet placement—Lime slurry injection 383 8.7 Ground improvement by reinforcement 384 8.7.1 Stone columns 384 8.7.2 Compacted sand piles 385 8.7.3 Rammed Aggregate Piers™ (RAPs) 385 8.7.4 Controlled Modulus Columns™ 388 8.7.5 Vibro-Concrete Columns 389 References 390

9 Problematic soils 9.1 9.2

9.3

9.4

401

Introduction 401 Highly compressible organic soils 402 9.2.1 Occurrence of organic soils 402 9.2.2 Identification of organic soils 403 9.2.3 Mitigation practices for organic soils 406 Liquefiable coarse-grained soils 406 9.3.1 Occurrence of liquefiable coarse-grained soils 406 9.3.2 Identification of liquefiable coarse-grained soils 407 9.3.3 Mitigation practices for liquefiable coarse-grained soils 410 Collapsible soils 410 9.4.1 Occurrence of collapsible soils 411 9.4.2 Identification of collapsible soils 413 9.4.2.1 Indirect methods 413 9.4.2.2 Direct methods 417

xiv

Contents

9.4.3 Collapse “Sensitivity” 424 9.4.4 Mitigation practices for collapsible soils 424 9.5 Expansive soils 425 9.5.1 Occurrence of expansive soils 425 9.5.2 Identification of expansive fine-grained soils 428 9.5.2.1 Indirect methods 428 9.5.2.2 Free Swell Test 431 9.5.2.3 Direct methods 435 9.5.3 Swell “Sensitivity” 440 9.5.4 Shrinkage behavior 442 9.5.4.1 Linear shrinkage test to describe shrinkage behavior 444 9.5.4.2 Shrink Test 445 9.5.4.3 Shrink “Sensitivity” 446 9.5.5 Mitigation practices for expansive soils 447 9.6 Highly sensitive “quick” clays 450 9.6.1 Occurrence of “quick” clays 450 9.6.2 Identification of “quick” clays 451 9.6.3 Mitigation practices for “quick” clays 452 9.7 Dispersive soils 452 9.7.1 Occurrence of dispersive soils 452 9.7.2 Identification of dispersive soils 453 9.7.2.1 Double Hydrometer Test 454 9.7.2.2 Pinhole Dispersion Test 455 9.7.3 Mitigation practices for dispersive soils 457 9.8 Corrosive soils 457 9.8.1 Introduction 457 9.8.2 Laboratory tests for corrosion potential 458 9.8.2.1 pH tests 458 9.8.2.2 Electrical resistivity 458 9.8.3 Corrosivity rating 459 9.9 High-sulfate soils 460 9.10 Unusual or special soil conditions 460 References 461

10 Inspection of geoconstruction 10.1 10.2

Introduction 471 Inspection of shallow foundation construction 472 10.2.1 Verify footing locations, dimensions, and elevations 472 10.2.2 Evaluate subgrade 472

471

Contents

xv

10.2.3

10.3

10.4

10.5

10.6

10.7 10.8

Index

Observation of placement of reinforcement and concrete 472 10.2.4 Quality assurance of concrete 472 10.2.5 Observation of backfill placement 473 10.2.6 After construction 474 Inspection of driven piles 474 10.3.1 Before installation 474 10.3.2 During installation 474 10.3.3 After installation 475 Inspection of drilled shafts, bored piles, and micropiles 475 10.4.1 Contractor setup 475 10.4.2 Shaft excavation 476 10.4.3 Reinforcement and concrete placement 476 10.4.4 Post-installation record 477 Inspection of helical piles 477 10.5.1 Before installation 478 10.5.2 During installation 479 10.5.3 After installation 480 Inspection of ground anchors 481 10.6.1 Contractor setup 481 10.6.2 Anchor-hole excavation 481 10.6.3 Reinforcement and concrete placement 481 10.6.4 Post-installation record 481 Inspection of ground improvement 482 Inspector’s toolkit 483

485

About the Author

Alan J. Lutenegger has more than 40  years of practical experience in geotechnical engineering and has been a professor at the University of Massachusetts for almost three decades. He has researched in situ testing and geotechnical site investigations, behavior of deep and shallow foundations, ground improvement, collapsible and expansive soils, helical piles and helical anchors, and problematic soils. He is an instructor for the National Highway Institute, FHWA, for professional training courses on design of shallow foundations and geotechnical subsurface investigations. He is a Professional Registered Engineer and has been an expert witness on a number of litigations involving ground movements and damage.

