Soil Mechanics
Principles and Practice
G. E. Barnes
MACMILLAN ©G.E.Barnes 1995 Acknowledgements
All rights reserved. No reproduction, copy or transmis- Extracts from British Standards are reproduced with sion of this publication may be made without written the permission of BSL Complete copies can be ob- permission. tained by post from BSI Sales, Linford Wood, Milton Keynes, MK146LE. Tables 13.1 and 13.7 are repro- No paragraph of this publication may be reproduced, duced with the permission of the Controller of Her copied or transmitted, save with written permission or Majesty's Stationery Office. Material from ASTM in accordance with the provisions of the Copyright, publications is reproduced with their permission. Full Designs and Patents Act 1988, or under the terms of versions can be obtained from American Society for any licence permitting limited copying issued by the Testing and Materials, 1916 Race Street, Philadelphia, Copyright Licensing Agency, 90 Tottenham Court Pa. 19103-1187, USA. Figure 12.10 is reproduced Road, London W1P9HE. with permission from the Boston Society of Civil Engineers Section, American Society of Civil Engi- Any person who does any unauthorised act in relation neers. The Journal of the Boston Society of Civil to this publication may be liable to criminal prosecu- Engineers is now known as 'Civil Engineering Prac- tion and civil claims for damages. tice'. Figures5.6,9.7,9.18,9.19,10.9and lO.lOhave been reproduced with permission from the American First published 1995 by Society of Civil Engineers. Figures 5.2, 8.19, 8.20, MACMILLAN PRESS LTD 10.9,10.10,10.11,10.20,12.4,12.17,13.11 andTable Houndmills, Basingstoke, Hampshire RG21 6XS 9.7 have been reproduced with permission from John and London Wiley and Sons, Inc., New York. Figure 3.15 is repro- Companies and representatives throughout the world duced with the permission of McGraw-Hill, Inc. Fig- ure 5.8 is reproduced with the permission of Engineer- ISBN 978-0-333-59654-8 ISBN 978-1-349-13258-4 (eBook) ing Publications Office, University of Illinois. Figure DOI 10.1007/978-1-349-13258-4 7.13 is reproduced with the permission of The Royal Society and Professor P.W. Rowe. Figure 9.1 is repro- A catalogue record for this book is available from the duced with the permission of Transportation Research British Library. Board, National Research Council, Washington, D.C. Figure 9.5 is reproduced with the permission of Mr. 10 987654321 F.G. Butler. Figures 10.15,10.16,10.18 and 10.19 are 04 03 02 01 00 99 98 97 96 95 reproduced with the permission of Dr T. Whitaker.
The author wishes to record his thanks to all of the other organisations who have granted permission to use material from their sources. Contents
Preface viii Soil Classification List of symbols IX Introduction 18 Note on units xiii Particle density 19 Particle shape 19 Particle size distribution 20 1 Soil Formation and Nature Density 22 Moisture content 24 Consistency and Atterberg limits Soil Formation 25 Plasticity chart Introduction 1 27 Man-made soils 1 Activity 28 Shrinkage limit Contaminated and polluted soils 1 28 Soil model Naturally-occurring soils 1 29 Worked Examples In situ soils - weathered rocks I 32 Exercises In situ soils - peat 3 35 Water-borne soils 3 Glacial deposits 3 Wind-blown soils 4 3 Permeability and Seepage Soil Particles Nature of particles 5 Permeability Clay minerals 5 Introduction 36 Groundwater 36 Soil Structure Flow problems 36 Introduction 8 Stability problems 37 Granular soils 8 Soil voids 38 Relative density 9 Pressure and head 39 Cohesive soils 10 Darcy's Law 39 Effect of temperature 40 Empirical correlations for k 40 2 Soil Description and Classification Layered soils 42 Laboratory tests 43 Soil Description Borehole tests 46 Introduction 12 Pumping tests 51 Classification 12 Seepage Made Ground 12 Seepage theory 51 Topsoil 12 Flow nets 53 Clay 13 Flow net construction 54 Silt 14 Seepage quantities 57 Sand and gravel 15 Total head, elevation head and pressure head 57 Cobbles and boulders 16 Pore pressure and uplift pressure 58 Peat and organic soils 16 Seepage force 58 Types of description 17 Quick conditions and boiling 58
iii iv Soil Mechanics - Princivles and Practice
