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Soil Mechanics

Concepts and applications2nd edition

William Powrie

First published 1997 by E&FN Spon

Second edition published 2004 by Spon Press 2 Park Square, Milton Park, Abingdon, Oxon, OXI4 4RN

Simultaneously published in the USA and Canada by Taylor & Francis

270 Madison Avenue, New York, NY 10016

Taylor & Francis is an imprint of theTaylor & Francis Group

This edition published in the Taylor & Francis e-Library, 2009.

To purchase your own copy of this or any of Taylor & Francis or Routledges collection of thousands of eBooks

please go to www.eBookstore.tandf.co.uk.

2004 William Powrie

All rights reserved. No part of this book may be reprinted or reproduced or utilized in any form or by any electronic, mechanical,

or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or

retrieval system, without permission in writing from the publishers.

Every effort has been made to ensure that the advice and information in this book is true and accurate at the time of going to press. However, neither the publisher nor the author can accept any legal responsibility or liability

for any errors or omissions that may be made. In the case of drug administration, any medical procedure or the use of technical equipment mentioned within this book,

you are strongly advised to consult the manufacturers guidelines.

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library

Library of Congress Cataloging in Publication Data Soil mechanics: concepts and applications/William Powrie.2nd ed.

p. cm. Includes bibliographical references and index.

ISBN 0-415-31155-1 (hb:alk. paper) ISBN 0-415-31156-X (pb: alk. paper)

I. Soil mechanics. I.Title. TA710.P683 2004

624.15136dc22 2004002128

ISBN 0-415-31155-1 (hbk)ISBN 0-415-31156-X (pbk)

ISBN 0-203-46152-5 Master e-book ISBN

ISBN 0-203-34010-8 (Adobe ebook Reader Format)

Contents

Preface xiii Preface to the first edition xvi

General symbols xix

1 Origins and classification of soils 1

1.1 Introduction: what is soil mechanics? 1

1.2 The structure of the Earth 4

1.3 The origin of soils 5 1.4 Soil mineralogy 11 1.5 Phase relationships for soils 16 1.6 Unit weight 19 1.7 Effective stress 25 1.8 Particle size distributions 29 1.9 Soil filters 37 1.10 Soil description 41 1.11 Index tests and classification of clay soils 41 1.12 Compaction 45 1.13 Houses built on clay 52 1.14 Site investigation 57 Key points 61 Questions 62 Notes 65 References 66

vi Contents

2 Soil strength 68

2.1 Introduction and objectives 68 2.2 Stress analysis 69 2.3 Soil strength 78 2.4 Friction 79 2.5 The shearbox or direct shear apparatus 83 2.6 Presentation of shearbox test data in engineering units 85 2.7 Volume changes during shear 86 2.8 Critical states 89 2.9 Peak strengths and dilatancy 93 2.10 Shearbox tests on clays 99 2.11 Applications 102 2.12 Stress states in the shearbox test 105 2.13 The simple shear apparatus 111 Keypoints 118 Questions 119 Note 123 References 123

3 Groundwater, permeability and seepage 125

3.1 Introduction and objectives 125 3.2 Pore water pressures in the ground 126 3.3 Darcys Law and soil permeability 132 3.4 Laboratory measurement of permeability 136 3.5 Field measurement of permeability 142 3.6 Permeability of laminated soils 146 3.7 Mathematics of groundwater flow 150 3.8 Planeflow 152 3.9 Conftned flownets 153 3.10 Calculation of pore water pressures using flownets 162

Contents vii

3.11 Quicksand 163 3.12 Unconfined flownets 165 3.13 Distance of influence 168 3.14 Soils with anisotropic permeability 168 3.15 Zones of different permeability 172 3.16 Boundary conditions for flow into drains 173 3.17 Application of well pumping formulae to construction dewatering 176 3.18 Numerical methods 178 3.19 Wells and pumping methods 181 Keypoints 186 Questions 187 Notes 193 References 193

4 One-dimensional compression and consolidation 195

4.1 Introduction and objectives 195 4.2 One-dimensional compression: the oedometer test 197 4.3 One-dimensional consolidation 208 4.4 Properties of isochrones 211 4.5 One-dimensional consolidation: solution using parabolic isochrones 213

4.6 Determining the consolidation coefficient cv from oedometer test data

218

4.7 Application of consolidation testing and theory to field problems 220 4.8 One-dimensional consolidation: exact solutions 239 4.9 Radial drainage 250

4.10 Limitations of the simple models for the behaviour of soils in one-dimensional compression and consolidation

253

Keypoints 255 Questions 257 Notes 262 References 263

viii Contents

5 The triaxial test and soil behaviour 265

5.1 Objectives 265 5.2 The triaxial test 266 5.3 Stress parameters 269 5.4 Stress analysis of the triaxial test 273

5.5 Determining the effective angle of shearing resistance from triaxial shear tests

282

5.6 Undrained shear strengths of clay soils 289 5.7 Isotropic compression and swelling 293

5.8 Sample preparation by one-dimensional compression and swelling: K0 consolidation 294

5.9 Conditions imposed in shear tests 296 5.10 Critical states 298 5.11 Yield 301

5.12 State paths during shear: normally consolidated and lightly overconsolidated clays

311

5.13 Peak strengths 323 5.14 Residual strength 327 5.15 Sensitive soils 329 5.16 Correlation of critical state parameters with index tests 330 5.17 Creep 332 5.18 Anisotropy 333 5.19 Partly saturated soils 335 5.20 The critical state model applied to sands 336 5.21 Non-linear soil models 338 Key points 340 Questions 341 Notes 346 References 347

Contents ix

6 Calculation of soil settlements using elasticity methods 350

6.1 Introduction 350 6.2 Selection of elastic parameters 352 6.3 Boussinesqs solution 357

6.4 Estimation of increases in vertical stress at any depth due to any pattern of surface load, using Newmarks chart

360

6.5 Estimation of settlements due to surface loads and foundations 365 6.6 Influence factors for stresses 369

6.7 Standard solutions for surface settlements on an isotropic, homogeneous, elastic half-space

374

6.8 Estimation of immediate settlements 375 6.9 Effect of heterogeneity 376 6.10 Cross-coupling of shear and volumetric effects due to anisotropy 377 Key points 378 Questions 379 Note 383 References 383

7 The application of plasticity and limit equilibrium methods to retaining walls

385

7.1 Engineering plasticity 385 7.2 Upper and lower bounds (safe and unsafe solutions) 387 7.3 Failure criteria for soils 388 7.4 Retaining walls 393 7.5 Calculation of limiting lateral earth pressures 399

7.6 Development of simple stress field solutions for a propped embedded cantilever retaining wall

403

7.7 Mechanism-based kinematic and equilibrium solutions for gravity retaining walls

414

7.8 Limit equilibrium stress distributions for embedded retaining walls 442 7.9 Soil/wall friction 447

x Contents

7.10 Earth pressure coefficients taking account of shear stresses at the soil/wall interface

448

Key points 454 Questions 455 Notes 461 References 461

8 Foundations and slopes 463

8.1 Introduction and objectives 463

8.2 Shallow strip foundations (footings): simple lower bound (safe) solutions

464

8.3 Simple upper bound (unsafe) solutions for shallow strip footings 467

8.4 Bearing capacity enhancement factors to account for foundation shape and depth, and soil weight

473

8.5 Shallow foundations subjected to horizontal and moment loads 477 8.6 Simple piled foundations: ultimate axial loads of single piles 486

8.7 or

488

8.8 Pile groups and piled rafts 496 8.9 Lateral loads on piles 497 8.10 Introductory slope stability: the infinite slope 502 8.11 Analysis of a more general slope 507 Key points 519 Questions 520 Notes 524 References 524

9 Calculation of bearing capacity factors and earth pressure coefficients for more difficult cases, using plasticity methods

526

9.1 Introduction and objectives 526

9.2 Stress discontinuities: analysis 528

9.3 Stress discontinuities: u analysis 530 9.4 Application to stress analysis 531

Contents xi

9.5 Shallow foundations 533 9.6 Calculation of earth pressure coefficients for rough retaining walls 545 9.7 Sloping backfill 556 9.8 A wall with a sloping (battered) back 558 9.9 Improved upper bounds for shallow foundations 564 Key points 573 Questions 574 Note 575 References 575

10 Particular types of earth retaining structure 576

10.1 Introduction and objectives 576

10.2 Design calculations for embedded retaining walls: ultimate limit states

577

10.3 Calculation of bending moments and prop loads: serviceability limit states

581

10.4 Embedded walls retaining clay soils 584 10.5 Geostructural mechanism to estimate wall movements 594 10.6 Effect of relative soil: wall stiffness 599 10.7 Compaction stresses behind backfilled walls 609 10.8 Strip loads 616 10.9 Multi-propped embedded walls 617 10.10 Reinforced soil walls 618 10.11 Tunnels 621 Keypoints 632 Questions 633 Note 635 References 636

11 Modelling, in situ testing and ground improvement 638

11.1 Introduction and objectives 638 11.2 Modelling 638

xii Contents

11.3 In situ testing 653 11.4 Ground improvement techniques 679 Key points 691 Questions 693 Notes 694

References 694

Index 700

Preface

In preparing the second edition of this book, I have taken the opportunity to

introduce a limited amount of new material, for example, on shallow foundations align the text, where appropriate, with the design philosophy of Eurocode 7 by

embracing more closely the concepts of limit state design and, in an ultimate limit state calculation, applying the factor of safety to the soil strength (rather than to the load or some other parameter)

update material where there have been other changes in the interpretation of knowl-edge or design guidance

re-order the material in Chapter 5 (the triaxial test and soil behaviour) to enable a completely traditional interpretation, not including Cam clay, to be followed more easily if required, and

correct a number of errors and ambiguities regrettably present in the first edition.