xvii

Chapter 1

Soils—The nature of ground; Soil properties and soil behavior

1.1 THE NATURE AND COMPOSITION OF SOILS

1.1.1 Types of soils Soils consist of the assemblage of small particles of mineral matter, usually disintegrated particles derived from rock deposits. The uniqueness of soils relative to other civil engineering materials is that they are naturally formed from geologic processes and they are a composite material, made up of solid mineral matter, water, and sometimes air. Soil deposits generally fall into one of two principal categories, depending on their formation: (1) Transported Soils, and (2) Residual Soils. 1.1.1.1 Transported soils Transported Soils are those soils whose particles are produced in one location and then transported by various geologic processes and deposited in another location. An example would be mid-continental glacial deposits in the U.S. that were produced in Canada during the Ice Age and then transported by glaciers to the Midwest. As the ice melted and retreated, the materials were deposited in their present location, leaving engineers to deal with this material as it currently exists. The geologic processes most responsible for Transported Soils include movement by wind (eolian deposits), water (alluvial, marine, and lacustrine [lake bed] deposits), ice (glacial deposits), gravity (colluvial, talus, and slope wash deposits), and combinations of these processes. Because the original composition of a soil can vary greatly as a result of variations in the source or parent material and because the transport processes can vary over time, the range of resulting soil deposits can also vary widely. Transported Soils generally do not reflect compositional characteristics of the underlying bedrock, especially if the transport distance has been on the order of 10s or 100s of miles. Transported Soils also can undergo post-depositional changes, i.e., alteration or weathering after deposition (Lutenegger 1995). Post-depositional 1

2

Soils and Geotechnology in Construction

processes that act on geologic deposits can include periodic freeze-thaw cycles, fluctuating ground-water levels, erosion, infiltration of water from rainfall and snow-melt, changes in water chemistry, oxidation, leaching, etc., as illustrated in Figure 1.1. These processes can alter the freshly deposited material with different intensity over time to create the current deposit that must be managed. Depending on the mechanisms involved and the time, the alteration of a fresh geologic deposit may be slight—for example, the leaching of carbonates—or it may be severe—for example, the complete weathering of individual particles into a new material.

Figure 1.1 Post-depositional weathering processes for transported soils. (From Lutenegger, A.J., Transport. Res. Rec., 1479, 61–74, 1995. Reproduced with permission from SAGE.)

Soils—The nature of ground; Soil properties and soil behavior

3

1.1.1.2 Residual soils Unlike Transported Soils, Residual Soils are those soils which are formed in place as a result of chemical and physical weathering of the underlying bedrock or weathering of an underlying geologic deposit. The resulting deposit derives much of its compositional characteristics directly from the underlying material. Because of the wide variety of bedrocks and other deposits and their composition, residual soils can be highly variable over short distances, especially if the underlying material is also highly variable at a site. One of the most important regions in the continental U.S. containing Residual Soils developed from bedrock is the Piedmont Physiographic Region of the eastern and southeastern U.S. that extends from about Philadelphia, Pennsylvania in the North to south of Atlanta, Georgia in the South. Residual Soils occur in many parts of the world: in Asia, South America, and Africa in moderate-to-temperate climates. In certain areas, some Residual Soils are sometimes referred to as Tropical Soils since they are the product of intense weathering in climates of high rainfall and warm temperatures, which tend to accelerate the weathering process. Most Residual Soil deposits show a gradual vertical transition with depth, from a highly weathered material at or near the ground surface to the underlying unweathered bedrock. This transition may occur over a few feet or several 10s of feet depending on the location and the nature and composition of the underlying bedrock and the degree of alteration. A classical schematic description of this transition is illustrated in Figure 1.2.

Figure 1.2 Transition from bedrock to residual soil: (a) metamorphic rocks and (b) intrusive igneous rocks. (From FHWA, FHWA-NHI-06-088, 1, 2006.)

4

Soils and Geotechnology in Construction

Figure 1.3 Exposure of residual soil in a fresh cut.