Piping adjacent to sheet piling 59 Worked Examples 100 Seepage through earth dams 60 Exercises 103 Seepage through flood banks, levees 60 Worked Examples 62 Exercises 68 6 Compressibility and Consolidation
Compressibility 4 Effective Stress and Pore Pressure Introduction 104 Void ratio/effective stress plot 104 Total stress 70 Reloading curves 105 Pore pressures below the water table 70 Preconsolidation pressure 105 Effective stress 70 Effect of sampling disturbance 106 Stress history 71 In situ curve for normally consolidated clay 106 Normally consolidated clay 72 In situ curve for overconsolidated clay 106 Overconsolidated clay 74 Isotropic compression 107 Overconsolidation ratio 74 Anisotropic compression 108 Desiccated crust 75 Consolidation Present state of stress in the ground 76 Terzaghi theory of one-dimensional Mohr's circle of stress 76 consolidation 109 Earth pressure at rest 76 Oedometer test 111 Changes in stress due to engineering works 79 Coefficient of consolidation 114 Pore pressure parameters 81 Rowe consolidation cell 116 Capillary rise above the water table 83 Two and three-dimensional consolidation 118 Effective stresses above the water table 84 Radial consolidation for vertical drains 119 Worked Examples 86 Worked Examples 122 Exercises 89 Exercises 128
5 Contact Pressure and 7 Shear Strength Stress Distribution General Contact pressure Introduction 130 Introduction 91 Effects of strain 130 Uniform loading 91 Failure criterion 132 Point loading 92 Stress paths 133 Stress distribution Effects of drainage 133 Introduction 92 Test procedures 136 Stresses beneath point load and line load 92 Shear strength of sand Assumptions 94 Stress-strain behaviour 139 Stresses beneath uniformly loaded areas 94 Shear box test 140 Bulbs of pressure 94 Effect of packing and particle nature 141 Stresses beneath a flexible rectangle 97 Constant volume condition 142 Principle of superposition 97 Stresses beneath flexible area of any shape 97 Shear strength of clay Stresses beneath a flexible rectangle - finite soil Effect of sampling 143 thickness 99 Undrained cohesion, Cll 143 Stresses beneath a rigid rectangle 99 Unconfined compression test 143 Contents v
Vane test 143 Worked Examples 187 Triaxial test 145 Exercises 191 Triaxial unconsolidated undrained test (UU) 146 Effect of clay content and mineralogy 148 Partially saturated clays 148 9 Shallow Foundations - Settlements Fissured clays 148 Variation with depth 149 Introduction 192 Frictional characteristics 151 Test procedures 151 Clays - immediate settlement Triaxial consolidated undrained test (CU) 151 General method 192 Triaxial consolidated drained test (CD) 153 Principle of superposition 192 Principle of layering 192 Critical state theory 153 Rigidity correction 195 Residual strength 159 Depth correction 195 Worked Examples 161 A verage settlement 195 Exercises 166 Modulus increasing with depth 195 Effect of local yielding 195 Estimation of undrained modulus 198 8 Shallow Foundations - Stability Clays - consolidation settlement Compression index Cc method 198 General Oedometer or mv method 200 Introduction 168 Clays - total settlement Spread foundations 168 Skempton-Bjerrum method 200 Design requirements 168 Elastic drained method 201 Types of shallow foundation 169 Estimation of drained modulus 203 Strip foundations 169 Pad foundations 171 Secondary compression Raft foundations 171 Introduction 203 Depth of foundations 172 General method 204 Estimation of C" or e" values 204 Bearing capacity Modes of failure 176 Sands Bearing capacity - vertical loads only 176 Introduction 205 Shape and depth factors 178 Methods of estimating settlements 206 Bearing capacity - overturning 178 Schmertmann's method 206 Eccentric loading 178 Burland and Burbridge's method 207 Inclined loading 179 Permissible settlements Different soil strength cases 181 Introduction 209 Effect of water table 181 Definitions of ground and foundation Net ultimate bearing capacity 182 movement 209 Factor of safety 182 Criteria for movements 210 Effect of compressibility of soil 183 Routine settlement limits 2 I I Sliding 184 Worked Examples 213 Allowable bearing pressure of sand Exercises 218 Settlement limit 184 Allowable bearing pressure 184 vi Soil Mechanics - Principles and Practice