The underlying philosophy of the book, however, remains the same.The main changes are as follows:

In Chapter 1, the discussion on the concept of effective stress has been expanded slightly (section 1.7); and the filter rules given in section 1.9 have been changed to reflect those recommended in the revised Construction Industry Research and Infor-mation Association Report C515 on groundwater control (Preene et al., 2000).

In Chapter 5, the sections on peak strength and undrained shear strengths (including the associated worked examples) have been placed before the sections leading up to Cam clay. As indicated above, this will enable lecturers should they so wish to offer their students a complete traditional interpretation of triaxial test data, including criti-cal, peak and undrained stengths, without going into the concepts of yield and Cam clay. For lecturers wishing to cover Cam clay more thoroughly, a new section has been added that includes the derivation of the Cam clay yield surface.

In Chapter 7, the long case study of the Cricklewood retaining wall has been split into four worked examples. A significant omission in the assessment of the long-term stability of the wall in overturning has been corrected, and I have revised both this and the short-term calculation to give a treatment of factor of safety that is more con-sistent with the idea of a factor of safety on soil strength (strength mobilization factor) promoted by BS8002, Eurocode 7 and the revised Construction Industry Research and Information Association Report C580 on embedded retaining walls (Gaba et al., 2003). I had considered replacing this case study with a number of simpler worked examples, but the opinion of other teachers of geotechnical engineering subjects was that a single problem containing all parts of the calculation was helpful.

xiv Preface

Chapter 8 contains a new section on shallow foundations subject to combined verti-cal, horizontal and moment loading, based on the work of Roy Butterfield and his colleagues. I hope that both teachers and students of geotechnical engineering will find this elegant approach both interesting and useful. The general thrust of this chap-ter has also been changed towards applying the factor of safety to the soil strength, rather than to the load as in the first edition. This is in line with the philosophy of Eurocode 7 (BSI, 1995), but as with the first edition this book is intended as a guide to soil mechanics principles and their application to geotechnical engineering, not codes of practice.

Chapter 10 has also been revised to align it more closely with the now generally accepted idea of applying the factor of safety for an embedded retaining wall mainly to the soil strength, in accordance with BS8002, Eurocode 7 and CIRIA report C580. I have therefore reduced substantially the discussion of the different definitions of factor of safety that used to be associated with retaining walls.

In addition, I have made numerous small but nonetheless important changes throughout the text in the interests of (I hope) improving clarity and giving references to relevant new developments in soil mechanics and geotechnical engineering. I have also removed the distinction between case studies and worked examples (they are all now worked examples), not least because the grey background to the case studies in the first edition did not make

Educationally, things have moved on since 1996 when the first edition came out. For students working towards chartered engineer status, a 4-year MEng is a much more com-mon route if not the norm. (At the time of writing, it is quite possible that revised profes-sional registration requirements will cause this to change.) There is also, quite rightly, an increased emphasis on a broader syllabus that encourages the development of students communications, IT, teamworking and other transferable skills. This changing educational context has influenced the course structure suggested in the preface to the first edition: at Southampton, our soil mechanics and geotechnical engineering syllabus is now as roughly as follows:

Year 1: Chapter 1, in courses on engineering geology and civil and environmental engineering materials.

Year 2: Chapter 2 up to and including section 2.11; Chapter 3 up to and including section 3.15; Chapter 4 up to and including parts of section 4.7 (section 4.8 is covered in parallel in the mathematics course on differential equations); Chapter 5 up to and including section 5.13, except for the derivation of the Cam clay model and detailed calculations of state paths using it; Chapter 6 up to and including section 6.5; and an introduction to retaining walls, foundations and slopes covering sections 7.1 to 7.6, 8.1 to 8.4, and 8.9.

Year 3: Retaining walls, foundations and slopes in more detail, that is, the parts of Chapters 7 and 8 not covered in the second year and sections 10.1 to 10.6, 10.8 and 10.9.

Year 4 (option):

Modelling and analysis including numerical predictions of state paths using Cam clay, Chapter 9 and selected sections from Chapter 11.

the text any easier to read. A free solutions manual is available at www.sponpress.com/supportmaterial.

Preface xv

This material is, of course, used and developed in design and individual projects in all years of the degree programme.

I am grateful to a number of people including Emmanuel Detournay, Andrew Drescher, Susan Gourvenec, Bill Hewlett, Adrian Oram and Toby Roberts for bringing to my atten-tion various errors and ambiguities in the first edition; Alan Bloodworth, Malcolm Bolton, Roy Butterfield, Asim Gaba, Richard Harkness, David Richards and Antonis Zervos for their help with some of the more major revisions I have made; and most especially to Joel Smethurst who commented on drafts of much of the new text, checked the new calculations and drew the new figures.

William Powrie 2 May 2004

Preface to the first edition

My original aims in writing this book were:

To encourage students of soil mechanics to develop an understanding of fundamen-tal concepts, as opposed to the formula-driven approach which seems to be used by many authors.

To assist the student to build a framework of basic ideas, which would be robust and adaptable enough to support and accommodate the more complex problems and analytical procedures which confront the practising geotechnical engineer.

To illustrate, with reference to real case histories, that the sensible application of simple ideas and methods can give perfectly acceptable engineering solutions to many classes of geotechnical problem.

To avoid the unnecessary use of mathematics. To cover the soil mechanics and geotechnical engineering topics usually included in

typical BEng-level university courses in civil engineering and related subjects, with-out the additional material which clutters many existing textbooks.

While these aims have probably not been compromised to any significant extent, reality is such that

Civil engineers must be numerate, and possess a reasonable degree of mathematical ability: they must be able to do sums.

Different lecturers will have different views on the content of a core syllabus in the soil mechanics/geotechnical engineering subject area.

Some material may not be suitable for formal presentation in lectures, but is none-theless essential background reading.

Furthermore, the current trend towards four-year MEng courses for the most academically gifted undergraduates means that some of the material which has traditionally been taught at MSc level will inevitably find its way into MEng syllabuses. The result of all this is that while perhaps 7580% of the book is indisputably core material at BEng level, there are some sections which are useful background, some sections which might be covered in some courses but not in others, and one or two sections (e.g. section 4.8) which are almost certainly for reference only.

I had originally thought that I would mark the noncore sections in the text, but eventu-ally decided not to do so. This is because of the subjectivity involved in deciding what should be included in a core syllabus, and the ensuing need to distinguish further between non-core sections which were (a) desirable background reading; (b) for possible future use; and (c) purely for reference. Instead, I have tried to ensure that separable topics and sub-topics are covered in separate sections and subsections, with clear and unambiguous titles and subtitles. This should enable a university lecturer to draw up a personalized reading schedule, appropriate to his or her own course.

Preface to the fi rst edition xvii

The book is based on the undergraduate courses in soil mechanics and geotechnical engineering that I helped to develop at Kings and Queen Mary and Westfield Colleges in the University of London, over the period 19851994. The material in the first eight chap-ters would probably comprise a core BEng-level syllabus, covering the subject in sufficient detail for those not specializing in geotechnical engineering. Some of the sectionspartic-ularly those at the end of Chapters 3 and 5would be considered to be background reading or for possible future use, while section 4.8 is included primarily for reference on a need to know basis. The material in Chapters 911 might well be included in 3rd year options for intending geotechnical specialists in BEng courses, or taught at MEng/MSc level.

My suggested course structure would be:

Year 1: Chapters 1 and 2, except for sections 2.12 and 2.13.

Year2: Chapter 3 (up to and including section 3.15); Chapter 4 (up to and including section 4.6, plus one of the case studies from section 4.7); Chapter 5 (up to and including sec-tion 5.13); Chapter 6 (up to and including section 6.5); Chapter 7 (up to and including section 7.7); and Chapter 8 (omitting sections 8.7, 8.8 and 8.10).

Year 3 (optional):

Selected material from Chapters 911, plus sections from Chapters 7 and 8 not already covered in Year 2.

Some of the early chapters are structured around the standard laboratory tests that are used to investigate a particular class of soil behaviour. These are Chapter 2 (the shearbox test; friction), Chapter 4 (the oedometer test, one-dimensional compression and consolidation) and Chapter 5 (the triaxial test, more general aspects of soil behaviour). This approach is not new, but is by no means universal. In my experience, the integration of the material covered in lectures with laboratory work, by means of coursework assignments in which laboratory test results are used in an appropriate geotechnical engineering calculation, is entirely beneficial.

