An Introduction to Soil Mechanics and Foundations THIRD EDITION C. R. SCOTT B.A., M.I.C.E., M.I.Struct.E. Senior Lecturer in Civil Engineering, The City University, London Springer-Science+Business Media, B.V. First edition 1969 Second edition 1974 Third edition 1980 Reprinted 1994 (!;) 1994 Springer Science+Business Media Dordrecht Originally published by Chapman & Hall in 1994 ISBN 978-0-419-16040-3 ISBN 978-1-4899-7250-7 (eBook) DOI 10.1007/978-1-4899-7250-7 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British library Preface This book is mainly intended to meet the needs of undergraduate students of Civil Engineering. A certain amount of factual information, in the form of design charts and tables, has been included. I hope, therefore, that the book wilt prove to be of use to the students when their courses are over, and will help to bridge the awkward gap between theory and practice. In preparing the first edition of this book, I had two principal aims: firstly to provide the student with a description of soil behaviour-and of the effects of the clay minerals and the soil water on such behaviour-which was rather more detailed than is usual in an elementary text, and secondly to encourage him to look critically at the traditional methods of analysis and design. The latter point is important, since all such methods require certain simplifying assumptions without which no solution is generally possible. Serious errors in design are seldom the result of failure to understand the methods as such. They more usually arise from a failure to study and understand the geology of the site, or from attempts to apply analytical methods to problems for which the implicit assumptions make them unsuitable. In the design of foundations and earth structures, more than in most branches of engineering, the engineer must be continually exercising his judgement in making decisions. The analytical methods cannot relieve him of this responsibility, but, properly used, they should ensure that his judgement is based on sound knowledge and not on blind intuition. In this third edition, a few minor alterations have been made to take account of recent research. The main changes, however, concern the core of the book, consisting of Chapters 5 to 8. These have been largely rewritten to place the study of soil strength and consolidation more firmly in the context of plasticity theory. Substantial changes have also been made in Chapters 9, 10 and 11 to allow a fuller discussion of the methods of analysis of lateral earth pressure, bearing capacity of foundations and the stability of slopes. I am grateful to the following for permission to reproduce diagrams and other material from their publications: the iii iv PREFACE Councils of the Institutions of Civil, Structural and Water Engineers, the Director of Road Research, the Clarendon Press, Butterworths and Co., McGraw-Hill Book Co., John Wiley and Sons, Addison-Wesley, Prof. A. Casagrande, and the Controller of H.M. Stationery Office. Extracts from British Standards and Codes of Practice are reproduced by permission of the British Standards Institution, 2 Park Street, London WI, from whom copies of the complete Standards and Codes of Practice may be obtained. Finally, I should like to thank my colleagues in The City University for their help and encouragement, and in particular Mr R. H. Foster, Mr J.D. Coleman, Mr G. A. Watt and Mr J. S. Evans for their generous assistance in the revision of the text and their many helpful suggestions. Contents Preface iii Chapter 1. The analysis and classification of soils Chapter .2. The clay minerals 31 Chapter 3. Pore pressure, effective stress and suction 49 Chapter 4. Permeability and seepage 63 Chapter 5. A model for soil consolidation 93 Chapter 6. A model for shear strength of soil 119 Chapter 7. General soil models 141 Chapter 8. Plasticity, limit analysis, and limiting equilibrium 177 Chapter 9. Lateral earth pressure 211 Chapter 10. Stability of slopes 251 Chapter 11. Bearing capacity of foundations 289 Chapter 12. Settlement of foundations 313 Chapter 13. Piled foundations 343 Chapter 14. Geotechnical processes 359 Chapter 15. Site investigation and in situ tests 379 Index 401 v List of symbols The symbols used in the text conform, as far as possible, with the recommendations of BS 1991: Part 4: 1961. Other symbols have been chosen to conform generally with established usage. The reader should notice that, in conformity with BS 1991, the bulk density (y) has, throughout this book, been defined as the weight density or unit weight (that is, the gravitational force per unit volume, measured here in kN/m3). The significance of the more important symbols is given below. A air void ratio: area: pore pressure coefficient. A f pore pressure coefficient A at failure. a area. B breadth: pore pressure coefficient. b breadth. C, compression index: coefficient of curvature. c. swelling index: compressibility of the soil skeleton. C" uniformity coefficient. Cv