Residual Soils tend to be highly variable over short vertical and horizontal distances, often making their geotechnical characterization a challenge. Figure 1.3 shows a fresh cut exposing residual soil. Another type of Residual Soil commonly encountered throughout the upper midwestern U.S. and many parts of the world and is the product of in-place weathering of glacial till deposits. Over time and exposure to the atmosphere after deposition, the surficial glacial till weathered in between successive glacial advances and became significantly altered from its original state. The product of this in-place weathering is essentially a soil, but since it is usually buried by subsequent younger geologic deposits, it is referred to as a “paleosol,” which means “ancient soil.” It is often exposed in excavations or side slopes and can create problems, especially if it is composed of highly expansive clay minerals. The types of soils that might be encountered at any particular site depend on the location, geologic history, and topography. The soil deposits at an individual project site may be quite uniform, or they may be highly variable. The purpose of site investigations (discussed in Chapter 2) is, in part, to determine the types of soils at a site and their degree of uniformity, i.e., to uncover what is covered up.

1.1.2 Fine-grained soils vs. coarse-grained soils For geotechnical work, it is important to be able to make the distinction between Coarse-Grained Soils and Fine-Grained Soils because their

Soils—The nature of ground; Soil properties and soil behavior

5

engineering behavior is very different and controlled by different factors. Additionally, their compressibility and load-carrying capacity can be very different when water is available. Coarse-Grained Soils are an assemblage of particulate materials that act individually but also as a group, depending on confinement, stress conditions, cementation, and other factors. FineGrained Soils are dominated by very small particles with large surface area and unbalanced electrical charges so that their behavior is largely influenced by microscopic surface phenomena, as will be described in later chapters. In general, the distinction between Coarse-Grained Soils and FineGrained Soils is based on the No. 200 sieve, which has an opening size of 0.074 mm. If a soil has 50% or more of the particles (by mass) larger than, or retained on, the No. 200 sieve, it is described as “Coarse-Grained”; typically, this includes sands and gravels. If a soil has 50% or more by mass that is finer than, or passes through, the No. 200 sieve, it is usually described as “Fine-Grained”; this includes silts and clays. Most of the individual particles of a Coarse-Grained Soil can be seen with the naked eye or perhaps a small magnifying lens, but the individual particles of a Fine-Grained Soil are so small that they can usually only be seen with a microscope. Since Coarse-Grained Soils, such as sands and gravels, develop engineering behavior as particulate materials, this means that individual grain characteristics influence engineering behavior. These characteristics include: mean grain size; grain-size distribution, grain shape (e.g., rounded vs. angular); and grain hardness (e.g., mineralogy). These characteristics, which are essentially compositional—in combination with particle packing, i.e., voids ratio and confining stress—account for the majority of variation in engineering behavior of Coarse-Grained Soils. To some degree, the stress history will also influence engineering behavior. In contrast, Fine-Grained Soils, such as silts and clays, develop engineering behavior based largely on the amount and type of clay minerals present. For the most part, the silt particles of Fine-Grained Soils are simply just very, very, very fine sand particles, similar to larger sand grains— e.g., quartz—but were small enough to pass through the No. 200  sieve. These silt particles are generally not electrically charged and are sometimes referred to as “surface dead,” and they have no inherent attraction for other soil particles or for water. Since clay minerals are negatively charged and have an affinity for water and other soil particles, the behavior of FineGrained Soils will vary with the amount (%) and type of clay in the soil and the amount and nature of the water. The amount and type of clay minerals influences the surface area, mineralogy, surface charge, and cation exchange capacity, all of which can be considered as compositional factors. In addition, the most important factor that controls the behavior of Fine-Grained Soils, other than composition and water content, is stress history. Table 1.1 summarizes the most important variables influencing the development of engineering behavior of Coarse-Grained and Fine-Grained soils.

6

Soils and Geotechnology in Construction

Table 1.1 Principle factors influencing the development of engineering behavior of soils Coarse-Grained Soils

Fine-Grained Soils

Parameter

Parameter

Composition

Mean Grain Size Grain-Size Distribution Grain Mineralogy Other Constituents (e.g., Carbonates, etc.)

Composition

State

Voids Ratio Relative Density State Parameter Stress History

State

Clay Fraction Clay Mineralogy Specific Surface Cation Exchange Other Constituents (Carbonates, etc.) Water Content Plasticity Liquidity Index Stress History

1.2 COMPOSITE NATURE OF SOILS —PHASE RELATIONSHIPS Soils can be thought of as a composite material consisting of three different components or phases: (1) solid soil mineral matter; (2) water; and (3) air. Sometimes a soil consists of only the solid and water components and there is no air, i.e., all of the space between solid particles is occupied by water—for example, below the water table. It is important that we be able to quantify the different amounts of each of these components in any given soil, and therefore, we need specific definitions for both mass amounts and volume amounts of each component. Figure 1.4 shows a cross-section of a soil block of unit volume and illustrates the way in which we can visualize the different components in the soil. Note that this discussion is related only to mass and volume and does not consider the different constituents within the solid phase (such as mineralogy) that might influence soil behavior.