The order in which the material is presented is based on the belief that students need time to assimilate new concepts, and that too many new ideas should not be introduced all at once. This may have led to the division of what might be seen as a single topic (for example, soil strength or retaining walls) between two chapters. Where this occurs, the topic is initially addressed at a fairly basic level, with more detailed or advanced coverage reserved for a later stage.

For example, while some authors deal with both the shearbox test and the triaxial test under the same heading (such as laboratory testing of soils or soil strength), I have cov-ered them separately. This is because the shearbox serves as a relatively straightforward introduction to the behaviour of soils at failure in terms of simple stress and volumetric parameters, and to the concept of a critical state. The triaxial test introduces more general stress states, the difference between isotropic compression and shear, the generation of pore water pressures in undrained tests, and the behaviour of clay soils before and after yield. Similarly, I have endeavoured to establish the basic principles of earth pressures and collapse calculations with reference to relatively simple retaining walls in Chapter 7, before addressing soil/wall friction more rigorously in Chapter 9, and more complex earth

xviii Preface to the fi rst edition

retaining structures in Chapter 10. In the course structure suggested above, the shearboxbasic soil behaviouris covered in the first year, and the triaxial testmore advanced soil behaviourin the second. Basic retaining walls, slopes and foundations are covered in the second year, and more complex situations and methods of analysis in the third.

The book assumes a knowledge of basic engineering mechanics (equilibrium of forces and moments, elastic and plastic material behaviour, Mohr circles of stress and strain etc.). Also, it is written to be followed in sequence. Where necessary, a qualitative description of an aspect of soil behaviour which has not yet been covered is given in order to allow the development of a fuller understanding of another. For example, the generation of excess pore water pressures during shear is mentioned qualitatively in Chapter 2 in order to explain the need for drained shear box tests on clay soils to be carried out slowly. For the experienced reader, it is hoped that the section and subsection headings are sufficiently descriptive to enable the required information to be extracted with the minimum of effort.

More than 50 worked examples and case studies are included within the text, with fur-ther questions at the end of each chapter. Some of these were provided by my colleagues Dr R.H.Bassett, Dr R.N.Taylor, Dr N.W.M.John, Dr M.R.Cooper and Professor J.B.Burland, to whom I would like to express my gratitude. So far as I am aware, the other examples, case studies and questions are original, but I apologize for any that I have inadvertently borrowed.

The book has been influenced by those from whom I have learnt about soil mechanics and geotechnical engineering, including (as teachers, colleagues or both) John Atkinson, Malcolm Bolton, David Muir Wood, Toby Roberts, Andrew Schofield, Neil Taylor and Jim White. I am indebted to them, and also to the undergraduate and postgraduate students at Kings College London, Queen Mary and Westfield College London, and Southampton University, from whom I have learnt a great deal about teaching soil mechanics and geo-technical engineering. I am grateful to Richard Harkness for his help with parts of Chapters 2 and 3. My special thanks go to Susan Gourvenec, who read through the penultimate draft of the book and suggested many changes which have (I hope) improved its clarity.

William Powrie 25 September 1996

Note: simple dimensions (A for cross-sectional area, D or d for depth or diameter, H for

scripts are not listed where their meaning is clear (e.g. crit for critical, max for maximum, ult for ultimate). Effective stresses and effective stress parameters are denoted in the text by a prime ().

A Air content of unsaturated soil (section 1.5); Activity (section 1.11); A soil parameter used in the description of creep (section 5.17)

Ac

Projected area of cone in cone penetration test (section 11.3.2)

An

Fourier series coefficient (section 4.8)An Area of shaft of cone penetrometer

B u/c in undrained isotropic loading (Chapter 5)

Bq Pore pressure ratio in cone penetration test (section 11.3.2)

C Tunnel cover (depth of crown below ground surface) (section 10.11)

C Parameter used in analysis of shallow foundations (section 8.5)

Cc

Compression indexslope of one dimensional normal compression line on a graph of e

against log10

Cs

Swelling indexslope of one dimensional unload/reload lines on a graph of e against log10

CN

Correction factor applied to SPT blowcount (section 11.3.1)

D Drag force (section 1.8)

D10

etc. Largest particle size in smallest 10% etc. of particles by massE Youngs modulus. Subscripts may be used as follows: h (horizontal); (vertical); u (und-

rained)

One-dimensional stiffness modulus

E* Rate of increase of Youngs modulus with depth

E Horizontal side force in slope stability analysis (section 8.10)

EI Bending stiffness of a retaining wall

Er

Youngs modulus of raft foundation

Es

Youngs modulus of soil

F Shear force

F Prop load (propped retaining wall)

F Factor of safety. A subscript may be used to indicate how the factor of safety is applied: see section 10.2

Fr

Normalized friction ratio in cone penetration test

General symbols

height, for radius etc.) and symbols used as arbitrary constants are not included. Sub-R

xx General symbols

Fs Factor of safety applied to soil strengthFF, FT Factor of safety or load factor against frictional failure (pull-out) and tensile failure respec-

tively, for a reinforced soil retaining wallG Shear modulusG* Modified shear modulus in the presence of shear/volumetric coupling (section 6.10); Rate

of increase of shear modulus with depth (section 10.6)Gs Relative density (=s/w) of soil grains (also known as the grain specific gravity)H Rating on mineral hardness scale (Chapter 1)H Horizontal load or forceH Overall head drop (e.g. across flownet)H Slope of Hvorslev surface on a graph of q against pH Hydraulic head at the radius of influence in a well pumping testH Limiting lateral load on a pile (section 8.9)ID Density indexIL Liquidity indexIP Plasticity index (=wLL PL)I, I

Influence factor for settlement and stress respectively (Chapter 6)

J Parameter describing effect of shear/volumetric coupling (section 6.10)K Intrinsic permeability (Chapter 3)K Earth pressure coefficient, . Subscripts may be used as follows: a (to denote active

conditions); p (passive conditions); i (prior to excavation in front of a diaphragm-type retaining wall); 0 (in situ stress state in the ground); nc (for a normally consolidated clay); oc (for an overconsolidated clay)

K Elastic bulk modulus (subscript u denotes undrained)K* Modified bulk modulus in the presence of shear/volumetric coupling (section 6.10)Kac, Kpc Multipliers applied to u in the calculation of active and passive total pressures respectively

(undrained shear strength model)KT Total stress earth pressure coefficient, h/v (section 10.7)L0 Distance of influence of a dewatering system idealized as a pumped wellLF Tunnel load factor (section 10.11)M Bending moment. Subscripts may be used as follows: des (to denote the design bending

moment); le (retaining wall bending moment calculated from a limit equilibrium analysis); p or ult (fully plastic or ultimate bending moment of beam or retaining wall)

M Moment loadM Mobilization factor on soil strength

Constrained or one-dimensional modulus

N Normal force, for example, on rupture surface or soil/structure interfaceN SPT blowcountNk Cone factor relating qc and u (section 11.3.2)

w

General symbols xxi

N1 SPT blowcount normalized to a vertical effective stress of 100kPa (section 11.3.1)N60 Corrected SPT blowcount, for an energy ratio of 60% (section 11.3.1)

N1, N2, N3

Interblock normal forces, mechanism analysis for shallow foundation (section 9.9)

NF, NH Number of flowtubes and potential drops respectively in a flownetNc Basic bearing capacity factor: undrained shear strength analysisNp Value of at In p=0 on isotropic normal compression line on a graph of against In p

(Chapter 5)Nq Basic bearing capacity factor: frictional soil strength analysisN

Term in bearing capacity equation to account for self-weight effects

OCR Overconsolidation ratioP Prop load; tensile strength of reinforcement strip (reinforced soil retaining wall)Q Ram load in triaxial test (Chapter 5)Q Equivalent toe force in simplified stress analysis for unpropped retaining wallQt Normalized cone resistance in cone penetration testR Proportional settlement /ultR Resultant force, for example, on rupture surface or soil/structure interfaceR Dimensionless flexibility number R=m (section 10.6)R Depth of tunnel axis below ground level (section 10.11)Rr, Rz Degree of consolidation due to radial and vertical flow alone, respectively (section 4.9)R0 Radius of influence of a pumped wellS, Sr Saturation ratioS Slope of graph; drain spacing (section 4.9); sensitivity (section 5.15)S Surface settlement due to tunnelling (section 10.11)T Total shear resistance of soil/pile interface (section 2.11)T Shear force, for example, on rupture surface or soil/structure interfaceT Surface tension at air/water interfaceT Dimensionless time factor cvt/d

2 in consolidation problemsT Anchor load (anchored retaining wall)T TorqueTC Tunnel stability number at collapse (section 10.11)Tdes Design anchor load (anchored retaining wall)Tr Dimensionless time factor for radial consolidation Tv Torque due to shear stress on vertical surfaces in shear vane test (section 11.3.4)U Coefficient of Uniformity=D60/D10U Average excess pore water pressure (consolidation problems)U Force, for example, on rupture surface or soil/structure interface due to pore water

pressure

xxii General symbols

Ue Pore water suction at air entryUr, Uz Average excess pore water pressure if drainage were by radial flow or vertical flow alone,

respectively (section 4.9)V Volume (total)V Electrical potential difference (voltage) (section 11.4.1)V Vertical load or force

