Craig's Soil Mechanics 7th Edition PDF

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Craig’s Soil Mechanics Craig’s Soil Mechanics Seventh edition R.F. Craig Formerly Department of Civil Engineering University of Dundee UK 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 edi...

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Craig’s Soil Mechanics

Craig’s Soil Mechanics

Seventh edition

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

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 Seventh edition 2004 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Spon Press 29 West 35th Street, New York, NY 10001 This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.”

Spon Press is an imprint of the Taylor & Francis Group ª 1974, 1978, 1983, 1987, 1992, 1997, 2004 R.F. Craig All rights reserved. No part of this book may be reprinted or reproduced or utilised 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 Cataloging in Publication Data Craig, R.F. (Robert F.) [Soil mechanics] Craig’s soil mechanics / R.F. Craig. — 7th ed. p. cm. Second ed.: Soil mechanics / R.F. Craig. Van Nostrand Reinhold, 1978. Includes bibliographical references and index. ISBN 0–415–32702–4 (hb.: alk. paper) — ISBN 0–415–32703–2 (pbk.: alk. paper) 1. Soil mechanics. I. Title: Soil mechanics. II. Craig, R.F. (Robert F.) Soil mechanics. III. Title. TA710.C685 2004 624.10 5136—dc22 2003061302 ISBN 0-203-49410-5 Master e-book ISBN

ISBN 0-203-57441-9 (Adobe eReader Format) ISBN 0–415–32702–4 (hbk) ISBN 0–415–32703–2 (pbk)

Contents

Preface

viii

1

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

1 1 6 7 9 17 21 28 29

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

30 30 31 37 42 49 52 53 55 66 66 67 70

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

71 71 71 74 79 80 88 90

vi

Contents

4 Shear strength 4.1 Shear failure 4.2 Shear strength tests 4.3 Shear strength of sands 4.4 Shear strength of saturated clays 4.5 The critical-state concept 4.6 Residual strength 4.7 Pore pressure coefficients Problems References

91 91 94 102 106 118 125 127 133 135

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

136 136 144 155 159 160

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

161 161 162 176 184 185 187 195 212 215 217 221 225

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

227 227 227 235 237 243 244 245 252 260 265 268 274 276

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

277 277 281 293 294 311 332 336 339 342 344

9

Stability of slopes 9.1 Introduction 9.2 Analysis for the case of
u ¼ 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

347 347 348 351 357 360 362 364 369 371

10

Ground investigation 10.1 Introduction 10.2 Methods of investigation 10.3 Sampling 10.4 Borehole logs 10.5 Geophysical methods 10.6 Ground contamination References

373 373 374 381 388 388 393 394

11

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

395 395 396 407 409 434

Principal symbols Answers to problems Index

436 440 443

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 geotechnical 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 as well as parts of 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. Both the traditional and limit state methods of design are included and some of the concepts of geotechnical engineering are introduced. The different types of field instrumentation are described and a number of case studies are included in which the differences between prediction and performance are discussed. References are included as an aid to the more advanced study of any particular topic. It is intended that the book will serve also as a useful source of reference for the practising engineer. The author wishes to record his thanks to the various publishers, organizations and individuals who have given permission for the use of figures and tables of data, and to acknowledge his dependence on those authors whose works provided sources of material. Extracts from BS 8004: 1986 (Code of Practice for Foundations) and BS 5930: 1999 (Code of Practice for Site Investigations) are reproduced by permission of BSI. Complete copies of these codes can be obtained from BSI, Linford Wood, Milton Keynes, MK14 6LE. Robert F. Craig Dundee March 2003

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 ¼ 1 kPa (kilopascal) 1 MN/m2 ¼ 1 MPa (megapascal)

Chapter 1

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 described as being of ‘bulky’ form and their shape can be indicated by terms such as angular, rounded, flat and elongated. 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 (Figure 1.1) is described as single grain, each particle being in direct contact with adjoining particles without there being any bond between them. The state of the particles can be described as dense, medium dense or loose, depending on how they 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 ( 90) upper plasticity range (wL > 35)

W P Pu Pg L I H V E U O

Figure 1.7 Plasticity chart: British system (BS 5930: 1999).

