Foundation Design & Construction by M.J.tomlinson

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Ffomigini urud{Seventh Edition

M. J. TomlinsonCEng, FICE, FIStructE

with contributions byR. Boorman BSc, MEng, MICE,FlStructE

IIli

An :mpnnt of Pearson EducationHarlow, England London New York Reading, Massachusetts San FranciscoToronto Don Mills, Ontario Sydney Tokyo Singapore Hong Kong SeoulTaipei Cape Town Madrid Mexico City Amsterdam Munich Paris Milan

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Pearson Education LtdEdinburgh GateHarlowEssex CM2O 2JE

England

and Associated Companies around the World

Visit us on the World Wide Web atwwwpearsoneduc corn

First published by Pitman Publishing Limited 1963Second edition 1969Third edition 1975Fourth edition 1980Fifth edition pubhshed under the Longman imprint 1986Sixth edition 1995Seventh edition 2001

M J Tomlmson and R Boorman 1986, 1995 Pearson Education Lmuted 2001

The rights of M J Tomhnson and R Boorman to be identified as theauthors of this Work have been asserted by them in accordance with theCopyright, Designs and Patents Act 1988

All rights reserved No part of this publication may be reproduced,stored in a retrieval system, or transmitted in any form or by any means,electromc, mechamcal, photocopying, recording or otherwise, withouteither the pnor written permission of the publisher or a licence permittingrestricted copying in the United Kingdom issued by the CopyrightLicensing Agency Ltd, 90 Tottenham Court Road, London WIP 012

ISBN 0130-31180-4

British Library Cataloguing-rn-Publicatwn DataA catalogue record for this book can be obtained from the British Library

Library of Congress Cataloging-in-Publication DataTomlinson, M J (Michael John)

Foundation design and construction / M J Tomlinson , withcontributions by R Boorman 7th ed

p cmIncludes bibliographical references and indexISBN 01303118041 Foundations I Boorman, R

TA775 T6 2001624 l'5dc2l

00051642

10 9 8 7 6 5 4 3 2 105 04 030201

Typeset in 95/1 ipt Times by 35Printed and bound m Great Britain byTI International Ltd, Padstow, Cornwall

'V


1 5 Exploration m rocks 161 6 Ground water 181 7 Borehole records 201 8 Investigations for foundations of works

over water 201 9 Geophysical methods of site

mvestigation 21110 Investigations of filled and contaminated

ground 21111 Laboratory tests on soils 221 12 Laboratory tests on rock specimens

obtained by rotary core drilling 26113 The foundation engineermg report 26114 Foundation properties of soil types 29115 Foundation properties of rock types 33

References 36

design 852.9 Examples 92

References 102

3 5 Ground movement due to mimngsubsidence 118

3 6 Foundations on filled ground3 7 Machinery foundations 132

References 136

Preface to the first edition ixPreface to the seventh edition xi

1 Site investigations and soil mechanics 11 0 General requirements 111 Information required from a site

investigation 21 2 Site investigations of foundation

failures 31 3 Borehole layout 41 4 Exploration in soils 5

2.5 Estimation of allowable bearing pressuresby prescriptive methods 55

2.6 Settlement of foundations 582.7 Settlement of foundations on rocks 812.8 The applicability of computerized

methods to foundation analysis and

3 Foundation design in relation to groundmovements 1053.1 Soil movements 1053.2 Ground movements due to water seepage

and surface erosion 1153 3 Ground movements due to

vibrations 1173 4 Ground movements due to hillside

creep 117

128

2 The general prindples of foundationdesign 382.1 Foundation types 382.2 Foundation design procedures 3923 Calculations for ultimate bearing capacity

by analytical methods 4524 Calculation of ultimate bearing capacity

by semi-empirical methods 54

4 Spread and deep shaft foundations 1374 1 Determination of allowable

bearing pressures for spreadfoundations 137

4.2 Structural design and construction 1424.3 Foundations to structural steel

columns 15244 Grillage foundations 15245 Raft foundations 15346 Deep shaft foundations 15647 Examples 164

References 174

V


vi Contents

5 Buoyancy rafts and basements(box foundations) 1755 1 General principles of design 1755 2 Drag-down effects on deep

foundations 176Buoyancy raft foundations 177Basement or box foundations 181

5 5 Piled basements 1945 6 Settlement-reducing piles rn piled rafts

and basements 1975 7 The structural design of basement rafts

and retaining walls 1985 8 The application of computer-based

methods to the design of rafts andpiled rafts 208

5 9 Waterproofing basements 2165 10 Worked example of basement

retaining wall 218References 221

6 Bridge foundations 2236 1 Introduction 22362 Code of Practice requirements 22363 Bridges on land 22464 Bndges over water 239

References 274

7 Piled foundations 1: the carrying capacity ofpiles and pile groups 2767 1 Classification of piles 2767 2 The behaviour of piles and pile groups

under load 2777 3 Deflmtions of failure load on piles 2797 4 Calculating ultimate loads on isolated

driven piles in coarse-grained soils 2807.5 Calculating ultimate loads on driven

and cast-in-place piles in coarsesoils 289

7 6 Calculating ultimate loads on bored andcast-in-place piles in coarse soils 290

77 Ultimate loads on piles driven intofine-grained soils 291

7 8 Driven and cast-rn-place piles infine-grained soils 295

7 9 Bored and cast-in-place piles infine-grained soils 295

7 10 Calculation of the carrying capacity ofpiles in soils intermediate between sandand clay, layered soils, and uncementedcalcareous sands 297

7 11 The settlement of a single pile at theworking load 298

7 12 The carrying capacity of piles foundedon rock 298

7 13 Piles in fillnegative skin fnction 3047 14 The carrying capacity of pile

groups 3067 15 The design of axially loaded piles

considered as columns 3117 16 Piles resisting uplift 3127 17 Piles subjected to honzontal or

inclined loads 3147 18 The behaviour of piles under

vibrating loads 3217 19 Computerized methods for predicting the

load/deformation behaviour of the singlepile and pile groups under axial andlateral loading 321

7 20 Calculation of carrying capacity and pilednveability by dynamic formulae 327

7 21 Procedure in correlating static methods ofcalculating pile resistance with drivingrecords 332