Va Volume of air voids in soil sampleVs Volume of soil solids in soil sampleVt Total volume occupied by a soil sampleVti Total volume of triaxial test sample as prepared (Chapter 5)Vto Total volume of triaxial test sample at start of shear test (Chapter 5)Vv Volume of voids in soil sampleVw Volume of water in soil sampleVL Volume loss in tunnelling (section 10.11)Vtunnel Nominal volume of tunnel (section 10.11)W Weight of a block of soilW Mass of falling weight used in heavy tamping (section 11.4.6)Wc Set of collapse loads for a structure (in plastic analysis)Wt Total weight of a soil sampleX Vertical side force in slope stability analysis (section 8.11)Z Coefficient of curvature=(D30)

2/(D60 D10)Z Reference depth in a Newmark chart analysis (Chapter 6)

a Acceleration (section 11.2.2)a Area ratio An/Ac in cone penetration test (section 11.3.2)av Subscript indicating the average value of a parameterb Parameter defining the intermediate principal stress (section 5.10)c Subscript denoting the initial statecrit Subscript denoting a critical conditioncurrent Subscript indicating the current value of a parameterc Intersection with -axis of extrapolated straight line joining peak strength states on a graph

of against chv, cv Consolidation coefficientvertical compression due to horizontal flow, and vertical com-

pression due to vertical flow, respectivelyd Equivalent particle size (section 1.8.1)d Prefix denoting infinitesimally small increment (e.g. of stress, strain or length)d Half depth of oedometer test sample (maximum drainage path length)

Depth factors (bearing capacity analysis)

General symbols xxiii

dq, d Depth factors (bearing capacity analysis)ds Subscript indicating parameter measured in direct sheare Void ratiof Subscript used to denote final conditions at the end of a testf Frequencyfc Sleeve friction (stress) in cone penetration test (section 11.3.2)ft Corrected sleeve friction (stress) in CPTf1 Parameter relating undrained shear strength to SPT blowcount (section 11.3.1)g Acceleration due to Earths gravity (=9.81m/s2)g Constant used to define Hvorslev surface (section 5.13)h Total or excess head, height of sample in shearbox testh Subscript: horizontal

hc Critical height of backfill in analysis of compaction stresses behind a retaining wall (sec-tion 10.7)

hcrit Critical hydraulic head drop across an element of soil at fluidizationhe Excess head (consolidation analysis)hi Height of triaxial test sample as prepared (Chapter 5)h0 Height of triaxial test sample at start of shear test (Chapter 5)h0 Initial depth of block of soil in analysis of settlement due to change in water content (sec-

tion 1.13); initial height of soil sample in shearbox test; drawdown at a line of ejector wells analysed as a pumped slot (section 4.7.3)

hw Head in a pumped or equivalent welli Hydraulic gradient. Subscripts x, y or z may be used to indicate the directioni Parameter quantifying width of settlement trough due to tunnelling (section 10.11)i Subscript denoting an initial state (the pre-excavation state in the case of an in situ retain-

ing wall)icrit Critical hydraulic gradient across an element of soil at fluidizationie Electrical potential (voltage) gradient (section 11.4.1)k Permeability used in Darcys Law. Subscripts may be used as follows: h (horizontal); v

(vertical); x, y or z (x-, y- or z-direction), t (transformed section); i or f (at start or end of a permeability test)

ke Electro-osmotic permeability (section 11.4.1)l Limit of range of a Fourier series (section 4.8)l Length of part of a slip surface (sections 8.11 and 9.9)m Soil stiffness parameter (section 10.6); Rate of increase of soil Youngs modulus with

depth (section 11.2)m A soil parameter used in the description of creep (section 5.17)ma Mass of air in soil samplems Mass of soil solids in soil sample

xxiv General symbols

mt Mass of tin or containermw Mass of water in soil samplemax Subscript indicating the maximum value of a parametern Porosityn Overconsolidation ratio based on vertical effective stressesnp Overconsolidation ratio based on average effective stressesn Number of squares of Newmark chart covered by a loaded area (Chapter 6)n Centrifuge model scale factor (Chapter 11)p, p Average principal total and effective stress, respectively: p=(1+2+ 3)/3

Maximum previous value of p; value of p at tip of current yield locus (Chapter 5)

pu, Lateral load capacity (per metre depth) of a pile (section 8.9)

p Subscript denoting prototype (section 11.2.2)p Cavity pressure in pressuremeter test (section 11.3.3)

pp Cavity pressure at onset of plastic behaviour in pressuremeter test (section 11.3.3)pL Extrapolated limit pressure in analysis of the plastic phase of the pressuremeter test (sec-

tion 11.3.3)pb Passive side earth pressure (section 10.6)

Equivalent consolidation pressure: value of p on isotropic normal compression line at cur-rent specific volume (Chapter 5)

ps Subscript indicating parameter measured in plane strainq Deviator stressq Volumetric flowrateq Surface surcharge or line loadqc Measured cone resistance (stress) (section 11.3.2)qt Corrected cone resistancer Wall roughness anglere Radius of an equivalent well used to represent an excavationrp Radius of plastic zone in the soil around a pressuremeter (section 11.3.3)ru Pore pressure ratio, ru=u/zr

Reduction factor (bearing capacity analysis)s Average total stress (1+3)/2: locates centre of Mohr circle on -axis

Shape factors (bearing capacity analysis)

sq, s Shape factors (bearing capacity analysis)s (section 11.3.3)s Average effective stress : locates centre of Mohr circle on -axist Time

General symbols xxv

t Radius of Mohr circle of stress, th, tm Parameters used in analysis of shallow foundations (section 8.5)tr Thickness of raft foundationtx Reference point on time axis used to determine consolidation coefficient cv from oedom-

eter test datau Pore water pressureu Subscript: undrainedue Excess pore water pressure (consolidation analysis)ued Excess pore water pressure at mid-depth of an oedometer test sample of overall depth

2d (Chapter 4)u2 Pore water pressure measured in cone penetration testult Subscript denoting the ultimate value of a parameter (e.g. settlement) Specific volume Particle settlement velocity (section 1.8); velocity of relative sliding (section 9.9)v Subscript: vertical, D Darcy seepage velocity. Subscripts x, y or z may be used to indicate the direction0 Reference velocity for mechanism analysis (section 9.9)k Intersection of unload/reload line with In p=0 axistrue True average fluid seepage velocity

w Water contentwLL, wPL Water content at liquid limit and plastic limit, respectivelyw Weight of a soil element (sections 8.10 and 10.11)x Relative horizontal movement in shearbox testy Upward movement of shearbox lidyc Outward movement of cavity wall in pressuremeter test (section 11.3.3)yrp Outward displacement of soil at the plastic radius rp in a pressuremeter test (section

11.3.3)z Depth coordinatezc Critical layer thickness for compaction of soil behind a retaining wall (section 10.7)z0 Depth of tunnel axis below ground level (section 10.11)zp Depth of pivot point below formation level (unpropped embedded retaining wall) Value of at In p=0 on critical state line on a graph of against In p space (Chapter 5) Prefix denoting increment (e.g. of stress, strain or length) Angle used in Mohr circle constructions for stress analyses (Chapter 9) Multipropped wall flexibility parameter (Chapter 10)M Slope of critical state line on a graph of q against p c (section 5.20)

xxvi General symbols

Vtc, Vtq Volume change of triaxial test sample during consolidation and shear, respectively (Chapter 5)

y, z Width and depth, respectively, of reinforced soil retaining wall facing panelpu/r,max Maximum reduction in cavity pressure in a pressuremeter test that can be applied with-

out causing plastic behaviour in unloading (section 11.3.3) Transformation factor for flownet in a soil with anisotropic permeability (Chapter 3) A soil parameter used in the description of creep (section 5.17) Angle of inclination of slip surface to the horizontal (section 8.11) Term applied to one of the two characteristic directions, along which the full strength of

the soil is mobilized (Chapter 9) Retained height ratio h/H of a retaining wall (section 10.6) Soil/wall adhesion reduction factor Angle between flowline and the normal to an interface with a soil of different perme-

ability Term applied to one of the two characteristic directions, along which the full strength of

the soil is mobilized (Chapter 9) Slope angle Parameter quantifying depth to anchor for an anchored retaining wall (section 10.6) Engineering shear strainy Unit weight (=g)dry Unit weight of soil at same void ratio but zero water contentf Unit weight of permeant fluid (section 3.3)sat Unit weight of soil when saturated

yw Unit weight of water Soil/wall interface friction angle Strength mobilized along a discontinuity (Chapter 9) Prefix denoting increment (e.g. of stress, strain or length) Displacementmob Mobilized soil/wall interface friction angle Direct strain. Subscripts may be used to indicate the direction as follows: h (horizontal);

v (vertical); r (radial); (circumferential)c Cavity strain in pressuremeter test (section 11.3.3)q Triaxial shear strain q=(2/3) (vh)vol Volumetric strain1, 3 Major and minor principal strains, respectively Electro-kinetic or zeta potential Stress ratio q/pf Dynamic viscosity Rotation of stress path on a graph of q against p