size and as sandy if more than 50% of the coarse fraction is of sand size. The alternative term M-SOIL is introduced to describe material which, regardless of its particle size distribution, plots below the A-line on the plasticity chart: the use of this term avoids confusion with soils of predominantly silt size (but with a significant proportion of clay-size particles) which plot above the A-line. Fine soils containing significant amounts of organic matter usually have high to extremely high liquid limits and plot below the A-line as organic silt. Peats usually have very high or extremely high liquid limits. Any cobbles or boulders (particles retained on a 63-mm BS sieve) are removed from the soil before the classification tests are carried out but their percentages in the total sample should be determined or estimated. Mixtures of soil and cobbles or boulders can be indicated by using the letters Cb (COBBLES) or B (BOULDERS) joined by

Soil description and classification

15

a þ sign to the group symbol for the soil, the dominant component being given first, for example: GW þ Cb – well-graded GRAVEL with COBBLES B þ CL – BOULDERS with CLAY of low plasticity. A general classification system is useful in developing an understanding of the nature of different soil types. However, in practice it is more appropriate to use systems based on properties related to the suitability of soils for use in specific construction situations. For example, the UK Department of Transport [6] has detailed a classification system for the acceptability of soils for use in earthworks in highway construction. In this system, soils are allocated to classes based mainly on grading and plasticity but, for some classes, chemical, compaction and strength characteristics are also specified.

Example 1.1 The results of particle size analyses of four soils A, B, C and D are shown in Table 1.4. The results of limit tests on soil D are: Liquid limit: Cone penetration (mm) Water content (%) Plastic limit: Water content (%)

15.5 39.3

18.0 40.8

23.9

24.3

19.4 42.1

22.2 44.6

24.9 45.6

The fine fraction of soil C has a liquid limit of 26 and a plasticity index of 9. (a) Determine the coefficients of uniformity and curvature for soils A, B and C. (b) Allot group symbols, with main and qualifying terms to each soil.

Table 1.4 BS sieve

Particle size*

Percentage smaller Soil A

63 mm 20 mm 6.3 mm 2 mm 600 mm 212 mm 63 mm

100 64 39 24 12 5 0 0.020 mm 0.006 mm 0.002 mm

Note * From sedimentation test.

Soil B

100 98 90 9 3

Soil C

Soil D

100 76 65 59 54 47 34 23 14 7

100 95 69 46 31

16

Basic characteristics of soils

The particle size distribution curves are plotted in Figure 1.8. For soils A, B and C the sizes D10, D30 and D60 are read from the curves and the values of CU and CZ are calculated:

Soil

D10

D30

D60

CU

CZ

A B C

0.47 0.23 0.003

3.5 0.30 0.042

16 0.41 2.4

34 1.8 800

1.6 0.95 0.25

For soil D the liquid limit is obtained from Figure 1.9, in which cone penetration is plotted against water content. The percentage water content, to the nearest integer, corresponding to a penetration of 20 mm is the liquid limit and is 42. The plastic limit is the average of the two percentage water contents, again to the nearest integer, i.e. 24. The plasticity index is the difference between the liquid and plastic limits, i.e. 18. Soil A consists of 100% coarse material (76% gravel size; 24% sand size) and is classified as GW: well-graded, very sandy GRAVEL. Soil B consists of 97% coarse material (95% sand size; 2% gravel size) and 3% fines. It is classified as SPu: uniform, slightly silty, medium SAND. Soil C comprises 66% coarse material (41% gravel size; 25% sand size) and 34% fines (wL ¼ 26, IP ¼ 9, plotting in the CL zone on the plasticity chart). The classification is GCL: very clayey GRAVEL (clay of low plasticity). This is a till, a glacial deposit having a large range of particle sizes. Soil D contains 95% fine material: the liquid limit is 42 and the plasticity index is 18, plotting just above the A-line in the CI zone on the plasticity chart. The classification is thus CI: CLAY of intermediate plasticity.

Figure 1.8 Particle size distribution curves (Example 1.1).

Phase relationships

17

Figure 1.9 Determination of liquid limit.

1.5

PHASE RELATIONSHIPS

Soils can be of either two-phase or three-phase composition. In a completely dry soil there are two phases, namely the solid soil particles and pore air. A fully saturated soil is also two phase, being composed of solid soil particles and pore water. A partially saturated soil is three phase, being composed of solid soil particles, pore water and pore air. The components of a soil can be represented by a phase diagram as shown in Figure 1.10(a). The following relationships are defined with reference to Figure 1.10(a). The water content (w), or moisture content (m), is the ratio of the mass of water to the mass of solids in the soil, i.e. w¼

Mw Ms

Figure 1.10 Phase diagrams.