7.22 Examples 332References 343

8 Piled foundations 2: structural design andconstruction methods 3458 1 Classification of pile types 34582 Pile-driving equipment 3468 3 Jetting piles 35184 Pile driving by vibration 35285 Pile driving over water 35286 Pile driving through difficult

ground 353Test piling 353Timber piles 359Precast concrete piles 361Jointed precast concrete piles 365

8788898 108118 128 13

Prestressed concrete piles 366Steel piles 369Types of driven and cast-in-placepile 373

8 14 Types of bored pile 3748 15 Types of composite pile 3828 16 The design of pile caps and capping

beams 3838 17 The economics of piled foundations 3858 18 The choice of type of pile 387

References 389

9 Foundation construction 3909 1 Site preparation 39092 Excavation methods 3919 3 Stability of slopes to open

excavations 39294 Trench excavation 3989 5 Support of excavations 401

5354


Contents vii

9.6 Structural design of supports toexcavations 416

97 Overall stability of struttedexcavations 424

9.8 Inward yielding and settlement of theground surrounding excavations 425

9.9 The use of finite element techmques forpredicting deformations around deepexcavations 428

9.10 Example 433References 435

10 Cofferdams 43610 1 Cofferdain types 436102 Design of single-wall sheet pile

cofferdams 444103 Construction of single-wall sheet pile

cofferdams 451104 Double-wall sheet pile cofferdams 459105 Cellular sheet pile cofferdams 460106 Concrete-walled cofferdams 461107 Movable cofferdams 464108 Underwater foundation construction 464109 Examples 470

References 479

11 Geotechnical processes 481111 Ground improvement by geotechmcal

processes 48111 2 Ground water inexcavations 48111 3 Methods of ground-water control 48911 4 The settlement of ground adjacent to

excavations caused by ground-waterlowenng 507

11 5 Ground water under artesian head beneathexcavations 509

11 6 The use of geotechmcal processes forground improvement 510References 516

12 Shoring and underpinning 51812 1 Requirements for shonng and

underpinmng 518122 Methods of shonng 51812 3 Methods of underpinmng 522124 Moving buildmgs 533

References 535

13 Protection of foundation structures againstattack by soils and ground water 53713 1 Causes of attack 53713 2 Soil and ground-water

investigations 53713 3 Protection of timber piles 53813 4 Protection of steel piling against

corrosion 54113 5 Protection of concrete structures 542

References 547

Appendix A Properties of matenals 549Appendix B Ground movements around

excavations 550Appendix C Conversion tables 552Author Index 555Subject Index 560


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IFireface Ito the fiirItedlifion

The author's aim has been to provide a manual of foun-dation design and construction methods for the practis-ing engineer The book is not intended to be a textbookon soil mechanics, but it does include examples of theapplications of this science to foundation engineenngThe principles of the science are stated only briefly,the reader should refer to the relevant textbooks forexplanations of its theory It is hoped that the liniita-tions and pitfalls of soil mechamcs have been clearly setout undue reliance on soil mechanics can be danger-ous if foundation designs are based on inadequate dataor on the use of wrong investigational techniques

Professor Peck has listed three attributes necessary tothe practice of subsurface engineering, these are a know-ledge of precedents, familiarity with soil mechanics,and a working knowledge of geology He believes thefirst of these to be by far the most important Regard-ing soil mechanics, he states

The everyday procedures now used to calculate bear-ing capacity, settlement, or factor of safety of a slope,are nothing more than the use of the frameworkof soil mechanics to organize experience If the tech-niques of soil testing and the theories had not led toresults in accord with experience and field observa-tions, they would not have been adopted for practical,widespread use Indeed, the procedures are validand justified only to the extent that they have beenverified by experience In this sense, the ordinaryprocedures of soil mechanics are merely devicesfor interpolating among the specific experiences ofmany engineers in order to solve our own problems,or which we recognize to fall within the limits ofprevious experience

The author has included information on ordinaryfoundations, including the economic design of housefoundations, as a help to architects and builders in the

Is

use of the present-day techmques of mvestigation andconstruction The application of soil mechanics scienceto the carrying capacity of pile foundations of all typesis a comparatively new development, but is coming tobe recognized as having advantages over older meth-ods usmg dynamic formulae, and this subject is fullytreated It is hoped that the information given on large-diameter bored-pile foundations will be helpful inaiding the design of foundations of tall multi-storeybuildings Experience in recent years has shown theeconomies which these high-capacity piles can give overthe more conventional types where heavy foundationloads are to be camed

The background information on soil mechanicshas been mainly drawn from Terzaghi and Peck's SoilMechanics in Engineering Practice (John Wiley) Forexamples of constructional problems the author hasdrawn freely on his experiences with George Wimpey& Co Ltd, and he is indebted to Dr L J Murdock,DSc, MICE, manager of their Central Laboratory, forpermission to publish this information, together withillustrations and photographs General informationon current design practice m Great Britain has beenobtained from the Institution of Civil Engineers Codeof Practice No 4 (1954) Foundations, with the kindperimssion of the Institution

The author gratefully acknowledges the help andcriticism of his colleagues in the preparation of thebook, and in particular of A D Rae, BSc, for checkingthe manuscript and proofs Thanks are especially dueto Professor H 0 Ireland, of the University ofllhnois, for critical reading of the manuscript and adviceon its application to American engineering practiceThe illustrations are the work of Mrs W Alder andMrs P Payne

Amersham 1963 MJT


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Pireface to the evelnlthedition

In this edition all chapters have been brought up-to-date with recent developments in foundation design andconstruction techmques These include the recent re-search undertaken by the Construction Industry Researchand Development Association (CIRIA) leadmg to revi-sions of the current methods for determinmg allowablebearing pressures of shallow foundations and the ulti-mate resistance of piles bearing in chalk Also includedm a new method for determining the resistance of pilesdriven to a deep penetration into sands This develop-ment is based on research undertaken at Imperial Col-lege, London, pnmanly for the foundations of offshorestructures. However, the method has useful applica-tions for structures on land wherever piles are requiredto be driven to a deep penetration to mobthze the re-quired resistance in shaft friction

In recent years, much attention has been given toobservations of ground movements around tunnels anddeep excavations, and to the measurement of forces inbracing members supporting excavations These obser-vations led to the conclusion that current methods forcalculating earth pressures on excavation supports wereunduly conservative Co-operative research betweenengmeers and contractors under the leadership of CIRIAhas resulted in the establishment of new design rulesdescribed in this edition