General symbols xxvii

Rotation of principal stress directions; included angle in a fan zone (Chapter 9) Slope of idealized unload/reload lines on a graph of against In p0 Slope of idealized unload/reload lines on a graph of against In Slope of critical state line and of one-dimensional and isotropic normal compression

lines on a graph of against In p Load factor in structural design0 Slope of one-dimensional normal compression line on a graph of against In Coefficient of frictionvr Poissons ratio of raft foundationvs Poissons ratio of soilv Poissons ratiovu Undrained Poissons ratio Mass density. Subscripts may be used as follows: b (for the overall or bulk density of

a soil); s (for the density of the soil grains); w (for the density of water=1000 kg/m3 at 4C)

Settlement Wall flexibility H4/EI; a subscript c may be used to denote a critical value Cavity radius in pressuremeter test (section 11.3.3); a subscript 0 may be used to denote

the initial value Parameter used in analysis of shallow foundations (section 8.5), Total and effective stress, respectively. Subscripts may be used to indicate the direction

as follows: a (axial, in a triaxial test); h (horizontal); h0 (horizontal, in situ); v (vertical); v0 (vertical, maximum previous); r (radial); (circumferential); n (normal)

c Cell pressure in a triaxial testNormal total and effective stress (respectively) on a shallow foundation at failure

Normal total and effective stress (respectively), acting on either side of a shallow foun-dation at failureTotal and effective stresses on the plane whose normal is in the x direction, acting in the x direction

1, 2, 3 Major, intermediate and minor principal total stress, respectivelyMajor, intermediate and minor principal effective stress, respectively

T Tunnel support pressure (section 10.11)TC Tunnel support pressure required just to prevent collapse (section 10.11)uc Unconfined compressive strength Shear stressc Shear stress at cavity wall in pressure meter test (section 11.3.3)u Undrained shear strengthu,design Design value of undrained shear strength

xxviii General symbols

w Shear strength mobilized on soil/wall interfacexy Shear stress on the plane whose normal is in the x direction, acting in the y direction

Soil strength or angle of shearing resistance (effective angle of friction)

Critical state strength

Design strength

Mobilized strength

Peak strength

Slope of best-fit straight line joining peak strength states on a graph of against

True friction angle between soil grain and wall materials

True friction angle of soil grain material

Parameter used in the description of unsaturated soil behaviour (section 5.19) Angle of dilation Angular velocity; angle of retaining wall batter (Chapter 9)0 Subscript denoting an initial state (at t=0), a value at x=0 or z=0, or the initial in situ

state in the ground

Chapter 1

Origins and classification of soils

1.1 Introduction: what is soil mechanics?

Soil mechanics may be defined as the study of the engineering behaviour of soils, with reference to the design of civil engineering structures made from or in the earth. Examples of these structures include embankments and cuttings, dams, earth retaining walls, tunnels, basements, sub-surface waste repositories, and the foundations of buildings and bridges. An embankment, cutting or retaining wall often represents a major component, if not the whole, of a civil engineering structure, and is usually (for better or for worse) clearly vis-ible in its finished form (Figure 1.1). Tunnels and basements are generally only visible from inside the structure, while foundations and underground waste repositoriesonce completedare not usually visible at all. By definition, foundations form only a part of the structure which they support. Although out of sight, the foundation is nonetheless important: if it is deficient in its design or construction, the entire building may be at risk (Figure 1.2).

Problems in soil mechanics had begun to be identified and addressed analytically by the beginning of the eighteenth century (Heyman, 1972). Despite this, the growth of soil mechanics as a core discipline within civil engineering, taught at universities with almost the same emphasis as structures and hydraulics, has taken place largely within the last fifty years or so. The expansion of the subject during this time has been very rapid, and the term geotechnical engineering has been introduced to describe the application of soil mechan-ics principles to the analysis, design and construction of civil engineering structures which are in some way related to the earth.

The development of geotechnical processes and techniques has been led primarily by innovation in construction practice. The terms ground engineering and geotechnology are often used to describe the study of geotechnical processes and practical issues, includ-ing techniques for which the only available methods of assessment are either qualitative or empirical.

If these somewhat arbitrary definitions are accepted, the various terms cover a spectrum from soil mechanics (at the theoretical end), through geotechnical engineering (which is analytical but applied) to ground engineering and geotechnology, where the methods used in design may be largely empirical. This book is concerned primarily with soil mechanics and its application to geotechnical engineering (although section 11.4, on ground improvement techniques, could probably be classed as ground engineering or geotechnology). It describes the mechanical (e.g. strength and stress-strain) behaviour of soils in general terms, and shows how this knowledge may be used in the analysis of geotechnical engineering structures.

2 Soil mechanics

Figure 1.1 A visible and, at the time of its construction, controversial road cutting (the M3 motorway at Twyford Down, near Winchester, Hampshire, England). (Photograph courtesy of Mott MacDonald.)

The book does not (apart from the very brief overview given in section 1.3) cover engineer-ing geology; nor does it examine the mineralogy, physics, chemistry or materials science of soils. The book takes a macroscopic view, and does not address at the microscopic level the issues which constitute what Mitchell (1993) calls the why aspect of soil behaviour. This is not to say that these issues are unimportant. A study of engineering geology, and the geo-logical history of an individual site, will give an invaluable understanding of the structure and characteristics of the soil and rock formations present. It might also lead the engineer to anticipate the presence of potentially troublesome features, such as buried river beds which form preferential groundwater flow paths, and historic landslips which give rise to pre-existing planes of weakness in the ground. At least a basic knowledge of soil mineral-ogy and soil chemistry is essential for anyone involved in the increasingly important issue of the movement of contaminants (e.g. from landfill sites) through the ground.

These subjects are covered in more detail by Blyth and de Freitas (1984: engineering geology); Marshall et al. (1996: soil physics); and Mitchell (1993: mineralogy and soil chemistry). Full references to these works are given at the end of this chapter.

Origins and classifi cation of soils 3

Figure 1.2 A well-known building with an inadequate foundation (Pisa, Italy). (Photograph

courtesy of Professor J.B.Burland.)

Objectives

After having worked through this chapter, you should have gained an appreciation of:

the origin, nature and mineralogy of soils (sections 1.21.4) the influence of depositional and transport mechanisms and soil mineralogy on soil

type, structure and behaviour (sections 1.3 and 1.4) the principles and objectives of a site investigation (section 1.14).

You should understand:

the three-phase nature of soil, including the relationships between the phases and how these are quantified (sections 1.5 and 1.6)

the need to separate the total stress into the component carried by the soil skeleton as effective stress and the pore water pressure u, by means of Terzaghis equa-tion, =+u (section 1.7)

4 Soil mechanics

the importance of soil description, and classification with reference to particle size and index tests (sections 1.8, 1.10 and 1.11).

You should be able to:

manipulate the phase relationships to obtain expressions for the unit weight of the soil (section 1.6)

determine water content, unit weight, grain specific gravity, saturation ratio, liquid and plastic limits, and optimum water content from laboratory test data (sections 1.5, 1.6, 1.11 and 1.12)

calculate the vertical total stress at a given depth in a soil deposit and, given the pore water pressure, the vertical effective stress (section 1.7)

construct a particle size distribution curve from sieve and sedimentation test data (section 1.8)

design a granular filter (section 1.9) apply the phase relationships to the practical situations of compaction of fill and the

settlement of houses founded on clay soils (sections 1.12 and 1.13).

1.2 The structure of the Earth

Robinson (1977) points out that the highest mountain (Everest) has a height of 8.7km above mean sea level, while the deepest known part of the ocean (the Mariana Trench, off the island of Guam in the Pacific) has a depth of 11.3 km. This gives a total range of 20 km, or about 0.3% of the radius of the Earth (which is approximately 6440 km). If a cross-section through the Earth were represented by a circle 10cm in diameter, drawn using a reasonably sharp pencil, the variation in the position of the Earths surface would be con-tained within the thickness of the pencil line. The depths of soil with which civil engineers are concernedusually only a few tens of metresare even smaller in comparison with the radius of the Earth. Even the deepest mines have not penetrated more than 6 km or so below the surface of the Earth.

Although the civil engineer is concerned primarily with the behaviour of the soils and rocks within 50 m or so of the surface of the Earth, an appreciation of the overall structure of the planet provides a useful starting point. In the descriptions which follow, it must be borne in mind that the theories concerning the nature and composition of the Earth beyond a depth of a few kilometres are based mainly on geophysical tests and the interpretation of geological evidence. They cannot be verified by direct visual observation, or even by the recovery and testing of material, and therefore remain, at least to some extent, conjectural.

The Earth consists of a number of roughly concentric zones of differing composition and thickness. It has been possible to identify the three main zonesthe crust, the mantle and the corebecause of the changes in the resistance to the passage of seismic (earth-quake) waves which occur at the interfaces. The interface between the crust and the mantle is known as the Mohorovicic discontinuity (sometimes abbreviated to Moho), while the interface between the mantle and the core is known as the Gutenburg discontinuity. In

Origins and classifi cation of soils 5

both cases, the interfaces or discontinuities are named after their discoverers. The crust is approximately 3248 km thick, and the mantle 2850 km. The Gutenburg discontinuity, which defines the interface between the mantle and the core, is therefore about 2890km below the Earths surface.

