Soil Mechanics, 6th edition

Soil Mechanics

Soil Mechanics Sixth edition

R.F.Craig Department of Civil Engineering University of Dundee UK

London and New York

First published 1974 by E & FN Spon, an imprint of Chapman & Hall Second edition 1978 Third edition 1983 Fourth edition 1987 Fifth edition 1992 Sixth edition 1997 Spon Press is an imprint of the Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2004. © 1974, 1978, 1983, 1987, 1992, 1997 R.F.Craig 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. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0-203-35875-9 Master e-book ISBN

ISBN 0-203-37502-5 (Adobe eReader Format) ISBN 0-419-22450-5 (Print Edition)

Contents Preface

viii

1

Basic characteristics of soils 1.1 The nature of soils 1.2 Particle size analysis 1.3 Plasticity of fine-grained soils 1.4 Soil description and classification 1.5 Phase relationships 1.6 Soil compaction Problems References

1 1 6 8 10 24 28 36 37

2

Seepage 2.1 Soil water 2.2 Permeability 2.3 Seepage theory 2.4 Flow nets 2.5 Anisotropic soil conditions 2.6 Non-homogeneous soil conditions 2.7 Transfer condition 2.8 Seepage through embankment dams 2.9 Grouting 2.10 Frost heave Problems References

38 38 39 46 50 58 61 62 64 75 75 76 80

3

Effective stress 3.1 Introduction 3.2 The principle of effective stress 3.3 Response of effective stress to a change in total stress 3.4 Partially saturated soils 3.5 Influence of seepage on effective stress Problems References

81 81 81 84 89 91 100 102

4

Shear strength 4.1 The Mohr-Coulomb failure criterion 4.2 Shear strength tests

103 103 106

vi

CONTENTS

4.3 4.4 4.5 4.6 4.7

Shear strength of sands Shear strength of saturated clays The critical state concept Residual strength Pore pressure coefficients Problems References

115 118 133 140 142 149 151

5

Stresses and displacements 5.1 Elasticity and plasticity 5.2 Stresses from elastic theory 5.3 Displacements from elastic theory Problems References

152 152 161 172 177 177

6

Lateral earth pressure 6.1 Introduction 6.2 Rankine’s theory of earth pressure 6.3 Coulomb’s theory of earth pressure 6.4 Application of earth pressure theory to retaining walls 6.5 Design of earth-retaining structures 6.6 Gravity walls 6.7 Embedded walls 6.8 Braced excavations 6.9 Diaphragm walls 6.10 Reinforced soil Problems References

179 179 181 195 201 204 206 214 232 234 237 241 246

7

Consolidation theory 7.1 Introduction 7.2 The oedometer test 7.3 Consolidation settlement: one-dimensional method 7.4 Settlement by the Skempton-Bjerrum method 7.5 The stress path method 7.6 Degree of consolidation 7.7 Terzaghi’s theory of one-dimensional consolidation 7.8 Determination of coefficient of consolidation 7.9 Correction for construction period 7.10 Numerical solution 7.11 Vertical drains Problems References

248 248 249 256 259 265 266 267 274 283 289 292 298 299

CONTENTS

vii

8

Bearing capacity 8.1 Foundation design 8.2 Ultimate bearing capacity 8.3 Allowable bearing capacity of clays 8.4 Allowable bearing capacity of sands 8.5 Bearing capacity of piles 8.6 Ground improvement techniques 8.7 Excavations 8.8 Ground anchors Problems References

302 302 305 318 319 339 361 363 366 370 372

9

Stability of slopes 9.1 Introduction 9.2 Analysis for the case of φ u=0 9.3 The method of slices 9.4 Analysis of a plane translational slip 9.5 General methods of analysis 9.6 End-of-construction and long-term stability 9.7 Embankment dams Problems References

376 376 377 380 386 388 390 394 399 401

10

Ground investigation 10.1 Introduction 10.2 Methods of investigation 10.3 Sampling 10.4 Borehole logs 10.5 Geophysical methods References