1.2.1 Phase relationships based on mass 1.2.1.1 Water content The only relationship between the various phases of soil based solely on mass composition is the Water Content, defined as the mass of water divided by the mass of solids, expressed as a percentage: Water Content = W = (Mw/Ms) × 100% where: Mw = Mass of water Ms = Mass of solids

(1.1)

Soils—The nature of ground; Soil properties and soil behavior

7

Figure 1.4 Phase composition of soil.

There are several methods for determining Water Content, the most common being to use a convection oven in the laboratory to drive off all the water. A known mass of wet soil is placed in the oven and allowed to dry for 24 hrs. After drying, the mass of dry soil is determined. The difference in mass before and after drying represents the water loss and the mass after drying represents the dry mass of soil. Based on Equation 1.1, the range of Water Content of soils could be from 0% for a completely oven-dry soil to well over 100% for a very wet soil. In the field, there is really no such thing as a truly “dry” soil, although the term “dry” is often used when creating a soil description. Because the definition is based on the ratio of two quantities, if the mass of solids is very low— for example, as with highly organic soils below the water table—the Water

8

Soils and Geotechnology in Construction

Content could easily be 400% or higher. For most soils, however, the natural Water Content typically ranges from about 5%–10% for soils in dry desert or arid environments to about 100%–120% for offshore marine deposits. Water Content is one of the most useful quantities for soils and should be routinely measured on all samples taken on projects. Water Content of Fine-Grained Soils is used to place them in the context of plasticity, i.e., Liquidity Index, as will be discussed later in this chapter.

1.2.2 Phase relationships based on volume As previously noted, and shown in Figure 1.4, a unit volume of soil is made up of either the solid phase and the non-solid or void phase. The void phase can have either air or water or a combination of these two components. There are several useful relationships based on the volumes of the various phases present in a given soil that can be defined: 1.2.2.1 Voids ratio The Voids Ratio, traditionally noted as e, is defined as the volume of voids divided by the volume of solids: Voids ratio = e = VV/VS

(1.2)

where: V V = Volume of voids VS = Volume of solids The Voids Ratio is always expressed as a decimal, usually to 3  decimal places. Loose soils have a high Voids Ratio; compact or dense soils have a low Voids Ratio. Typical ranges are from about 0.300 to 1.200 for different soils, although highly organic soils can have a Voids Ratio of 4.000 or higher. 1.2.2.2 Porosity Porosity, a term generally used more by geologists than engineers, is defined as the volume of voids divided by the total volume, expressed as a percentage: Porosity = n = (Vv/Vt) × 100%

(1.3)

where: Vv = Volume of voids Vt = Total volume Porosity can only vary from 0% for a solid mass of material (like a steel block) to 100% for air, but the practical range for most soils is somewhere

Soils—The nature of ground; Soil properties and soil behavior

9

between about 10%–60%. Using Equations 1.2 and 1.3, we may develop a relationship between Voids Ratio and Porosity: n = e/(1 + e)

(1.4)

1.2.2.3 Saturation Saturation, like Water Content, helps describe how much water is present in the soil, but in this case expressed in terms of volume. Saturation is expressed as a percentage and describes what percentage of the void space (i.e., the volume of the non-solid phase) is filled with water. Saturation is defined as: Saturation = S = (Vw/Vv) × 100%

(1.5)

where: Vw = Volume of water Vv = Volume of voids For a completely oven-dry soil with W = 0%, S = 0%. For a soil in which all of the void space is completely filled with water, S = 100%. Therefore, the entire range of saturation is theoretically from 0%–100%, but a more practical range of natural and compacted soils in the field is probably somewhere between about 20% and 100%, because a soil in the field is never completely dry, as previously described. Generally, it can be assumed for practical purposes that soil samples obtained from below the groundwater table are saturated and S = 100%. If S = 100%, we say that the soil is “saturated”; if S  6 CC > 4

CU = 1–3 CU = 1–3

Soils—The nature of ground; Soil properties and soil behavior

17

Table 1.5 Grain-size characteristics of the two sands in Figure 1.5 Soil

D50 (mm)

CU

CC

Fines (%)

Gradation

FHWA Sand DOE Sand

0.31 0.85

1.14 0.86

2.06 6.47

2.5 2.7

Uniform Well-Graded

Table 1.6 Descriptive terms for different soil particle sizes Soil Coarse-Grained