10 Pile Foundations Effect of wall friction 243 Coulomb theory - active thrust 243 Single piles Coulomb theory - passive thrust 245 Introduction 220 Earth pressure coefficients 246 Loading conditions 220 Effect of cohesion intercept c' 248 Types of pile 220 Minimum equivalent fluid pressure 248 Design of single piles 221 Effect of water table 248 Load capacity of single piles 221 Undrained conditions 248 Tension cracks 250 Bored piles in clay Loads applied on soil surface 251 End bearing resistance 222 Adhesion 222 Retaining structures Introduction 251 Driven piles in clay Basement walls 252 End bearing resistance 223 Bridge abutments 254 Adhesion 223 Gabions and cribwork 254 Effective stress approach for adhesion 225 Stability of gravity walls Driven piles in sand Introduction 254 Effects of installation 226 Rotational failure 255 End bearing resistance 226 Overturning 255 Critical depth 226 Bearing pressure 255 Skin friction 227 Sliding 256 Bored piles in sand 229 Internal stability 257 Factor of safety 229 Sheet pile walls Introduction 257 Pile groups Cantilever sheet pile walls 258 Pile spacing 230 Factor on embedment method 258 Stressed zone 230 Gross pressure method 258 Load variation 230 Single anchor or propped sheet pile walls 259 Efficiency 231 Factor on embedment method 259 Ultimate capacity 232 Gross pressure method 259 Settlement ratio 233 Anchorages for sheet piling 260 Settlement of pile groups 234 Worked Examples 236 Strutted excavations Exercises 239 Introduction 261 Strut loads 261 Reinforced earth 11 Lateral Earth Pressure and Introduction 263 Design of Retaining Structures Effects of reinforcement 263 Internal stability 263 External stability 265 Lateral earth pressures Worked Examples 266 Introduction 240 Exercises 273 Effect of horizontal movements 240 Effect of wall flexibility and propping 242 Contents vii
12 Slope Stability Soil compaction Introduction 313 General Factors affecting compaction 313 Introduction 274 Field compaction Types of mass movement 274 Introduction 314 Natural slopes 274 Compaction plant 314 Artificial slopes or earthworks 274 Specification of compaction requirements 316 Short-term and long-term conditions 276 Control of compaction in the field 319 Methods of analysis Laboratory compaction Plane translational slide 280 Introduction 319 Circular arc analysis - undrained conditon 281 Laboratory tests 319 Tension crack 283 Air voids lines 321 Undrained analysis - stability charts 283 Correction for stone content 321 Effective stress analysis - method of slices 284 Worked Examples 322 Fellenius method 286 Exercises 327 Bishop simplified method 286 Pore pressure ratio ru 287 Effective stress analysis - stability 14 Site Investigation coefficients 287 Submerged slopes 290 Site investigation Rapid drawdown 290 Introduction 328 Non-circular slip surfaces - lanbu method 291 Stages of investigation 328 Wedge method - single plane 292 Desk study 328 Wedge method - multi-plane 292 Site reconnaissance 329 Factor of safety 294 Worked Examples 295 Ground investigation Exercises 300 Extent of the ground investigation 329 Depth of exploration 330 Choice of method of investigation 330 13 Earthworks and Soil Compaction Methods of ground investigation 331 Undisturbed sampling - sampling quality 335 Earthworks Types of samples 335 Introduction 303 Methods of in situ testing 339 Construction plant 303 Groundwater observations 342 Purpose and types of materials 304 Investigation of contaminated land 345 Material requirements 304 Site investigation reports Acceptability of fill 306 Introduction 345 Acceptability of granular soils 306 Factual report 346 Acceptability of cohesive soils 307 Interpretative report 348 Efficiency of earthmoving 309 Material problems 309 Answers to exercises 350 Softening 310 References 352 Bulking 311 Index 359 Preface