The requirements of the draft Eurocode 7 Founda-tions, were the subject of comment in the previousedition Subsequently a revised draft has been publishedby the British Standards Institution in the form of aDraft for Development with the title Eurocode 7Geotechnical Design The code requirements as theyapply to the geotechnical design of shallow founda-tions, retaining walls and piles are discussed in thisedition together with examples comparing the lunit statedesign methods of the code with permissible stressmethods in general use

The author is grateful to Drs Fiona Chow, DavidHight, and Richard Jardine for helpful discussions andcorrespondence concerning the behaviour of pilesdriven into sands and clays, to Mr Malcolm Brittain, lateof Wimpey Group Services, for revising illustrations ofreinforced concrete raft foundations, and to Mr RogerBoorman for once again revising his contributions oncomputer-aided design, for his assistance with workedexamples, and for providmg new examples of computer-based design

In the previous edition I paid tribute to the help andencouragement of my wife in the preparation of thesixth and all earlier editions of this book over a periodof 30 years since the first edition was written Sadly,she died in 1999 after helping me with the early stagesof this present work

Grateful acknowledgement is made to the follow-ing firms and individuals who have given pernussionto reproduce illustrations, photographs and otherinformation

American Petroleum Institute Table 7 1 Reproducedcourtesy of the American Petroleum Institute

American Society of Civil Engineers Figs 1 4, 2 14,228,245,255,523(a), 631,716,726,744,746,7.50, 945, 1012, 1133

Bachy Ltd Figs 8 28, 1030Baumaschine und Bautechnik Fig 653DrM D BoltonFigs6l3,614,615Boston Society of Civil Engineers Table 29BSP International Foundations Ltd Figs 8 3, 8 5, 86,

8.8Building Research Establishment Figs 3 1, 3 3, 3 4,

3.5,3 8, 39,434,131Butterworth-Heinemann Ltd Figs 2 16, 2 51, 10 31,

Table 13 1Burlington Engineers Ltd Figs 8 2, 8 4, 8 7


xii Preface to the seventh edition

Cementation Pihng and Foundations Ltd Figs 8 31,1140

Construction Industry Research and InformationAssociation Figs 5 25, 5 27, 5 30, 5 35, 5 36, 5 40,714,916,918,919,935,936,11.7, ll9repro-duced by kind permission of CIRIA

Columbia Umversity Press Fig 106Corns UK Ltd Tables 84 and 10 1Construction Research Commumcations Tables 3 1,

5 3, Fig 5 28The late W K Cross Esq Figs 638, 639Crown Copynght, Controller of JIM Stationery Office

Figs 3 1, 3.3,34,35CSIRO, AustraliaFig 3.7Danish Geotechnical Institute Figs 2.7,2 8,29, 2 10,

2 11, 2 12,737D'Appolonia Consulting Engmeers Inc Fig 942Edmund Nuttall Ltd Figs 651, 652, 9 33Engineering News Record Figs 641Fugro-McClelland Limited Fig 7 8Honshu-Shikoku Bridge Authority Fig. 656Indian Roads Congress Figs 632, 635Institution of Structural Engineers Figs 4 36, 5 12,

121,124,1225,1226Japan Society ISSMFE Fig 942Jim Mackintosh Photography Figs 5 7, 1220David Lee Photography Fig 643The Marine Technology Directorate Ltd Fig 74Mabey Hire Limited Fig 932Metropolitan Expressway Public Corporation, Yoko-

hamaFig 649Nanyang Technological Institute Figs 10 13, 1024,

1025National House Building Council Fig 3 6National Research Council of Canada Figs 2 37, 2 46,

718,721,722Director of Techrncal Services, Northumberland County

Council Figs 6 19, 6 20, 6 21Norwegian Geotechmcal Institute Fig 2 31Offshore Technology Conference Figs 7 11,7 47,7 48,

7 53Pearson Education Ltd Fig 651Professor Osterberg Fig 7 16Ove Arup and Partners Figs 4 38, 12 8Ove Arup Computing Systems Fig 9 10Pentech Press Figs 2 20, 2 36, 3 2, 3.4

J. F S Pryke Esq Figs 12 12, 1224Ronald White Fig 116S SerotaEsqFig. 1031Roger Bullivant Ltd Fig 8 29Professor I M. Smith Figs 251, 2.52Soil Mechanics Limited Fig 8.24Speed-Shore (UK) Ltd Fig 9 17Dr S M Spnngman Figs 6 13, 6 14, 6 15Steen Consultants Fig 441Structural Engineers Trading Organisation Ltd Figs

439,610,121,124, Table28Swedish Geotechnical Institute Fig 1 5Thomas Telford Publications (and Institution of Civil

Engineers) Figs 1 6, 1 7, 2 21, 222, 2 26, 2 27,233,243,244,253,32,41,4.34,435,58,59,543, 5.44, 618,624, 644, 6.48, 655, 657, 741,95,9 39,947,948, 1022, 1023, 1032, 1033, 1034,11 14, 11 42, 12 21, 12 22, 12 23, Tables 5 5, 5 6, 102

Trafalgar House Technology Fig 628Transport Research Laboratory Figs 6 13, 6 14, 6 15Tudor Engineering Company Fig 6 31Turner Visual Group Limited Fig 97U S Army, Waterways Experiment Station Figs 745,

1111U S National Committee, ISSMFE Fig 2 34Westpile Limited Fig 8 27John Wiley & Sons Inc Figs 2 13, 2 15(a), 1112John Wiley and Sons Limited Figs 2 51, 2 52, 5 23(b)Wimpey Group Services Lmnted Figs 116,5 7,5 16,

517,820,94,97,1010,1121,1122,1220Zublin Spezialtiefbau GmbH Fig 8 11

Extracts from British Standards are reproduced withthe permission of BSI under hcence number SK2000/0490 Complete copies of the standard can be obtainedby post from BSI Customer Services, 389 ChiswickHigh Road, London W4 4AL

Figs 2 34, 4 37, 4 38, and 7 17 are reproduced withpermission from A A Balkema, P0 Box 1675, NL-3000 BR, Rotterdam, The Netherlands