Fine-Grained

Particle Size (mm)

Descriptive Term

>12 in. (305 mm) 3 in.–12 in. (76 mm–305 mm) ¾ in.–3 in. (19 mm–76 mm) No. 4 Sieve–¾ in. (4.75 mm–19.0 mm) No. 10–No. 4 Sieve (2 mm–4.75 mm) No. 4–No. 10 Sieve (0.42 mm–2 mm) No. 200–No. 10 Sieve (0.075 mm–0.042 mm) 0.075–0.002 mm   1), the value of KO is related to both the Normally Consolidated KO value as well as the OCR, as shown in Figure 1.20. One convenient approach presented by Mayne and Kulhawy (1982) suggests that the overconsolidated value of KO, KO(OC), may be obtained from: KO(OC) = KO(NC) (OCR)sin φ′TX

(1.28)

Early work had suggested that there was some variation in the value of KO(NC) for different Fine-Grained Soils, as might be expected if the value of φ′TX varies. Some early correlations had suggested that KO(NC) was related to the Plasticity Index or the Liquid Limit; however, there now appears to be considerable scatter in such a correlation. It has been

40

Soils and Geotechnology in Construction

Figure 1.20 Variation in KO with stress history. (Courtesy of Mayne, P. and Kulhawy, F., Journal of the Geotechnical Engineering Division, ASCE, 108, GT6, pp. 851–872, 1982.)

suggested (Kulhawy and Mayne 1990) that for many Fine-Grained Soils, a value of φTX′ = 30° provides a reasonable fit to many lab test results. Using this value gives: KO(NC) = 1 − sin φTX′ = 1 − 0.5 = 0.5

(1.29)

KO(OC) = 0.5(OCR)0.5

(1.30)

and

Values of KO(OC) obtained from Equation 1.29 are based on simple mechanical unloading. Some natural soils have very complex loading history, and therefore, it is likely that values obtained using Equation 1.29 will represent lower bound values. Real soils may have values much higher depending on the mechanism of overconsolidation. It is also likely that values of KO(NC) will be slightly higher for soils with higher plasticity. 1.6 SHEAR STRENGTH OF SOILS The shear strength of soils is a complex issue and the details are well beyond the scope of this book, but it is useful to review some of the basic principles of shear strength. Differences between the behavior of Coarse-Grained and

Soils—The nature of ground; Soil properties and soil behavior

41

Fine-Grained Soils and drained versus undrained strength are fundamental to soil behavior and performance of constructed facilities. It should also be kept in mind that shear strength parameters are not unique soil properties but depend on a number of factors, including strain rate, stress path to failure, stress level, anisotropy, scale effects, and aging. In general, a simple way to consider the shear strength of soils is to separate the behavior into drained and undrained behavior and to separate the soils into coarsegrained and fine-grained.

1.6.1 Drained shear strength Soils with high permeability that are freely draining when loaded, such as sands and gravels, are generally treated as exhibiting drained behavior. Drained shear strength parameters are also used when loads are applied very slowly over long periods of time, and any Pore Water Pressure generated in a saturated soil is allowed to dissipate. The shear strength is a function of the applied normal stress and often described using a linear approximation, as shown in Figure 1.21. The failure envelope, which describes the state of stresses at failure, is called the Mohr-Coulomb failure envelope and is somewhat analogous to sliding friction between materials, in this case, soil-to-soil. The envelope is defined using two components as: τ = c′ + σ′tan ϕ′

(1.31)

where: τ = shear strength c′ = cohesion intercept φ′ = friction angle of the soil The prime designated for cohesion and friction angle indicates effective stress (drained) conditions. Not all soils have both components of shear strength indicated in Equation 1.30. The cohesion intercept of the Mohr-Coulomb failure envelope describes the available shear strength when the effective normal stress

Figure 1.21 General Mohr-Coulomb approximation to shear strength envelope for soil.