The main aims of this book are to provide an under• A good geotechnical engineer will have a know ledge standing of the nature of soil, an appreciation of soil of mathematics, science and be proficient with soil behaviour and a concise and clear presentation of the mechanics but a knowledge of geology, soil profiles basic principles of soil mechanics. and groundwater conditions is fundamental to the The subject of soil mechanics attempts to provide a application of soil mechanics. For this reason the book framework for understanding the behaviour of the aims to consider soil mechanics with more emphasis ground by considering the principles which apply to on its application in the ground and less emphasis on soils. The geotechnical engineer must then use judge• the behaviour of soils in the unnatural environment of ment to determine how to apply these principles in real the laboratory. situations. The book contains a range of worked examples to It is often said that soil mechanics is a 'black art' assist the learning of the subject and illustrate the because these principles may not apply universally, applications of the various analytical approaches. To and there is a considerable amount of empirical knowl• consolidate this understanding, problem exercises have edge which has been built up over the years but which been included for students to attempt themselves. still serves the engineer well. This is also probably a I am most grateful to all those researchers, writers and result of trying to apply a purely scientific approach to practising engineers who have investigated the subject a material which has not been controlled during its and collected information over the last seventy years or manufacture by human interference. Instead the ground so, without whom no standard text-book could be is a natural material, variable, unique, not fully under• written. In particular, I wish to record my thanks to stood and sometimes surprising in its behaviour. those publishers, organisations and individuals who The book is intended as a main text for undergraduate have granted permission to use material from their civil and ground engineering students to provide the publications. basic principles and to illustrate how, why and with I wish to express my gratitude to Dr Bob Saxton from what limitations these principles can be applied in Plymouth University for reviewing the draft manu• practice. It is also intended to be retained by these script and making valuable comments. I wish to thank students when they become practitioners and for pro• Professor Clive Melbourne, Head of School of Civil fessionals already in practice as a reference source Engineering at Bolton Institute for his support and providing guidance and information for the solution of encouragement. Thanks are are also due to Miss real geotechnical problems. Joanne Carney for typing the draft manuscript. It is assumed that the reader will have a basic under• Finally, my sincere thanks go to my wife, Linda, for her standing of mathematics and science and a good support and understanding during the preparation of understanding of applied mechanics. In civil engineer• the book. ing undergraduate degree courses there is often insufficient emphasis on the need to provide a sound Graham Barnes knowledge of geology and, in particular, the superfi• cial geology, in other words, the soils! This material too often gets in the way for many geologists who are mainly interested in the rocks.
Vlll List of symbols
A Activity Dr Relative density A Area E' Young's modulus in terms of effective stress A' Effective area (drained condition) A Pore pressure parameter Ep Pressuremeter modulus Ae Ash content Eu Young's modulus in terms of total stress Ab Pile base area (undrained condition) Ar Pore pressure parameter at failure E Lateral force on side of slice Ar Area ratio ESP Effective stress path As Pile shaft area e Eccentricity Av Air voids content e Void ratio a Slope stability coefficient eo Initial void ratio B % of particles passing maximum size ef Final void ratio B Width of foundation emax Void ratio at loosest state
B Pore pressure parameter emin Void ratio at densest state B Pore pressure parameter F Factor of safety, length factor B' Effective width F Force b Slope stability coefficient Fd Enlargement factor C N Correction for overburden pressure Fa Correction for roughness Cw Correction for water table FD Correction for depth of embedment Ce Compression index f Shape factor or intake factor Cc Coefficient of curvature fo Slope stability correction factor Ca Coefficient of secondary compression Is Skin friction, sleeve friction Cs Soil skeleton compressibility Is Shape factor Cs Swelling index it Correction for time Cw Compressibility of pore water it Permissible tensile strength of reinforcement CD Consolidated drained fy Yield factor CU Consolidated undrained j; Thickness factor CI Consistency index G Shear modulus CSL Critical state line Gs Specific gravity of particles Ca Adhesion g Gravitational acceleration (9.81 mls2)