The crust and the core may each be subdivided into inner and outer layers. The outer crust is composed primarily of crystalline granitic rock, with a comparatively thin and discontinuous covering of sedimentary rocks1 (e.g. sandstone, limestone and shale). The rocks forming the outer crust are composed primarily of silica and aluminium, and have a relative density (or specific gravity) generally in the range 2.02.7 (i.e. they are 2.0 to 2.7 times denser than water). The outer crust is known as sial, from si for silica and al for aluminium. Below the outer crust, there is a layer of denser basaltic rocks, which have a specific gravity of about 2.73.0. These denser rocks are composed primarily of silica and magnesium, and the lower crust is known as sima (si for silica and ma magnesium). The inner crust or sima is continuous, while the outer crust or sial is discontinuous, and appears to be confined to the continental land masses: it is not generally present under the sea. For this reason, the denser sima is known as oceanic crust, while the overlying sial is known as continental crust.

The mantle consists mainly of the mineral olivine, a dense silicate of iron and magne-sium, possibly in a fairly fluid or plastic state. The specific gravity of the mantle increases from about 3 at the Mohorovicic discontinuity (approximately 40km deep) to about 5 at the Gutenburg discontinuity (approximately 2890km deep).

The core is composed largely of an alloy of nickel and iron, which is sometimes given the acronym nife (from ni for nickel and fe for iron). As the core does not transmit the transverse or S-waves which arise from earthquakes, at least part of it must be in liquid form. There is some evidence that the outer core may be liquid, while the inner core (with a radius of 1440km or so) is solid. The temperature of the core is estimated to be in excess of 2700C. The specific gravity of the core material varies from 5 to 13 or more.

According to the theory of plate tectonics, the crust is divided into a number of large slabs or plates, which float on the mantle and move relative to each other as a result of convection currents within the mantle. Although individual plates are fairly stable, rela-tive movements at the plate boundaries are responsible for many geological processes. Sideways movements create tear faults and are responsible for earthquakes: an example of this is the San Andreas fault in California. Where the plates tend to move away from each other or diverge, new oceanic crust is formed by the emergence of molten material from the mantle through volcanoes. Where the plates tend to collide or converge, the oceanic plate (sima) is forced down into the mantle where it tends to melt. The continental plate (sial) rides over the oceanic plate, and is crumpled and thickened to form a mountain chain (e.g. the Andes in South America). The geological process of mountain building is known as orogenesis.

1.3 The origin of soils

Soil is the term given to the unbonded, granular material which covers much of the surface of the Earth that is not under water. It is worth mentioning here that civil and geotechnical

6 Soil mechanics

engineers are not usually interested in the properties of the top metre or so of soilknown as topsoilin which plants grow, but in the underlying layers or strata of rather older geological deposits. The topsoil is not generally suitable for use as an engineering material, as it is too variable in character, too near the surface, too loose and compressible, has too high an organic content and is too susceptible to the effects of plants and animals and to seasonal changes in groundwater level.

Soil consists primarily of solid particles, which may range in size from less than a micron to several millimetres. Because many aspects of the engineering behaviour of

Figure 1.3 Classification of soils according to particle size.

soils depend primarily on the typical particle size, civil engineers use this criterion to clas-sify soils as clays, silts, sands or gravels. The system of soil classification according to particle size used in the UK is shown in Figure 1.3. There are other systems in use around the worldparticularly in the USAwhich differ slightly in detail, but the principle is the same (e.g. Winterkorn and Fang, 1991).

Most soils result from the breakdown of the rocks which form the crust of the Earth, by means of the natural processes of weathering due to the action of the sun, rain, water, snow, ice and frost, and to chemical and biological activity. The rock may be simply broken down into particles. It may also undergo chemical changes which alter its chemical composition or mineralogy. If the soil retains the characteristics of the parent rock and remains at its place of origin, it is known as a residual soil. More usually, the weathered particles will be transported by the wind, a river or a glacier to be deposited at some new location. During the transport process, the particles will probably be worn and broken down further, and sorted by size to some extent.

Many soil deposits may be up to 65 million years old. Geotechnical engineers frequently encounter sedimentary rocks, such as chalk, limestone and sandstone, which may be hun-dreds of millions of years old. The Earth itself is thought to be 45000 million years old, and anything which occurred after the end of the last glacial period of the Ice Age (10 000 years ago) is described by geologists as Recent. Soils and rocks are classified by geologists according to their age, with reference to a geological timescale divided into four eras. The eras are named according to the life-forms which existed at the time. They are:

Archaeozoic (before any form of life, as evidenced by observable fossil remains): more usually known as Pre-Cambrian. This period covers perhaps 3900 million years, from the creation of the Earth up to about 570 million years before the present.

Origins and classifi cation of soils 7

Palaeozoic (ancient forms of life, also known as Primary): 225570 million years ago.

Mesozoic (intermediate forms of life, also known as Secondary): 65225 million years ago.

Cainozoic (recent forms of life): commonly but probably artificially subdivided into the Quaternary (02 million years ago) and the Tertiary (265 million years ago).

The four eras are subdivided into periods on the basis of the animal and plant fossils pres-ent. The periods are in turn subdivided into rock series. During a given period of time within an era, a series of rocks (e.g. shales, sandstones, limestones), containing certain types of fossil, was deposited.

The periods are named in different ways, which may describe the types of rock laid down (cretaceous for chalk, carboniferous for coal); the nature of the fossil content (e.g. holocene, meaning recent); the names of the places where the rocks were first recognized (e.g. Devonian for Devon, Cambrian for Wales); tribal names (Silurian from the Silures and Ordovician from the Ordovices, both ancient Celtic tribes in Wales); or the number of series within the period (e.g. Triassic for three). The names of the eras and periods, together with an indication of the major geological activities, rocks and forms of life, are given in Table 1.1.

In view of the age of most soil deposits, the environment in which a particular soil deposit was laid down is unlikely to be the same as the environment at the same place today. Nonetheless, the transport process and the depositional environment of a particular stratum or layer of soil have a significant influence on its structure and fabric, and probably on its engineering behaviour. They are therefore worthy of some comment.

1.3.1 Transport processes and depositional environments

Water

Small particles settle through water very slowly. They therefore tend to remain in suspen-sion, enabling them to be transported much further by rivers than larger particles. The larg-est particles are carriedif at allby being washed along the bed of a river, rather than in suspension. Pebbles, gravels and coarse sands tend to be deposited on the bed of the river along most of its course. As the river changes its course due to the downstream migration of meanders (bends), or erodes a deeper channel in a process known as rejuvenation (fol-lowing, e.g. a fall in sea level), the coarse material is left behind to form a terrace. Silts and fine particles may also be deposited on either side of the river following a flood, because the floodwater is comparatively still. A soil deposited along the flood plain of a river is known as alluvium, or an alluvial deposit.

A river tends to flow more rapidly in its upper reaches than in its lower course. For example, the Amazon has a gradient of about 1 in 70000 in its lowest reaches, compared with gradients as high as 1 in 100 in many of the upper streams (Robinson, 1977). This means that particles which were carried in suspension in the upper reaches of a river begin to be deposited downstream as the flow velocity falls. At the mouth of the river, sediment

Tabl

e 1.

1 Si

mpl

ified

geo

logi

cal c

lass

ifica

tion

of so

ils a

nd ro

cks i

n te

rms o

f era

s and

per

iods

of t

ime

(fro

m R

obin

son,

197

7)

Origins and classification of soils 9

builds up on the river bed, and constant dredging is usually required if shipping channels are to remain navigable. Sediment is also carried into the sea and deposited: if it is not removed by the tide, a build-up of sediment known as a delta is formed, gradually extend-ing seaward from the coast.

The structure of a typical deltaic deposit is illustrated in Figure 1.4. The bottomset bedsare made up of the finer particles, which have been carried furthest in suspension beyond the delta slope before settling out. The foreset beds are made up of coarser material, which has carried along the river bed before coming to rest on the advancing face of the delta. The topset beds are deposited on top of the foreset beds, in much the same way as the alluvial deposits further upstream. Deltaic deposits generally comprise clays and silts, with some sands and organic matter.

Figure 1.4 Structure of a deltaic deposit (after Robinson, 1977).

Wind

Approximately one-third of the Earths land surface is classed as arid or semi-arid. Although it is likely that the original weathering processes took place when the climate was more humid than it is now, the primary transport process for soils in desert regions is the wind. Sand dunes gradually migrate in the direction of the wind. Fine particles may be carried for hundreds of kilometres as wind-borne dust. Dust may eventually arrive at a more humid area where it is washed out of the atmosphere by rain. It then settles and accumulates as a non-stratified, lightly cemented material known as loess. The cementing is due to the presence of calcium carbonate deposits, from decayed vegetable matter. If the soil becomes saturated with water, the light cementitious bonds are destroyed, and the structure of loess collapses. Extensive deposits of loess are found in north-western China. A soil which has been laid down by the wind is known as an aeolian deposit.