42

Soils and Geotechnology in Construction

is zero. Many Coarse-Grained Soils, such as dry or saturated clean sands, exhibit zero cohesion, that is, there is nothing holding them together when the effective normal stress is zero and the shear strength is simply defined as: τ = σ′tan ϕ′

(1.32)

When the sand is moist, the water film between particles produces a force which tends to hold particles together for a short period of time and gives a small positive cohesion intercept. But this force disappears when the sand is dry or when the sand is saturated. It is often called apparent cohesion since it is not a true permanent bond between particles. Apparent cohesion is the phenomenon that allows us to build sand castles on the beach out of moist sand. The shear strength of Coarse-Grained Soils really consists of two components (Bolton 1986) and is often described from: ϕ′P = ϕ′CV + ψ(DR, σ0 )

(1.33)

where: ϕ′P = peak friction angle ϕ′CV = constant volume or critical state friction angle ψ = dilatancy component (which is a function of Relative Density and stress level) The value of ϕ′CV is predominantly a function of the mineralogy of the sand grains and the grain-size distribution and can be estimated approximately from the angle of repose of a sand pile, whereas ψ depends on the Relative Density and mean stress level. Equation 1.32 indicates that the degree by which the peak friction angle exceeds the constant volume friction angle depends on the dilational characteristics of the material; loose sands having less dilational tendency than dense sands. Table 1.14 lists a number of factors than can influence the shear strength behavior of Coarse-Grained Soils. Drained shear strength behavior can also be exhibited by Fine-Grained Soils, such as silts and clays. When loaded very slowly or when pore water

Table 1.14 Factors that influence the shear strength of coarse-grained soils Compositional Factors Mineral Type Mean Grain Size Grain-Size Distribution Cementation (e.g., carbonates)

State Factors Relative Density Stress Level Stress Path Test Type (e.g., Triaxial Compression, Direct Shear, Plain Strain)

Soils—The nature of ground; Soil properties and soil behavior

43

pressure is allowed to dissipate, the drained shear strength can usually be described by Equation 1.30, and these soils often contain both strength components, i.e., cohesion and friction.

1.6.2 Undrained shear strength Saturated clays and other Fine-Grained Soils of low permeability essentially are considered to exhibit undrained shear behavior when loaded quickly and there is no time for excess pore water pressures to dissipate. Historically, this behavior was often expressed as the ϕ′ = 0 behavior so that the undrained shear strength was only defined by the cohesion intercept. As noted, undrained shear strength is not a unique soil property, but depends on a number of factors. There are many laboratory and field methods used to determine undrained shear strength, each of which gives different values of strength. Figure 1.22 shows the difference in undrained shear strength (including remolded) obtained from different test methods in soft clay. Instead of considering the absolute undrained shear strength of saturated Fine-Grained Soils, it is more convenient to think in terms of the “normalized” undrained strength, i.e., Su σ′VO . Most sedimentary clays

Figure 1.22 Undrained shear strength of clay from different test methods. (After Mayne, P. et al., State-of-the-Art paper: GeoMaterial behavior and testing, Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering, IOS Press, 2777–2872, 2009. Reproduced with permission from IOS Press.)

44

Soils and Geotechnology in Construction

exhibit normalized behavior and the undrained shear strength is a function of the stress history through the Overconsolidation Ratio, OCR: Su σ′VO = S(OCR)m

(1.34)

where: S = Normally Consolidated value of Su σ′VO m = exponent that describes the rate of increase in normalized strength with OCR For many clays, a reasonably accurate value of the normally consolidated normalized strength can be taken as S = (Su σ′VO )NC = 0.23. The upper bound value of m is 1.0; however, experimental data have shown that a reasonable value of m may be taken as m = 0.8 which gives: Su σ′VO = 0.23(OCR)0.8

(1.35)

Values of S and m vary somewhat with plasticity, test method, and strain rate. Equation 1.34 may also be simplified and stated in terms of the preconsolidation stress, σ′P as: Su = 0.23 × σ′P

(1.36)

Values of undrained shear strength estimated from Equations 1.34 or 1.35 should be checked against site-specific laboratory tests conducted on highquality undisturbed samples or appropriate in situ tests.

1.6.3 Sensitivity of fine-grained soils An important behavioral property of some Fine-Grained Soils is the loss of undrained strength when disturbed. This loss is quantified by the term Sensitivity, St, defined as the ratio of undisturbed to fully remolded undrained shear strength at the same water content: St = sUU/sUR

(1.37)

where: sUU = undisturbed undrained shear strength sUR = fully remolded undrained shear strength The remolded undrained shear strength is used as the reference point, since it represents the lowest possible strength that a Fine-Grained Soil could have. Sensitivity is typically measured in soft soils using the field vane test, but in stiffer soils, it may also be measured using undisturbed and remolded soils (at the same water content and void ratio) using the lab vane, fall cone, or

Soils—The nature of ground; Soil properties and soil behavior

45

Table 1.15 Definitions of sensitivity Sensitivity

Classification

St  16

Insensitive Clays Clays of Low Sensitivity Clays of Medium Sensitivity Sensitive Clays Extra-Sensitive Clays Quick-Clays