Fig 74 is reproduced with kind pemussion from KluwerAcademic Publishers

Mi T. 2001, Richmond-upon-Thames


1 Site invetigaticns andik mechanics

1.0 General requirements

A site investigation in one form or another is alwaysrequired for any engineering or building structure. Theinvestigation may range in scope from a simple exam-mation of the surface soils with or without a few shallowtrial pits, to a detailed study of the soil and ground waterconditions to a considerable depth below the surfaceby means of boreholes and in-situ and laboratory testson the materials encountered The extent of the workdepends on the importance and foundation arrangementof the structure, the complexity of the soil conditions,and the information which may be available on thebehaviour of existing foundations on similar soils

The draft of Eurocode 7, Geotechnical Design''places structures and earthworks into three 'geotechnicalcategories' Light structures such as buildmgs with col-umn loads up to 250 kN or walls loaded to 100kN/m,low retaining walls, and single or two-storey housesare placed in geotechnical category 1 Provided that theground conditions and design requirements are knownfrom previous experience and the ground is not slopingto any significant degree, the qualitative investigationsin this category can be limited to verifying the designassumptions at the latest during supervision of construc-tion of the works Verification is deemed to consist ofvisual inspection of the site, sometimes with mspectionof shallow trial pits, or sampling from auger borings

Category 2 structures include conventional types onsites where there are no abnormal risks or unusual orexceptionally difficult ground or loading conditions

Conventional substructures such as shallow spreadfootings, rafts, and piles are included in this category,as well as retaining walls, bridge piers and abutments,excavations and excavation supports, and embankmentsQuantitative geotechnical information is required, but

routine procedures for field and laboratory testing andfor analysis and design are deemed to be satisfactory

Structures in category 3 are very large or unusualtypes or those involving abnormal risks, or unusual orexceptionally difficult ground or loading conditionsStructures in highly seismic areas are included in thiscategory

The investigations required for category 3 are thosedeemed to be sufficient for category 2, together withany necessary additional specialized studies If test pro-cedures of a specialized or unusual nature are required,the procedures and interpretations should be documentedwith reference to the tests

Thorough investigations are necessary for buildingsand engineering structures founded m deep excavationsAs well as providing information for foundation design,they provide essential information on the soil andground-water conditions to contractors tendering for thework Thus, money is saved by obtaining realistic andcompetitive tenders based on adequate foreknowledgeof the ground conditions A reputable contractor willnot gamble on excavation work if its cost amounts toa substantial proportion of the whole project, a corres-pondingly large sum will be added to the tender tocover for the unknown conditions Hence the saying,'You pay for the borings whether you have them ornot'

It follows that contractors tendering for excavationwork must be supplied with all the details of the siteinvestigation It is not unknown for the engineer to with-hold certain details such as ground-water level observa-tions under the mistaken idea that this might save moneyin claims by the contractor if water levels are subse-quently found to be different from those encountered inthe borings This is a fallacy, the contractor will eitherallow in his tender for the unknown risks involved or

t


2 Site investigations and soil mechanics

will take a gamble If the gamble fails and the waterconditions turn out to be worse than assumed, then aclaim will be made

An engmeer undertaking a site investigation mayengage local labour for trial pit excavation or handauger bonng, or may employ a contractor for boringand soil sampling If laboratory testing is required theboring contractor can send the samples to his own or toan mdependent testing laboratory The engineer thenundertakes the soil mechanics analysis for foundationdesign Alternatively a specialist organization offeringcomprehensive facilities for boring, sampling, field andlaboratory testing, and soil mechanics analysis may un-dertake the whole investigation A smgle organizationhas an advantage of providing the essential continuityand close relationship between field, laboratory, andoffice work It also permits the boring and testing pro-gramme to be readily modified in the light of informa-tion made available as the work proceeds Additionalsamples can be obtained, as necessary, from soil layersshown by laboratory testing to be particularly signifi-cant In-situ testing can be substituted for laboratorytesting if desired In any case, the engineer responsiblefor the day-to-day direction of the field and laboratorywork should keep the objective of the investigationclosely in mind and should make a continuous appraisalof the data in the same way as is done at the stage ofpreparing the report In this way vital information is notoverlooked, the significance of such features as weaksoil layers, deep weathering of rock formations, andsub-artesian water pressure can be studied m such greaterdetail, as may be required while the fieldwork is still inprogress

Whatever procedure the engineer adopts for carryingout his investigation work it is essential that the indi-viduals or organizations undertaking the work shouldbe conscientious and completely rehable The engineerhas an important responsibility to his employers inselecting a competent organization and in satisfyinghimself by checks in the field and on laboratory oroffice work that the work has been undertaken withaccuracy and thoroughness

1.1 Information required from a siteinvestigation

For geotechnical categories 2 and 3 the following in-formation should be obtained in the course of a siteinvestigation for foundation engineenng purposes

(a) The general topography of the site as it affectsfoundation design and construction, e g surfaceconfiguration, adjacent property, the presence of

watercourses, ponds, hedges, trees, rock outcrops,etc, and the available access for construction veh-icles and plant

(b) The location of buried services such as electricpower, television and telephone cables, water mains,and sewers.

(c) The general geology of the area with particular ref-erence to the main geological formations underly-ing the site and the possibility of subsidence frommineral extraction or other causes

(d) The previous history and use of the site includinginformation on any defects or failures of existingor former buildings attributable to foundation condi-tions, and the possibility of contamination of thesite by toxic waste materials

(e) Any special features such as the possibility of earth-quakes or climatic factors such as flooding, seasonalswelling and shrinkage, permafrost, or soil erosion

(f) The availability and quality of local constructionalmatenals such as concrete aggregates, building androad stone, and water for constructional purposes

(g) For maritime or river structures information onnormal spring and neap tide ranges, extreme highand low tidal ranges and river levels, seasonal riverlevels and discharges, velocity of tidal and rivercurrents, wave action, and other hydrographic andmeteorological data

(h) A detailed record of the soil and rock strata andground-water conditions within the zones affectedby foundation bearing pressures and constructionoperations, or of any deeper strata affecting the siteconditions in any way

(j) Results of field and laboratory tests on soil androck samples appropnate to the particular foundationdesign or constructional problems

(k) Results of chemical analyses on soil, fill matenals,and ground water to determine possible deleteriouseffects on foundation structures

(1) Results of chemical and bactenological analyseson contaminated soils, fill materials, and gas emis-sions to determine health hazard risks