Ice

Ice sheets and valley glaciers are particularly efficient at both eroding rock and transporting the resulting debris. Material may be carried along on top of, within, and underneath an ice sheet or glacier as it advances. The effectiveness of ice as a mechanism of transportation does not (unlike water and wind) depend on particle size. It follows that deposits which

10 Soil mechanics

have been laid down directly by ice action (known as moraines) are generally not sorted, and so encompass a large range of particle size. A mound deposited at the end of a glacier is termed a terminal moraine, while the sheet deposit below the glacier is known as a ground moraine (Figure 1.5). Unsorted glacial moraine is known as glacial till or boulder clay. The particles found in glacial tills are generally fairly angular, in contrast to the more rounded particles associated with typical water-borne deposits.

Ice and water

Material from on top of or within a melting glacier or ice sheet might be carried away by the meltwater before finally coming to rest. This would result in a degree of sorting according to particle size, with the finer materials being carried further from the end of the glacier. Soils which have been transported, sorted and deposited in this way are described as fluvio-glacial materials. The outwash from an ice sheet can cover a considerable area, forming an extensive out-wash plain of fluvio-glacial material (Figure 1.5).

Figure 1.5 Depositional mechanisms associated with glaciers and ice sheets.

In some cases, the till may be carried by the meltwater into a lake formed by water trapped near the end of the retreating glacier or ice sheet. The larger particles then settle relatively quickly, forming a well-defined layer on the bottom of the lake. The smaller particles settle more slowly, but eventually form an overlying layer of finer material. With the next influx of meltwater, the process is repeated. Eventually, a soil deposit builds up which consists of alternating layers of fine and coarse material, each perhaps only a few millimetres thick (Figure 3.15). This layered or varved structure can have a significant effect on the engineering behaviour of the soil, as discussed in section 3.6.

Material transported by ice, and deposited either directly or sorted and re-laid by out-wash streams, is known as drift. The principal depositional mechanisms associated with glaciers and ice sheets are summarized in Figure 1.5.

In this section we have discussed the breakdown of rocks into soils. We should note in passing that this is only one-half of the geological cycle. As soils become buried by the deposition of further material on top, they can be converted back into rocks (sedimentary or metamorphic) by the application of increased pressure, and perhaps chemical changes. They might also be converted into igneous rocks, by means of tectonic activity. However,

Origins and classifi cation of soils 11

this book is concerned with soils rather than rocks, and a discussion of the formation of rocks is beyond its scope.

1.4 Soil mineralogy

1.4.1 Composition of soils

Soils are composed of minerals,2 which are in turn made up from the elements present in the crust of the Earth. These elements are primarily oxygen (approximately 46.6% by mass), silicon (27.7%), aluminium (8.1%), iron (5.0%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (2.1%) (Robinson, 1977; Blyth and de Freitas, 1984). Many of the other elements (such as gold, silver, tin and copper) are rare in a global sense, but are found in concentrated deposits from which they can be extracted economically. The most common elements occur in rocks as oxides, 75% of which are oxides of silicon and aluminium.

Most soils are silicates, which are minerals comprising predominantly silicon and oxy-gen. The basic unit of a silicate is a group comprising one silicon ion surrounded by four oxygen ions at the corners of a regular tetrahedron: (SiO4)

4. The superscript 4 indicates that the silica tetrahedron has a net negative charge equivalent to four electr ons, or valency 4. This is because the silicon ion is Si4+, while the oxygen ion is O2. In order to become neutrally charged, the silica tetrahedron would need to combine with, for example, two metal ions of valency +2, such as magnesium Mg2+, to give Mg2SiO4 (olivine).

The (SiO4)4 groups may link together in different ways with metal ions and with each

other, to form different crystal structures. Although there are many silicate minerals, their properties (such as hardness and stability) depend primarily on their structure.

The (SiO4)4 tetrahedra may be independentjoined entirely with metal ions, rather

than to each otheras in the olivine group of minerals. Alternatively, they may be joined at the corners to form pairs (amermanite: each silica tetrahedron shares one oxygen ion), sin-gle chains (pyroxenes: each tetrahedron shares two oxygen ions), double chains or bands (amphiboles: two or three oxygen ions shared, depending on the position of the tetrahedron in the band) or rings (e.g. beryl: two oxygen ions shared). Some of the silicon ions (Si4+) may be replaced by aluminium ions (Al3+), as in augite and hornblende. In this case, the additional negative charge (which arises because of the different valencies of aluminium and silicon) can be balanced by the incorporation of metal ions such as sodium Na+ and potassium K+.

Sheet silicates (also known as phyllosilicates or layer-lattice minerals), such as mica, chlorite and the clay minerals, are formed when three of the four oxygen ions are shared with other tetrahedra. Sheet silicates are generally soft and flaky.

The strongest silicate minerals are those in which all four oxygen ions of each (SiO4)4

tetrahedron are shared with other tetrahedra, resulting in a three-dimensional framework structure.

The arrangements of the silica tetrahedra found in the various silicate minerals are shown diagrammatically in Figure 1.6.

12 Soil mechanics

1.4.2 The clay minerals

The clay minerals represent an important sub-group of the sheet silicates or phyllosilicates. In the context of soil mineralogy, the term clay is used to denote particular mineralogi-cal properties, in addition to a small particle size. These include a net negative electrical charge, plasticity when mixed with water and a high resistance to weathering. A further distinction between clay and non-clay minerals is that particles of non-clay minerals are generally bulky or rotund, while clay mineral particles are usually flat or platey.

Essentially, the clay minerals can be considered to be made up of basic units or lay-ers comprising two or three alternating sheets of silica, and either brucite [Mg3(OH)6] or gibbsite [Al2(OH)6]. Generally, the bonding between the sheets of silica and gibbsite or brucite within each layer is strong, but the bonding between layers may be weak. (Note that the terms sheet and layer are used quite distinctly: a layer of the mineral is made up of two or three sheets of silica and gibbsite/brucite.) The most common clay mineral groups are kaolinite, montmorillonite or smectite, and illite. Some clay minerals contain loosely

Figure 1.6 Chemistry and structure of silicate minerals. (Redrawn from J.E.Gillott, clay in engineering Geology, 1968, pp. 9697,with kind permission from Elsevier ScienceNL, Sara Burgerhartstraat 25, 1055KV Amsterdam, The Netherland.)

bonded metal ions (cations), which can easily be exchanged for other species (e.g. sodium is readily displaced by calcium), depending on local ion concentrations (e.g. in the pore water). This process is known as base exchange.

Origins and classifi cation of soils 13

Kaolinite

Kaolinite has a two-sheet structure, comprising silica and gibbsite. It is the principal com-ponent of china clay and results from the destruction of alkali feldspars (section 1.4.3) under acidic conditions. It has few or no exchangeable cations, and the interlayer bonds are reasonably strong. For these reasons, kaolin might be described as the least clay-like of the clay minerals, and it tends to form particles whichfor a clayare relatively large. Particles of well-crystallized kaolin appear as hexagonal plates, with lateral dimensions in the range 0.14m, and thicknesses of 0.05 to 2m. Poorly crystallized kaolinite tends to form platey particles which are smaller and less distinctly hexagonal.

Montmorillonite

The montmorillonite or smectite group of clay minerals have a three-sheet structure com-prising a sheet of gibbsite sandwiched between two silica sheets. Montmorillonites (smec-tites) have a similar basic structure to the non-clay mineral group known as pyrophyllites. The difference is that in smectites there is extensive substitution of silicon (by aluminium) in the silica sheets, and of aluminium (by magnesium, iron, zinc, nickel, lithium and other cations) in the gibbsite sheet. The additional negative charges which result from these substitutions are balanced by exchangeable cations, such as sodium and calcium, located between the layers and on the surfaces of the particles. The interlayer bonds are weak, and layers are easily separated by cleavage or by the adsorption of water. Thus smectite par-ticles are very small (often only one layer or 1nm thick), and can swell significantly by the adsorption of water. Soils which contain montmorillonites (smectites) exhibit a consider-able potential for volume change: because of this characteristic, they are sometimes known as expansive soils.

Bentonite is a particular type of montmorillonite which is used extensively in geotech-nical engineering. A suspension of 5% bentonite (by mass) in water will form a viscous mud, which is used to support the sides of boreholes and trench excavations, which are later filled with concrete to create deep foundations (known as piles: Chapter 8) and cer-tain types of soil retaining wall. It has many other uses, including the sealing of boreholes and the construction of barriers to groundwater flow, known as cut-off walls (section 3.3, Example 3.1).

Montmorillonite particles are generally 12m in length. Particle thicknesses occur in multiples of 1nmthe thickness of a single silica/gibbsite/silica layerfrom 1nm up to about 1/100 of the particle length.

Illite

Illite also has a three-sheet structure, comprising a sheet of gibbsite sandwiched between two silica sheets. In illite, the layers are separated by potassium ions, whereas in montmo-rillonite the layers are separated by cations in water. Illites have the same basic structure as the non-clay minerals muscovite mica and pyrophyllite. Muscovite differs from pyrophyl-

14 Soil mechanics

lite in that 25% of the silicon positions are taken by aluminium, and the resulting excess negative charges are balanced by potassium ions between the layers. Illite differs from muscovite in that fewer of the Si4+ positions are taken by Al3+, so that there is less potas-sium between the layers. Also, the layers are more randomly stacked, and illite particles are smaller than mica particles. Illite may contain magnesium and iron as well as aluminium in the gibbsite sheet. Iron-rich illite, which has a distinctive green hue, is known as glau-conite.