Source: Skempton, A.W. and Northey, R.D., Geotechnique, 3, 30–53, 1952.

other lab undrained strength tests, such as Unconfined Compression Tests. Table 1.15 gives suggested descriptive terms for use with different ranges of measured Sensitivity. Sensitivity of natural soils can range from about 1 for insensitive stiff clays to over 1000 for highly sensitive marine clays that essentially have close to zero undrained shear strength in the remolded state. This indicates that they are very “sensitive” to disturbance, hence the term Sensitivity. Generally, in marine clays such as those which occur throughout the Scandinavian countries and southern Ontario and Quebec in Canada—where the sensitivity is in part the result of leaching of some of the salts by fresh-water infiltration after deposition—the Sensitivity has sometimes been related to Liquidity Index. As the Liquidity Index increases, the Sensitivity also increases. However, in other non-marine clay deposits, there is much more scatter in such a relationship, as Sensitivity is developed from a breakdown of natural soil structure and not the product of post-depositional alteration of the composition, and there appears to be no unique relationship between Sensitivity and Liquidity Index. Soils can also have an Acquired Sensitivity; that is, they may develop Sensitivity after being remolded and then allowed to age, as in compacted clay.

1.6.4 Thixotropy of fine-grained soils Another important characteristic of most Fine-Grained Soils is the ability to regain some strength after disturbance. The regain in strength is timedependent and depends on the mineralogy and soil composition. Generally, the regain in strength will not reach the original undisturbed strength but may approach this value after a very long (geologic) time. Figure 1.23 illustrates the regain in strength of two Fine-Grained Soils with elapsed time after initial remolding. 1.7 HYDRAULIC CONDUCTIVITY An important characteristic of soils that may impact some construction projects is hydraulic conductivity, the rate at or ease with which water will flow through soil. Water flowing into excavations or into the bottom of drilled

46

Soils and Geotechnology in Construction

Undrained Strength (kPa)

14

Boston Blue Clay Onsoy Norway Clay

12 10 8 6 4 2 0.1

1

10

100

1000

10000

Aging (Hours) Figure 1.23 Thixotropic behavior of two fine-grained soils.

shaft holes may impact the quality of construction and can be a source of additional construction cost for a project. There are a number of laboratory and field methods to determine hydraulic conductivity which generally involve measuring the volume of water that will flow under specific conditions of head through a soil sample of known dimensions or geometry. Table 1.16 gives some typical values of laboratory-measured hydraulic conductivity. Some soils exhibit hydraulic conductivity anisotropy, having different flow characteristics in the horizontal direction as compared to the vertical direction. Horizontal or radial hydraulic conductivity may be important for certain projects or problems, as in the case of radial flow of groundwater to wells or wick drains. Horizontal hydraulic conductivity may also be important in analyzing slope stability problems. Natural hydraulic conductivity anisotropy has been reported for many clays and other Fine-Grained Soils. Macro-features, such as fissures, joints, or layering within a soil may also Table 1.16 Typical values of hydraulic conductivity for different soils Soil Type Uniform Coarse Sand Uniform Fine Sand Loess Connecticut Valley Varved Clay Leda Clay Nebraska Alluvial Clay Iowa Glacial Till

Hydraulic Conductivity (cm/sec.) 1.5–3.0 × 100 1.5–2.9 × 10−2 3.1 × 10−4 1.0 × 10−6–1.7 × 10−7 1.5 × 10−7–3.8 × 10−8 1.1 × 10−7–1.6 × 10−8 1.4 × 10−7–1.7 × 10−8

Soils—The nature of ground; Soil properties and soil behavior

47

Table 1.17 Reported hydraulic conductivity anisotropy Soil Varved Clay Glacial Till Loess Marine Clay Boston Blue Clay

rk = kh/kv

References

2−10 0.9–2.2 0.5–2.6 0.7–1.4 0.7–4.0

Chan and Kenney (1973) Wu et al. (1978) Lumb and Holt (1968) Olsen (1962) Haley and Aldrich (1969)

produce differences in hydraulic conductivity in the horizontal and vertical directions. For example, the presence of varves in lacustrine deposits produces anisotropy with horizontal hydraulic conductivity being greater than vertical hydraulic conductivity. Induced anisotropy may occur in compacted clays or clays subjected to consolidation if there is preferred orientation of the clay particles in the direction perpendicular to the compaction or applied vertical stress. The ratio of horizontal to vertical hydraulic conductivity is defined as the anisotropy ratio, rk = kn kv (Chapuis and Gill 1989). Typical ranges of values for different soil types are provided in Table 1.17. 1.8 SOIL IDENTIFICATION, DESCRIPTIONS, AND CLASSIFICATION For most geo-construction and any work involving soils, it is important to have accurate and consistent descriptions of the soils at a site. Disputes that arise between the contractor and engineer often are related to the correct identification of a particular soil layer or deposit based on the description given in the site investigation or engineering recommendations.