Items (a)(g) above can be obtained from a generalreconnaissance of the site (the 'walk-over' survey), andfrom a study of geological memoirs and maps and otherpublished records A close inspection given by walkingover the site area will often show sigmficant indicationsof subsurface features. For example, concealed swallowholes (sink holes) in chalk or limestone formationsare often revealed by random depressions and markedirregularity in the ground surface, soil creep is in-dicated by wrinkling of the surface on a hillside slope,or leaning trees, abandoned mine workings are shown


Information required from a site investigation 3

by old shafts or heaps of mineral waste, glacial depositsmay be indicated by mounds or hummocks (drumlins)in a generally flat topography, and river or lake de-posits by flat low-lymg areas in valleys The surface indi-cations of ground water are the presence of springs orwells, and marshy ground with reeds (indicating thepresence of a high water table with poor drainage andthe possibihty of peat) Professional geological adviceshould be sought in the case of large projects coveringextensive areas

Information should be sought on possible long-termchanges in ground-water levels, cessation of abstractionof ground water for industrial purposes from boredwells, or pumping from deep mine-shafts can cause aslow rise in ground water over a wide area.

On extensive sites, aerial photography is a valuableaid in site investigations Photographs can be taken frommodel aircraft or balloons Skilled interpretations ofaerial photographs can reveal much of the geomor-phology and topography of a site Geological mappingfrom aerial photographs as practised by specialist firmsis a well-established science

Old maps as well as up-to-date publications shouldbe studied, since these may show the previous use ofthe site and are particularly valuable when investigatingbackfllled areas Museums or libraries in the localityoften provide much information in the form of maps,memoirs, and pictures or photographs of a site in pasttimes Local authorities should be consulted for detailsof buried services, and in Bntain the Geological Surveyfor information on coal-mine workings Some partsof Britain were worked for coal long before records ofworkings were kept, but it is sometimes possible toobtain information on these from museums and librariesIf particular information on the history of a site has animportant bearing on foundation design, for examplethe location of buried pits or quames, every endeavourshould be made to cross-check sources of mformationespecially if they are based on memory or hearsayPeople's memones are notoriously unreliable on thesematters

Items (h), (j), and (k) of the list are obtained fromboreholes or other methods of subsurface exploration,together with field and laboratory testing of soils orrocks It is important to describe the type and consistencyof soils in the standard manner laid down in standardcodes of practice In Bntain the standard descriptionsand classifications of soils are set out in the BntishStandard Code of Practice Site Investigations, BS 5930

Rocks should be similarly classified in accordancewith the standard procedure of codes of practice, BS5930 requires rocks to be descnbed in the followingsequence

ColourGrain size (the grain size of the mineral or rock

fragments comprising the rock)Texture (e g crystalline, amorphous, etc)Structure (a descnption of discontinuities, e g

laminated, foliated, etc)State of weatheringROCK NAMEStrength (based on the uniaxial compression test)Other characteristics and properties

Of the above descnptions, the four properties of par-ticular relevance to foundation engineering are thestructure, state of weathering, discontinuity spacing, anduniaxial compression strength

The discontinuity spacing is defined in two ways

(1) The rock quality designation (RQD) which is thepercentage of rock recovered as sound lengths whichare 100 mm or more in length

(2) The fracture index which is the number of naturalfractures present over an arbitrary length (usuallyim)

The structure is of significance from the aspect ofease of excavation by mechanical plant, and also thefrequency and type of discontinuity affects the com-pressibility of the rock mass The state of weathering,discontinuity spacing, and umaxial compression strengthcan be correlated with the deformation characteristicsof the rock mass and also with the skm fnction andend-bearing of piles

In stating the description of rock strength in boreholerecords the classification adopted in BS 5930 should befollowed Thus

Classification Uniaxial compressivestrength (MN/rn2)

Very weakWeakModerately weakModerately strongStrongVery strongExtremely strong

Under 1 251 255

512 512550

50100100-200Over 200

1.2 Site mvestigations of foundation failures

From time to time it is necessary to make investiga-tions of failures or defects in existing structures Theapproach is somewhat different from that of normalsite investigation work, and usually takes the form oftrial pits dug at various points to expose the soil atfoundation level and the foundation structure, together



with deep trial pits or borings to investigate the fulldepth of the soil affected by bearing pressures A care-ful note is taken of all visible cracking and movementsin the superstructure since the pattern of cracking ismdicative of the mode of foundation movement, e g bysagging or hogging It is often necessary to make long-continued observations of changes in level and of move-ment of cracks by means of tell-tales Glass or papertell-tales stuck on the cracks by cement pats are of littleuse and are easily lost or damaged The tell-tales shouldconsist of devices specially designed for the purpose ornon-corrodible metal plugs cemented into holes drilledin the wall on each side of the crack and so arrangedthat both vertical and horizontal movements can bemeasured by micrometer gauges Similarly, points fortaking levels should be well secured against removal ordisplacement They should consist preferably of steelbolts or pins set in the foundations and surrounded by avertical pipe with a cover at ground level The levelsshould be referred to a well-established datum point atsome distance from the affected structure, ground move-ments which may have caused foundation failure shouldnot cause similar movement of the levelling datumThe Building Research Establishment in Britain hasdeveloped a number of devices such as tiltmeters andborehole extensometers for monitonng the movementsof structures and foundations

A careful study should be made of adjacent struc-tures to ascertain whether failure is of general occur-rence, as in mining subsidence, or whether it is due tolocalized conditions The past history of the site shouldbe investigated with particular reference to the formerexistence of trees, hedgerows, farm buildings, or wastedumps The proximity of any growing trees should benoted, and mformation should be sought on the seasonaloccurrence of cracking, for example if cracks tend toopen or close in winter or summer, or are worse in dryyears or wet years Any industrial plant in which forginghammers or presses cause ground vibrations should benoted, and inquiries should be made about any construc-tion operations such as deep trenches, tunnels, blasting,or piling which may have been camed out in the locality