Cb Adhesion at underside of foundation g Soil constant for the Hvorslev surface
Cr Remoulded undrained cohesion H Height, thickness, horizontal force Cu Undrained cohesion He Constant head above the water table Cv Coefficient of consolidation, vertical direction Ho Initial head above the water table CH Coefficient of consolidation, horizontal H, Head at time t direction h Head difference Cw Adhesion between soil and waIl he Capillary rise c' Cohesion in effective stress terms hm Mean head D Depth of foundation hp Pressure head D Depth factor of slip circle hs FuIly saturated capillary zone d Diameter, depth of penetration, particle size hw Depth to water table d Length of drainage path h, Elevation or position head do Initial depth of embedment I Influence value or factor
ix x Soil Mechanics - Principles and Practice
ICL Isotropic nonnal consolidation line OCR Overconsolidation ratio Ie Compressibility index PL Plastic limit Ip Plasticity index (or PI) PI Plasticity index (or Ip)
I z Strain influence factor P Force Hydraulic gradient Pa Resultant active thrust or force ie Critical hydraulic gradient Pp Resultant passive thrust or force ic Exit hydraulic gradient Pan Nonnal component of active thrust irn Mean hydraulic gradient Ppn Nonnal component of passive thrust J Seepage force Pw Horizontal water thrust K Absolute or specific penneability p Pressure, contact pressure Ko Coefficient of earth pressure at rest p Stress path parameter (Total stress) KoCL Ko nonnal consolidation line p' Stress path parameter (Effective stress) Ka Coefficient of active earth pressure Pc' Preconsolidation pressure Kae Earth pressure coefficient Pc' Initial isotropic stress Kp Coefficient of passive pressure Po' Present overburden pressure (Effective stress) Kpc Earth pressure coefficient Po Total overburden pressure Ks Coefficient of horizontal pressure Q Steady state quantity of flow k Coefficient of penneability Quit Ultimate load k Coefficient for modulus increasing with depth Qs Ultimate shaft load L, I Length, lever arm Qb Ultimate base load L' Effective length Q Line load surcharge LL Liquid limit q Flow rate Ll Liquidity index q Unifonn surcharge M Moment q Stress path parameter (Total stress) M Gradient of the critical state line on p'- q' plot q' Stress path parameter (Effective stress) MCV Moisture condition value qa Allowable bearing pressure m Mass qapp Applied pressure (or q) m Slope stability coefficient qb End bearing resistance mv Coefficient of volume compressibility qc Cone penetration resistance N Nonnal total force qrnax Maximum bearing pressure N Stability number qs Safe bearing capacity 2 N Specific volume at p' = 1.0 kN/m on ICL quit Ultimate bearing capacity N Standard penetration test result, No. of blows R Resultant force, distance N' Corrected SPT value R Dial gauge reading N' Nonnal effective force R Radius of influence of drain Ne Bearing capacity factor Rr Friction ratio Nq Bearing capacity factor Rs Pile group settlement ratio
Ny Bearing capacity factor R 3,Rt Time correction factors NC Nonnally consolidated RT Correction for temperature No Specific volume at p' = 1.0 kN/m2 on KoCL r Radial distance, or radius Ns Stability number rd Radius of well or drain n Porosity, number of piles ru Pore pressure ratio n Slope stability coefficient Sf Degree of saturation n Ratio R1rd SL Shrinkage limit nd Number of equipotential drops s Spacing of drains, spacing of piles, anchors nf Number of flow paths s Stress path parameter (Total stress) Oe Organic content s' Stress path parameter (Effective stress) OC Overconsolidated T Shear force, surface tension force, torque List of symbols xi
T Tensile force in reinforcement a Shaft adhesion factor