1.8.1 Soil classification It is important that engineers have a common language with which to talk about soils to avoid confusion with the wrong soil being allowed on a site or the wrong soil used as a foundation layer. Of course, there are already several different classification systems available that make use of the results of laboratory tests, such as Grain-Size Distribution and Atterberg Limits, to classify soils into specific categories based on their measured properties and expected behavior. However, there are many instances when projects do not have the luxury of having laboratory results available for every part of the soil to be used or encountered. This means that the engineer must rely on descriptions of the soil to avoid potentially costly mistakes. Soil Classification is the systematic grouping of soils using specific properties that give an indication of the anticipated engineering behavior.

48

Soils and Geotechnology in Construction

Two common classification systems used in geotechnical engineering are the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials System (AASHTO). The USCS is most often used by geotechnical engineers in the private sector, while the AASHTO system is used by most state highway departments. Both of these systems, which are very useful, require the results of laboratory tests in order to classify soils. They both require Atterberg Limits (Liquid and Plastic Limits) and Grain-Size Distribution. In many situations, these results are not available during field work, which means that drillers, engineers, and contractors cannot actually classify soils in the field. Naturally, they could guess at the classification, but without the results of the laboratory tests, they could not verify their guess. For this reason, it is useful to have a simple and reliable system to help quickly identify soil constituents in the field and be able to prepare a written description of the materials for the record. One approach that has been developed is a scheme of identification and description development based strictly on inspection of the materials and a few simple field procedures, without performing laboratory tests. A system that is being used by the Federal Highway Administration and many state highway departments and contractors for field personnel is the Modified Unified System of Soil Descriptions. This system essentially uses the terms from the USCS without the laboratory test results to develop a description or word picture of soils. This scheme is therefore not a classification scheme (which requires laboratory test results), but a scheme to identify the various materials present and the describe what has been identified.

1.8.2 Soil identification and descriptions—The modified unified soil description system For many years, the Unified Soil Classification System (USCS) has been used successfully by practicing geotechnical engineers to classify soil samples. The major advantage of this system is the easily understood word picture used to describe soil samples for classification. The major disadvantage of the USCS is the number of time-consuming laboratory tests (Grain-Size Distribution and Atterberg Limits) that must be performed in order to develop the classification. At present, numerous private firms and state agencies are using the nomenclature of the Unified Soil Classification System without the classification testing to describe soils in the field. This process of visually identifying and describing soil samples is known as the Modified Unified Description system or MUD. The procedure involves visually and manually examining soil samples with respect to color, texture, and plasticity and provides a method for building a “word picture” of a sample for entering on a subsurface exploration log or other appropriate data sheet. The procedure can be applied to soil descriptions made either in the field or in the laboratory, as it does not

Soils—The nature of ground; Soil properties and soil behavior

49

rely on laboratory test results. It should be understood that soil descriptions developed using the MUD procedure are based upon the judgment of the individual making the descriptions and are therefore somewhat subjective. Results of classification tests are not intended to be used to verify the MUD description, but to provide further information for analysis of soil design problems or for possible use of the soil as a construction material. The intent of this method is to describe only the constituent soil sizes that have a significant influence on the visual appearance and behavior of the soil. The description system is intended to provide the best visual description of the sample for those involved in the planning, design, construction, and maintenance of geo-related facilities. The MUD system requires a simple series of decisions based on simple soil characteristics, such as: Primary Characteristic 1. Composition 2. Color 3. Consistency 4. Moisture

Secondary Characteristic 5. Plasticity 6. Structure 7. Particle Shape 8. Other Descriptors (as needed)

A detailed procedure for the MUD system used by FHWA is given in Appendix A. Table 1.18 gives a summary of the basic approach to identifying and describing soils in the field. Table 1.18 Basic field soil identification and descriptions for site investigations No. 200 Sieve

Primary Soil Type

>50% Passing

Fine-Grained Soil