1.3 Borehole layoutWhenever possible boreholes should be sunk close tothe proposed foundations This is important where thebearing stratum is irregular in depth For the samereason the boreholes should be accurately located inposition and level in relation to the proposed structuresWhere the layout of the structures has not been decidedat the time of making the investigation a suitable patternof boreholes is an evenly spaced grid of holes For

extensive areas it is possible to adopt a grid of boreholeswith some form of zn-situ probes, such as dynamic orstatic cone penetration tests, at a closer spacing withinthe borehole grid EC 7 recommends, for category 2investigations, that the exploration points forming thegrid should normally be at a mutual spacing of 2040 m Trial pits for small foundations, such as stripfoundations for houses, should not be located on orclose to the intended foundation position because of theweakening of the ground caused by these relatively largeand deep trial excavations

The required number of boreholes which need to besunk on any particular location is a difficult problemwhich is closely bound up with the relative costs of theinvestigation and the project for which it is undertakenObviously the more boreholes that are sunk the more isknown of the soil conditions and greater economy canbe achieved in foundation design, and the risks of meet-ing unforeseen and difficult soil conditions which wouldgreatly increase the costs of the foundation work be-come progressively less However, an economic limitis reached when the cost of borings outweighs any sav-ings in foundation costs and merely adds to the overallcost of the project For all but the smallest structures, atleast two and preferably three boreholes should be sunk,so that the true dip of the strata can be establishedEven so, false assumptions may still be made aboutstratification

The depth to which boreholes should be sunk is gov-erned by the depth of soil affected by foundation bear-ing pressures The vertical stress on the soil at a depthof one and a half times the width of the loaded area isstill one-fifth of the applied vertical stress at foundationlevel, and the shear stress at this depth is still appreci-able Thus, borings in soil should always be taken to adepth of at least one to three times the width of theloaded area In the case of narrow and widely spacedstrip or pad foundations the borings are comparativelyshallow (Fig 1 1(a)), but for large raft foundations theborings will have to be deep (Fig 1 1(b)) unless rock ispresent within the prescribed depth Where strip or padfootings are closely spaced so that there is overlappingof the zones of pressure the whole loaded area becomesin effect a raft foundation with correspondingly deepborings (Fig 1 1(c)) In the case of piled foundationsthe ground should be explored below pile-point levelto cover the zones of soil affected by loading trans-mitted through the piles EC 7 recommends a depth offive shaft diameters below the expected toe level It isusual to assume that a large piled area in uniform soilbehaves as a raft foundation with the equivalent raftat a depth of two-thirds of the length of the piles(Fig 1 1(d))


Figure 1.1 Depths of boreholes for various foundationconditions

The 'rule-of-thumb' for a borehole depth of one anda half times the foundation width should be used withcaution Deep fill material could be present on somesites and geological conditions at depth could involve arisk of foundation instability

Where foundations are taken down to rock, either inthe form of strip or pad foundations or by piling, it isnecessary to prove that rock is in fact present at theassumed depths Where the rock is shallow this can bedone by direct examination of exposures in trial pits ortrenches, but when bonngs have to be sunk to locateand prove bedrock it is important to ensure that boul-ders or layers of cemented soils are not mistaken forbedrock This necessitates percussion boring or rotarydiamond core drilling to a depth of at least 3 m in bed-rock in areas where boulders are known to occur Onsites where it is known from geological evidence thatboulders are not present a somewhat shallower penetra-tion into rock can be accepted In some areas boulderslarger than 3 m have been found, and it is advisable tocore the rock to a depth of 6 m for important structuresMistakes in the location of bedrock in boreholes havein many cases led to costly changes in the design ofstructures and even to failures.

It is sometimes the practice, when preparing boreholerecords, to define rockhead or bedrock as the level atwhich auger or percussion boring in weak rock hasceased and coring in stronger rock has commencedThis practice is quite wrong The decision to change tocore drilling may have nothing to do with the strengthof the rock It may depend on the availability of a coredrill at any given time or on the level at which the

Exploration in soils

borehole has reached at the end of the morning or after-noon's work Rockhead or bedrock should be definedas the interface between superficial deposits and rock,irrespective of the state of weathenng of the latter

Direct exposure of rock in trial pits or trenches ispreferable to boring, wherever economically possible,since widely spaced core dnlhngs do not always give atrue indication of shattenng, faulting, or other struc-tural weakness in the rock Where rock lies at somedepth below ground level, it can be examined in large-diameter boreholes dnlled by equipment described inSection 8 14 Because of the cost, this form of deepexploration is employed only for important structures

EC 7 recommends that where the possibihty of baseuplift in excavations is being investigated the pore-water pressures should be recorded over a depth belowground-water level equal to or greater than the excava-tion depth Even greater depths may be required wherethe upper soil layers have a low density When boreholesare sunk in water-bearing ground which will be subse-quently excavated, it is important to ensure that theyare backfilled with concrete or well-rammed puddledclay If this is not done the boreholes may be a sourceof considerable inflow of water into the excavationsIn a report on an investigation for a deep basementstructure in the Glasgow area the author gave a warningabout the possibihty of upheaval of clay at the bottomof the excavation, due to artesian pressure in the under-lying water-bearing rock After completing the base-ment the contractor was asked whether he had had anytrouble with this artesian water The answer was that'the only trouble we had with water was up throughyour borehole' In another case, large bored piles withenlarged bases were designed to be founded within animpervious clay layer which was underlain by sandcontaimng water under artesian pressure The nsks ofsomewhat greater settlement due to founding in thecompressible clay were accepted to avoid the difficultyof constructing the piles in the underlying, less com-pressible sand However, considerable difficulty wasexpenenced in excavating the base of one of the pilesbecause of water flowing up from the sand strata throughan unsealed exploratory borehole

1.4 Exploration in soils

1.4.1 Investigation methods

Methods of determining the stratification and engineer-ing characteristics of subsurface soils are as follows

Trial pitsHand auger borings

5

(d)



Mechamcal auger bonngsLight cable percussion boringsRotary open hole drillingWash bonngsWash probingsDynanuc cone penetration testsStatic cone penetration testsVane shear testsPressuremeter testsDilatometer testsPlate bearing tests

Detailed descriptions of the above methods as usedin British practice are given in BS 5930 Site Investiga-tions. Brief comments on the applicability of thesemethods to different soil and site conditions are givenin the following Sections 1 421 45

1.4.2 Trial pits and borings

Trial pits are generally used for geotechmcal category1 investigations They are useful for examining thequality of weathered rocks for shallow foundations Tnalpits extended to trenches provide the most reliable meansof assessing the state of deposition and characteristicsof filled ground (see Section 110)