TSP Total stress path a F Settlement interaction factor Ty Time factor for one-dimensional consolidation lXv Peak adhesion factor TR Time factor for radial consolidation f3 Angle, relative rotation t Time f3 Skin friction factor t Stress path parameter (Total stress) X Proportion of cross-section occupied by water t' Stress path parameter (Effective stress) 0 Angle of wall friction, base sliding, piles V Water force ,1 delta, change in, increment of V,Vc Uniformity coefficent ,1 Relative deflection Ve Combined or overall degree of consolidation op Differential settlement VR Degree of radial consolidation OPh Differential heave Vy Degree of one-dimensional consolidation Ea Coefficient of secondary compression Vy Average degree of consolidation
Pall Allowable settlement Pi Immediate settlement Pc Consolidation settlement A. Heave Ps Secondary settlement PI Consolidation settlement at time t Pr Total settlement
Py Immediate settlement including yield a Total stress aN Normal total stress a' Effective stress aN' Normal effective stress am Mean stress
a l ,a2,a3 Major, intermediate and minor principal total stresses
a l ',a/,a/Major, intermediate and minor principal effective stresses aH,aH' Total and effective horizontal stresses aV'a,' Total and effective vertical stresses r Shear stress ry Yield stress w Tilt, correction for strength of fissured clays \If Flow function Notes on units
SI Units The acceleration due to gravity on the earth's surface The International System of units (SI) has been used (g) is usually taken as 9.81 mls2 so on the earth's throughout in this book. A complete guide to the surfacel kg mass gives a force of9.81 N. system appears in ASTM E-380 published by the The unit of force is the newton (N) with multiples of American Society for Testing and Materials. The following is a brief summary of the main units. kilonewton (kN) = 1000 N The base units used in soil mechanics are meganewton (MN) = 106 N
Quantit): Unit S):mbol Measuring scales or balances in a laboratory respond to length metre m force but give a measurement in grams or kg, in other mass kilogram kg words, in mass terms. time second s
Other commonly used units are: Stress and pressure for length: These have units of force per unit area (N/m2). The SI micron (~) unit is the pascal (Pa). millimetre (mm) 1 N/m2 = IPa for mass: 1 kN/m2 = 1 kPa gram (g) (kilopascal or kilonewton per square metre) megagram (Mg) 1 MN/m2 = I MPa 1 Mg = 1000 kg = 1 tonne or 1 metric ton
for time: Density and unit weight minutes (min) Density is the amount of mass in a given volume and is hours, days, weeks, years best described as mass density (p). The SI unit is kilogram per cubic metre (kg/m3). Other units are megagram per cubic metre (Mg/m3). Density is commonly used in soil mechanics because Mass, force and weight laboratory balances give a measure of mass. Mass represents the quantity of matter in a body and Unit weight (')1 is the force within a unit volume this is independent of the gravitational force. Weight where represents the gravitational force acting on a mass. Unit force (l N) imparts unit acceleration (1 mls2) to y=pg unit mass (l kg). Newton's Law gives The common unit for unit weight is kilonewton per Weight = mass x gravitational constant cubic metre (kN/m3) or sometimes MN/m3• Unit weight is a useful term in soil mechanics since it gives vertical stress directly when multiplied by the depth.
Xlll Other titles of interest to Civil Engineers
Understanding Hydraulics Les Hamill
Prestressed Concrete Design by Computer R. Hulse and W. H. Mosley
Reinforced Concrete Design by Computer R. Hulse and W. H. Mosley
Reinforced Concrete Design, Fourth Edition W. H. Mosley and J. H. Bungey
Civil Engineering Contract Administration and Control, Second Edition 1. H. Seeley.
Civil Engineering Quantities, Fifth Edition 1. H. Seeley
Understanding Structures Derek Seward
Fundamental Structural Analysis W. J. Spencer
Surveying for Engineers, Third Edition J. Uren and W. F. Price
Engineering Hydrology, Fourth Edition E. M. Wilson
Civil Engineering Materials, Fifth Edition Edited by N. Jackson and R. K. Dhir
Timber - Structure, Properties, Conversion and Use, Seventh Edition H. E. Desch and J. M. Dinwoodie
Highway Traffic Analysis and Design R. J. Salter and N. B. Hounsell
Plastic Methods for Steel and Concrete Structures, Second Edition S. S. 1. Moy
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