Illites usually occur as small, flaky particles mixed with other clay and non-clay min-erals. Illite particles range generally from 0.1m to a few micrometre in length, and may be as small as 3nm thick. Unlike kaolinite and montmorillonite, their occurrence in high-purity deposits is unknown.

Other clay minerals

There are two other groups of clay minerals: vermiculites, which have a similar tendency to swell as montmorillonites; and palygorskites, which are not common, and have a chain (rather than a sheet) structure.

1.4.3 Non-clay minerals

The most abundant non-clay mineral in soils generally is quartz (SiO2). Quartz is a frame-work silicate, in which the silica tetrahedra are grouped to form spirals. Small amounts of feldspar and mica are sometimes present, but pyroxenes and amphiboles (single and double chain silicates) are rare. This is very different from the typical composition of igneous rock, the parent material from which many soils were broken down, which might be 60% feld-spars, 17% pyroxenes and amphiboles, 12% quartz and 4% micas (Mitchell, 1993).

Quartz is quite hard (rated H=7 on an arbitrary 10-point scale where diamond, the hard-est, has H=10 and talc, the softest, has H=1) and resistant to abrasion. It is also chemically and mechanically very stable, as it is already an oxide and has a structure without cleavage planes, along which the material can easily be split. These factors explain its persistence and prevalence in non-clay soils (sands and gravels), which have a comparatively large particle size.

Feldspars also have three-dimensional framework structures, but some of the silicon ions have been replaced by aluminium. The resultant excess negative charge is balanced by the inclusion of cations such as potassium, sodium and calcium. This leads to a more open structure, with lower bond strengths between structural units. Thus feldspars are not as hard as quartz (they will cleave or split along weakly bonded planes) and they are more easily broken down. This is why they are not as prevalent in soils generally as they are in igneous rocks. Pyroxenes, amphiboles and olivines are also relatively easily broken down, which is again why they are absent from many soils.

1.4.4 Surface forces

In a solid material, atoms are bonded together in a three-dimensional structure. At the sur-face of the solid, the structure is interrupted, leaving unbalanced molecular forces.

Origins and classifi cation of soils 15

Table 1.2 Specific surface area of sand and clay particles (data from Mitchell, 1993)

Mineral group Partide length Particle thickness Spedfic surface area (m2/g)Sand 2mm 2mm 5104

Sand 1mm 1mm 103

Kaolinite 0.14m 0.052m 1020Illite 0.14m 3nm 65100Montmorillonite 12m 120nm up to 840

Equilibrium across the surface may be restored by the attraction and adsorption of mol-ecules from the adjacent phase (in soils, from the pore water); by cohesion (i.e. sticking together) with another mass of the same material; or by the adjustment of the molecular structure at the surface of the solid.

An unbalanced bond force is significant in comparison with the weight of a molecule, but not in comparison with the weight of a soil particle which is as large as a grain of sand. However, as the particle size is reduced, the ratio of the surface area to the volume or mass of the particle increases dramatically, as indicated in Table 1.2. The total surface area of the particles in 10g of montmorillonite is equivalent to a football pitch.

It might, therefore, be supposed that surface forces could have a significant influence on the behaviour of clay soils. At low stressesfor example, when clay particles are dispersed in a column of waterthis is indeed the case, and many clays behave as colloids in these circumstances (i.e. the clay particles are able to remain suspended in water, because the forces which tend to support them are greater than the gravitational force which tends to cause them to settle out). This is partly due to the small size of the clay particles, and partly due to the electrical surface forces which result from the substitution of ionsfor example, Al3+ for Si4+within their structure.

In most geotechnical engineering applications, however, the appropriate comparison is between the surface forces and the gravitational force due not just to the mass of a single particle, but to the total mass of soil above the particle in a deposit. This is more or less the same, whether the deposit is a sand or a clay. Thus in soil mechanics and geotechnical engi-neering, the surface forces between clay particles are not generally significant, and they do not have to be taken into account by means of some special form of analysis. Although surface forces and pore water chemistry might influence the structure of a newly deposited clay, the same laws apply in practical terms to soils made up of clay particles as to soils made up of non-clay particles. Certain effects might be more pronounced in clays than in sands (see, in particular, Chapter 4), but this is due to the difference in particle size, rather than to the influence of surface chemistry.

The strength of an assemblage of soil particles, be they sand or clay, comes primarily from interparticle friction. In some natural deposits the particles may be lightly cemented together, but this is more common in sands than in clays. Although a lump of moist clay can be moulded in the hand (whereas a lump of moist sand would fall apart) this is not due to interparticle or cohesive bonds. If it were, the clay would remain intact if it were immersed in water for a week or so. (Unless the particles are cementedin which case,

16 Soil mechanics

the soil will probably be too hard or brittle to mould by handa small lump of sand or clay will disintegrate very easily if it is kept immersed in water for long enough.) Clay soils can be moulded in the hand because the spaces or voids between the clay particles are small enough to hold water at a negative pressure, essentially by capillary action. (It may be, however, that the negative pore water pressures in a stiff clay paste which has just been made by mixing clay particles with water result from the tendency of the clay particles to adsorb water, and is therefore due to surface effects.) This negative pore water pres-sureor suctionpulls the particles together, giving the soil mass some shear strength. This concept is discussed in section 1.7.

1.4.5 Organic (non-mineral) soils

Some soils (notably peat) do not result from the breakdown of rocks, but from the decay of organic matter. Like topsoil, these soils are not suitable for engineering purposes. Peat is very highly compressible, and will often have a mass density which is only slightly greater than that of water. Unlike topsoil, organic soils may be naturally buried below the surface, and their presence is not necessarily obvious. This can make life difficult for the civil engi-neer, because it is important that these soils are detected at an early stage of a project, and if necessary, removed. They should not be relied on for anything, except to cause trouble.

Most of the factual content of section 1.4 is taken from the account given by Mitchell (1993), to which book the reader is referred for further details.

1.5 Phase relationships for soils

Soil is made up essentially of solid particles, with spaces or voids in between. The assem-blage of particles in contact is usually referred to as the soil matrix or the soil skeleton. In conventional soil mechanics, it is assumed that the voids are in general occupied partly by water and partly by air. This means that an element of soil (by which we mean the solid particles plus the substance(s) in the voids they enclose) may be a three-phase mate-rial, comprising some solid (the soil grains), some liquid (the pore water) and some gas (the pore air). This is illustrated schematically in Figure 1.7. A given mass of soil grains in particulate form occupies a larger volume than it would if it were in a single solid lump, because of the volume taken up by the voids.

Figure 1.7 Soil as a three-phase material.

Origins and classifi cation of soils 17

Figure 1.7 gives rise to a number of fundamental definitions, known as phase relation-ships, which tell us about the relative volumes of solid, liquid and gas. The phase relation-ships are important in beginning to characterize the state of the soil. They are:

1. The void ratio, which is defined as the ratio of the volume of the voids to the volume of solids (i.e. the soil particles), and is conventionally given the symbol e:

Void ratio e=volume of voidsvolume of solids=Vv/Vs (1.1)

2. The specific volume, which is defined as the actual volume occupied by a unit volume of soil solids. It is conventionally given the symbol :

Specific volume =total volumevolume of solids

=(Vs+Vv)/Vs=1+e (1.2)

3. The porosity, which is defined as the volume of voids per unit total volume, and is given the symbol n:

Porosity n=volume of voidstotal volume

= Vv/(Vs+Vv)=e/(1+e)=(1) (1.3)

4. The saturation ratio, which is defined as the ratio of the volume of water to the vol-ume of voids, and is usually given the symbol S or Sr:

Saturation ratio Sr=volume of watervolume of voids

=Vw/Vv (1.4)

The saturation ratio must lie in the range 0Sr1. If the soil is dry, Sr=0. If the soil is fully saturated, Sr=1.

Alternatively, the state of saturation of the soil may be quantified by means of the air content A, which is defined as the ratio of the volume of air to the total volume,

Air content A=volume of airtotal volume

=Va/(Vs+Vv)

Substituting Va=VvVw, and dividing through the top and the bottom of the definition of A by the volume of voids Vv, it can be shown that

A=(1)(1Sr)/=n(1Sr)

5. The water content (or moisture content), which is defined as the ratio of the mass of water to the mass of soil solids, and is given the symbol w:

Water content w=mw/ms (1.5)

18 Soil mechanics

The void ratio, the specific volume and the porosity are all indicators of the efficiency with which the soil particles are packed together. They are not independent: if one is known, the other two may be calculated. The choice of which one to use is largely a matter of personal preference. The specific volume and the void ratio e are more commonly used than the porosity n. The specific volume is often the most mathematically convenient.

Sands normally have specific volumes in the range 1.32.0 (e=0.31.0). For clays, the specific volume depends on the current stress state and the stress history, and also the mineralogy. The specific volume of a montmorillonite (such as the bentonite mud used as a temporary support for boreholes and trench excava