Hand and mechanical auger borings are also suit-able for category 1 investIgations in soils which remainstable in an unlmed hole When carefully done augermgcauses the least soil disturbance of any bonng method

Light cable percussion borings are generally used inBntish practice The simple and robust equipment iswell suited to the widely varying soil conditions inBntain, including the very stiff or dense stony glacialsoils, and weathered rocks of soil-like consistency

Large-diameter undisturbed samples (up to 250 mm)can be recovered for special testing

Rotary open hole drilling is generally used m USA,Middle East, and Far Eastern countries The rotary dnllsare usually tractor or skid-mounted and are capable ofrock dnlhng as well as drilling in soils Hole diametersare usually smaller than percussion-drilled holes, andsample sizes are usually limited to 50 mm diameter

Bentonite slurry or water is used as the drilling fluid,but special foams have been developed to assist inobtaining good undisturbed samples

Wash borings are small-diameter (about 65 mm) holesdnlled by water flush aided by chiselling Sampling isby 50 mm internal diameter standard penetration testequipment (see below) or 5075 mm open-drive tubes

Wash probings are used in over-water soil investiga-tions They consist of a small-diameter pipe jetted downand are used to locate rock head or a strong layer over-

lain by loose or soft soils, for example in investigationsfor dredging There is no positive identification of thesoils and sampling is usually impracticable

1.4.3 Soil sampling

There are two main types of soil sample which can berecovered from boreholes or trial pits

(a) Disturbed samples, as their name implies, aresamples taken from the bormg tools examplesare auger parings, the contents of the split-spoonsampler in the standard penetration test (see Section1 4 5), sludges from the shell or wash-water return,or hand samples dug from trial pits

(b) Undisturbed samples, obtained by pushing or driv-ing a thin-walled tube into the soil, represent asclosely as is practicable the in-situ structure andwater content of the soil. It is important not to over-drive the sampler as this compresses the contentsIt should be recogmzed that no sample taken bydriving a tube into the soil can be truly undisturbed

Disturbance and the consequent changes in soil prop-erties can be minimized by careful attention to main-tainmg a water balance in the borehole That is, thehead of water in the borehole must be maintained, whilesampling, at a level corresponding to the piezometricpressure of the pore water in the soil at the level ofsampling This may involve extending the boreholecasing above ground level or using bentomte slurryinstead of water to balance high piezometric pressuresThe care in sampling procedure and the elaboratenessof the equipment depends on the class of work whichis being undertaken, and the importance of accurateresults on the design of the works.

BS 5930 recommends five quality classes for soilsampling following a system developed in GermanyThe classification system, the soil properties which canbe determined reliably from each class, and the appro-priate sampling methods are shown in Table 11

In cohesive soils sensitive to disturbance, qualityclasses 1 and 2 require a good design of sampler suchas a piston or thin-walled sampler which is jacked orpulled down into the soil and not driven down by blowsof a hammer Class 1 and 2 sampling in soils insensi-tive to disturbance employs open-drive tube samplerswhich are hammered mto the soils by blows of a slidinghammer or careful hand-cut samples taken from trialpits There is a great difference in cost between pistonand open sampling, but the engineer should recogmzethe value of good quality if this can result in economiesin design, for example, good-quality sampling meanshigher indicated shear strengths, with higher bearing


Exploration in soils 7

Quality class(as BS 5930)

Soil properties thatcan be determinedreliably (BS 5930)

Sampling method

I Classification,moisture content,density, strength,deformation, andconsolidationcharacteristics

Soils sensitive todisturbancethin wall piston samplerSoils insensitive todisturbancethick or thin wall opensamplerSoils containingdiscontinuities (fabnc)affecting strength,deformation, andconsolidationlarge-diameter thin or thickwall sampler (piston oropen)

2 Classification,moisture content,density

Thin- or thick-wall opensampler

3 Classification andmoisture content

Disturbed sample ofcohesive soils taken fromclay cutter or auger in dryborehole

4 Classification Disturbed sample ofcohesive soils taken fromclay cutter or auger inboreholes where water ispresent

None, sequence ofstrata only

Disturbed samples of non-cohesive soils taken fromshell in cable percussionbonng or recovered asdebns flushed from rotarydnlling or wash bonng

pressures and consequently reduced foundation costsIn certain projects good sampling may mean the differ-ence between a certain construction operation bemgjudged possible or impossible, for example the placingof an embankment on very soft soil for a bndgeapproach If shear strength as indicated by poor-quahtysampling is low, then the engineer may decide it isimpossible to use an embanked approach and will haveto employ an expensive piled viaduct On the otherhand in 'insensitive' clays such as stiff glacial till thesampling procedure has not much effect on shearstrength and thick-wall open samplers may give quiteadequate information Also, elaborate samplers such asthe fixed piston types may be incapable of operation inclays containing appreciable amounts of large gravel

The presence of discontinuities in the form of pocketsor layers of sand and silt, laminations, fissures, and root

holes in cohesive soils is of sigmficance to their per-meability, which in turn affects their rate of consohda-tion under foundation loading, and the stability of slopesof foundation excavations The use of large-diametersample tubes may be justified to assess the significanceof such discontinuities or 'fabnc' to the particular foun-dation problem i 2

The engineer should study the foundation problemand decide what degree of elaborateness in samphngis economically justifiable, and he should keep in mindthat zn-situ tests such as the vane or cone tests may givemore reliable information than laboratory tests onundisturbed samples If zn-situ tests are adopted, elab-orateness an undisturbed sampling is unnecessary andthe 'simple' class is sufficient to give a check on iden-tification of soil types A good practice, recommendedby Rowei2 is to adopt continuous sampling in the firstboreholes dnlled on a site An open-dnve sampler withan internal split sleeve is used to enable the samplesto be split longitudinally for examination of the soilfabric The critical soil layers can be identified and theappropriate class of sampling or zn-situ testing adopted

BS 5930 gives details and dimensions of five typesof soil samplers for use in boreholes These are

Thin-walled samplersGeneral-purpose 100mm diameter open-tube samplerSplit-barrel standard penetration test samplerThin-walled stationary piston samplerContinuous sampler

Thin-walled samplers which are pushed rather thanhammered into the soil cause the minimum of moisturecontent changes and disturbance to the fabnc of thesoil Sample diameters are generally 75100 mm, buttubes up to 250 mm can be provided for special pur-poses The thin-wall sampler is suitable for use in verysoft to soft clays and silts