compressibility of the pore fluid. c apparent cohesion (in terms of total stress). c' apparent cohesion (in terms of effective stress). c' e Hvorslev's effective cohesion. c', residual value of apparent cohesion (in terms of effective stress). cu apparent cohesion under undrained conditions (in terms of total stress). cv coefficient of consolidation. c. coefficient of swelling. c.., adhesion between soil and a retaining wall. b depth: diameter. Df depth factor. D effective particle size. 10 d depth: diameter: length of drainage path. E Young's modulus. e void ratio. e, critical void ratio. F factor of safety. F, factor of safety with respect to cohesion. F¢ factor of safety with respect to friction. f plastic potential function. G. specific gravity of soil particles. H height. vii viii LIST OF SYMBOLS He critical height of an unsupported vertical bank. h height: total hydraulic head. influence coefficient for stress below a foundation. /11 /P influence coefficient for settlement of a foundation. i hydraulic gradient. ic critical hydraulic gradient for unstable upward flow. K coefficient of lateral earth pressure. Ka Rankine's coefficient of active earth pressure. Kao Kaq• Kay general coefficients of active earth pressure. KP Rankine's coefficient of passive earth pressure. Kpc, Kpq' KPY general coefficients of passive earth pressure. K coefficient of earth pressure at rest. 0 k permeability. L length. Ll liquidity index. LL liquid limit. I length. mv coefficient of volume change. N Taylor's stability number for earth slopes. Nc, Nq, NY bearing capacity factors. n porosity. P force. active thrust of earth on a retaining wall. passive resistance of earth to a movement of a retaining wall. PI plasticity index. PL plastic limit. p pressure: mean normal stress (=a 1 + a 2 + a 3). p' mean effective stress (=a' 1 + a' 2 + a' 3). Pa active lateral earth pressure. p'c preconsolidation pressure (spherical consolidation). p'e equivalent consolidation pressure (spherical consolidation). soil suction. initial value of p '. passive lateral earth pressure. fi roc} q rate of flow: deviator stress ( = allowable bearing pressure of a foundation. gross ultimate bearing capacity of a foundation. net foundation pressure. surcharge load on the ground surface. relative density. overconsolidation ratio (spherical consolidation). overconsolidation ratio (one-dimensional consolidation). r radius. LIST OF SYMBOLS lX pore pressure ratio. degree of saturation. shrinkage limit. torque: time factor: surface tension. time factor (one-dimensional consolidation). time. degree of consolidation (one-dimensional). mean degree of consolidation. pore pressure. pore air pressure. pore water pressure. volume. volume of solids. volume of voids. velocity: specific volume ( = l +e). weight. IV water content. saturated water content. body force in the (negative) x direction. body force in the (negative) z direction. f3 slope angle: inclination of x ax1s to horizontal: dilatancy angle. specific volume at the critical state for p' = 1·0. angle of wall friction. bulk density (weight density): shear strain. submerged density (weight density). shear strain rate (with respect to time). plastic shear strain. dry density (weight density). density of fluid. density of soil particles. saturated density (weight density). density of water (weight density). normal strain. plastic component of normal strain. normal strain rate (with respect to time). rr dynamic viscosity: efficiency of a pile group. K slope of an overconsolidation line on the v·: log.p' plane. slope of the normal (spherical) consolidation line on the v: log.p' plane. M slope of the critical state line in a plane of constant specific volume. slope of the state boundary surface in a plane of constant specific volume. coefficient of friction. J1. X LIST OF SYMBOLS N specific volume for normal (spherical) consolidation atp' = 1·0. \' Poisson's ratio. p surface settlement. 11 total normal stress. a' effective normal stress. 111, 112, 113 principal normal stress components. 11",11, axial and radial stress components in a triaxial compression test. a'c preconsolidation pressun~. } a ' .. equivalent consolidation pressure (one-dimensional). initial consolidation pressure. component of stress normal to a surface of sliding. 110"' octahedral normal stress. a, vertical component of normal stress. r shear stress components. shear strength. rmax maximum shear strength. octahedral shear stress. potential function angle of shearing resistance (in terms of total stress). angle of shearing resistance (in terms of effective stress). ¢'" angle of shearing resistance at constant volume (in terms of effective stress). Hvorslev's effective angle of internal friction. angle of shearing resistance for undrained conditions (in terms of total stress). residual angle of shearing resistance (in terms of effective stress). angle of friction at an intergranular contact. coefficient defining effective stress in partially saturated soils. stream function.