403 403 404 412 418 420 425

11

Case studies 11.1 Introduction 11.2 Field instrumentation 11.3 The observational method 11.4 Illustrative cases References

426 426 427 440 442 472

Principal symbols Answers to problems Index

474 478 481

Preface

This book is intended primarily to serve the needs of the undergraduate civil engineering student and aims at the clear explanation, in adequate depth, of the fundamental principles of soil mechanics. The understanding of these principles is considered to be an essential foundation upon which future practical experience in soils engineering can be built. The choice of material involves an element of personal opinion but the contents of this book should cover the requirements of most undergraduate courses to honours level (and now some Masters courses). It is assumed that the reader has no prior knowledge of the subject but has a good understanding of basic mechanics. The book includes a comprehensive range of worked examples and problems set for solution by the student to consolidate understanding of the fundamental principles and illustrate their application in simple practical situations. A list of references is included at the end of each chapter as an aid to the more advanced study of any particular topic. It is intended also that the book will serve as a useful source of reference for the practising engineer. In the sixth edition the aims and structure of the book remain unchanged. The main feature of the new edition is an additional chapter on case studies. The various types of instrumentation used to monitor in-situ performance in geotechnical engineering are described initially in this chapter then a number of real engineering situations are described and discussed to indicate how soil mechanics principles relate to practice. The other important feature of the sixth edition is an introduction to limit state design which recently has been introduced in new European geotechnical codes. Some minor updating and additions have also been made throughout the book. The author wishes to record his thanks to the various publishers, organizations and individuals who gave permission for the use of figures and tables of data, and to acknowledge his dependence on those authors whose works provided sources of material. The author also wishes to express his gratitude to lan Christie for his helpful comments on the drafts of the earlier editions and to all who worked on the manuscripts of the six editions. Extracts from BS 8004:1986 (Code of Practice for Foundations) and BS 5930:1981 (Code of Practice for Site Investigations) are reproduced by

PREFACE

permission of BSI. Complete copies of the codes can be obtained from BSI, Linford Wood, Milton Keynes, MK14 6LE. Robert F.Craig Dundee July 1996

ix

The unit for stress and pressure used in this book is kN/m2 (kilonewton per square metre) or, where appropriate, MN/m2 (meganewton per square metre). In SI the special name for the unit of stress or pressure is the pascal (Pa) equal to 1 N/m2 (newton per square metre). Thus: 1 kN/m2=1 kPa (kilopascal) 1 MN/m2=1 MPa (megapascal)

Basic characteristics of soils

1.1 THE NATURE OF SOILS To the civil engineer, soil is any uncemented or weakly cemented accumulation of mineral particles formed by the weathering of rocks, the void space between the particles containing water and/or air. Weak cementation can be due to carbonates or oxides precipitated between the particles or due to organic matter. If the products of weathering remain at their original location they constitute a residual soil. If the products are transported and deposited in a different location they constitute a transported soil, the agents of transportation being gravity, wind, water and glaciers. During transportation the size and shape of particles can undergo change and the particles can be sorted into size ranges. The destructive process in the formation of soil from rock may be either physical or chemical. The physical process may be erosion by the action of wind, water or glaciers, or disintegration caused by alternate freezing and thawing in cracks in the rock. The resultant soil particles retain the same composition as that of the parent rock. Particles of this type are approximately equidimensional and are described as being of ‘bulky’ form: the particles may be angular, subangular or rounded. The particles occur in a wide range of sizes, from boulders down to the fine rock flour formed by the grinding action of glaciers. The structural arrangement of bulky particles (Fig. 1.1) is described as single grain, each particle being in direct contact with adjoining particles without there being any bond or cohesion between them. The structure may be loose, medium dense or dense, depending on the way in which the particles are packed together. The chemical process results in changes in the mineral form of the parent rock due to the action of water (especially if it contains traces of acid or alkali), oxygen and carbon dioxide. Chemical weathering results in the formation of groups of crystalline particles of colloidal size (