ð1:3Þ

18

Basic characteristics of soils

The water content is determined by weighing a sample of the soil and then drying the sample in an oven at a temperature of 105–110  C and reweighing. Drying should continue until the differences between successive weighings at four-hourly intervals are not greater than 0.1% of the original mass of the sample. A drying period of 24 h is normally adequate for most soils. (See BS 1377.) The degree of saturation (Sr) is the ratio of the volume of water to the total volume of void space, i.e. Sr ¼

Vw Vv

ð1:4Þ

The degree of saturation can range between the limits of zero for a completely dry soil and 1 (or 100%) for a fully saturated soil. The void ratio (e) is the ratio of the volume of voids to the volume of solids, i.e. e¼

Vv Vs

ð1:5Þ

The porosity (n) is the ratio of the volume of voids to the total volume of the soil, i.e. n¼

Vv V

ð1:6Þ

The void ratio and the porosity are inter-related as follows: e¼

n 1n

ð1:7Þ

n¼

e 1þe

ð1:8Þ

The specific volume (v) is the total volume of soil which contains unit volume of solids, i.e. v¼1þe

ð1:9Þ

The air content or air voids (A) is the ratio of the volume of air to the total volume of the soil, i.e. A¼

Va V

ð1:10Þ

The bulk density () of a soil is the ratio of the total mass to the total volume, i.e. ¼

M V

ð1:11Þ

Convenient units for density are kg/m3 or Mg/m3. The density of water (1000 kg/m3 or 1.00 Mg/m3) is denoted by w .

Phase relationships

19

The specific gravity of the soil particles (Gs) is given by Gs ¼

Ms s ¼ Vs w w

ð1:12Þ

where s is the particle density. Procedures for determining the value of particle density are detailed in BS 1377 (Part 2) [2]. If the units of s are Mg/m3 then s and Gs are numerically equal. Particle density is used in preference to Gs in British Standards but it is advantageous to use Gs (which is dimensionless) in deriving relationships from the phase diagram. From the definition of void ratio, if the volume of solids is 1 unit then the volume of voids is e units. The mass of solids is then Gs w and, from the definition of water content, the mass of water is wGs w . The volume of water is thus wGs. These volumes and masses are represented in Figure 1.10(b). The following relationships can now be obtained. The degree of saturation can be expressed as Sr ¼

wGs e

ð1:13Þ

In the case of a fully saturated soil, Sr ¼ 1; hence e ¼ wGs

ð1:14Þ

The air content can be expressed as A¼

e  wGs 1þe

ð1:15Þ

or, from Equations 1.8 and 1.13, A ¼ nð1  Sr Þ

ð1:16Þ

The bulk density of a soil can be expressed as ¼

Gs ð1 þ wÞ w 1þe

ð1:17Þ

or, from Equation 1.13, ¼

Gs þ Sr e w 1þe

ð1:18Þ

For a fully saturated soil (Sr ¼ 1) sat ¼

Gs þ e w 1þe

ð1:19Þ

20

Basic characteristics of soils

For a completely dry soil (Sr ¼ 0) d ¼

Gs w 1þe

ð1:20Þ

The unit weight () of a soil is the ratio of the total weight (a force) to the total volume, i.e. ¼

W Mg ¼ V V

Equations similar to 1.17–1.20 apply in the case of unit weights, for example ¼

Gs ð1 þ wÞ w 1þe

ð1:17aÞ

¼

Gs þ Sr e w 1þe

ð1:18aÞ

where w is the unit weight of water. Convenient units are kN/m3, the unit weight of water being 9.8 kN/m3 (or 10.0 kN/m3 in the case of sea water). When a soil in situ is fully saturated the solid soil particles (volume: 1 unit, weight: Gs w ) are subjected to upthrust (w ). Hence, the buoyant unit weight ( 0 ) is given by 0 ¼

Gs w  w Gs  1 ¼ w 1þe 1þe

ð1:21Þ

i.e.  0 ¼ sat  w

ð1:22Þ

In the case of sands and gravels the density index (ID) is used to express the relationship between the in-situ void ratio (e), or the void ratio of a sample, and the limiting values emax and emin. The density index (the term ‘relative density’ is also used) is defined as ID ¼

emax  e emax  emin

ð1:23Þ

Thus, the densi...