One type of thin-walled sampler, not descnbed inBS 5930, is the Laval sampler developed in Canada forsampling soft clays ' It has been shown to providesamples of a quality equal to those obtained by conven-tional hand-cut block sampling The tube is hydraulic-ally pushed into a mud-supported borehole to recoversamples 200 mm in diameter and 300 mm long Thetube is over-cored before withdrawal

The general-purpose 100 mm diameter open-tubesampler was developed in the UK as a suitable devicefor sampling the very stiff to hard clays, gravelly glacialtill, and weak weathered rocks such as chalk and marlIn this respect the detachable cutting shoe is advan-tageous It can be discarded or reconditioned enablingmany reuses of the equipment However, the relativelythick-walled tube and cutting shoe do cause some

Table 1.1 Quality classification for soil sampling

5



I Proposed foundationI I___ ___-09

02mFoundation

" [1 07mIrm

I depth for ciaL..L..L

U (100) samples at 1 5 mnominal spacing

2 2m

Figure 1.2 Lack of information on shear strength at foundation level due to adoption of umform sampling depths in all boreholes

disturbance of the fabnc of the soil and moisture con-tent changes witlun the sample The equipment is suit-able for geotechmcal category 2 investigations

The split-barrel standard penetration test sampler isused to make the in-situ soil test descnbed in Section1 45 The tube has an mtemal diameter of 35 mm andrecovers a disturbed sample suitable for classes 3 and 4in Table 11 Some mdication of layenng or laminationscan be seen when the sampler is taken apart

Thin-walled stationary piston samplers are suitablefor quality class 1 m Table 11 and for geotechnicalcategory 3 investigations Diameters range from 75 to100 mm with special types up to 250 mm They recovergood samples of very soft to soft clays and silts, andsandy soils can sometimes be recovered Special thin-wall piston samples are used in stiff clays

The DeIft continuous sampler is an example of thistype It is made in 29 and 66 mm diameters with apenetration generally up to 18 m, but samples up to30 m can be recovered in favourable soil conditionsIt is designed to be pushed into the ground using the200 kN thrust of the standard cone penetration testsounding machine (see below)

The samples from the 66 mm tubes are retained mplastic liners which can be split longitudinally to exam-ine the stratification and fabric of the soil

1.4.4 Spacing of soil samples

It is frequently specified that soil samples should betaken at intervals of 1 5 m and at each change of strata

in boreholes While this spacing may be adequate if alarge number of boreholes is to be drilled, there can bea senous deficiency in quantitative soil information ifthe size of the area under investigation warrants only afew boreholes The lack of information is particularlynoticeable where structures with shallow foundationsare proposed Thus it is quite usual for the first sampleto be taken just below the topsoil, say from 02 to 07 mThe next, at the 1 5 m spacing, is from 1 7 to 2 2 mExactly the same depths are adopted for all the boreholeson the site It is normal to place foundations in clay at adepth of 09 or 1 0 m Thus there is no information onsoil shear strength and compressibility at and for a dis-tance of 0 8 m below foundation level, probably withina zone where there are quite large variations in soilcharactenstics due to the effects of surface desiccation(Fig. 1 2) Where only a few boreholes are to be sunk itis a good practice to adopt continuous sampling forthe first few metres below ground level or to stagger thesampling depths where the 1 5 m spacing is adopted.

1.4.5 In-situ testing of soils

Tests to detenmne the shear strength or density of soilsin situ are a valuable means of investigation since thesecharacteristics can be obtained directly without the dis-turbing effects of boring or sampling They are particu-larly advantageous in soft sensitive clays and silts orloose sands They must not be used as a substitute forborings but only as a supplementary method of investi-gation One cannot be sure of identifying the types of

0 Shear strength

information for foundationdesign not available overthis depth for all boreholes 1 7m

Zone ofdesiccation

Normal shearstrength profile

10

20

30

11

U

3 2m

3 7m


Ha, a,

o a,

00

0U


soil they encounter and the tests give no information onground-water conditions

The vane shear rest apparatus was developed to meas-ure the shear strength of very soft and sensitive clays,but in Scandinavian countries the vane test is also re-garded as a reliable means of determining the shearstrength of stiff-fissured clays The standard equipmentand test procedure are descnbed in BS 1377 (Test 18)The vane test is performed by rotating a four-bladedvane, 101 6 mm long x 508 mm wide overall, in thesoil below the bottom of a borehole or by pushing downand rotating the vane rod independently of bonng Thusthe test is performed in soil unaffected by bonng distur-bance However, it has been observed that the undrainedshear strength of a clay as measured by the vane testcan differ quite appreciably from the actual field strengthas measured from the behaviour of full-scale earthworksBjerrum'4 concluded that the difference is caused bythe anisotropy of the soil and the difference in the rateof loading between a rapidly executed field vane andthe slow application of loading from foundationsand earthworks Bjemim's correction factors to vane testresults correlated with the plasticity index of clays areshown in Fig 1 3 These factors should be applied tovane test results to obtain the equivalent undrained shearstrengths for foundation beanng capacity calculationsusing the methods descnbed in Section 23

From the results of this test or subsequent laboratorytests the clays are classified m accordance with BS 5930as follows

Term Undrained shear strength(kN/m2)

Very softSoftSoft to firmFirmFirm to stiffStiffVery stiff or hard

Less than 202040405040757510075150Greater than 150

The standard penetration test (SF1') (BS 1377 (Test19)) is made in boreholes by means of the standard50 8 mm outside and 33 8 mm inside diameter split-spoon sampler (sometimes known as the Raymondsampler) It is a very useful means of determining theapproximate zn-situ density of cohesionless soils and,when modified by a cone end, the relative strengthor deformability of rocks The sampler is dnven topenetration of 450 mm by repeated blows of a 63 5 kgmonkey falling freely through 760 mm, actuated by anautomatic trip device Only the number of blows forthe last 300 mm of dnving is recorded as the standardpenetration number (N-value) It is standard practice tocount the number of blows for every 75 mm of penetra-tion in the full 450 mm of dnving By this means thedepth of any disturbed soil in the bottom of the boreholecan be assessed and the level at which any obstructionsto dnving, such as cobbles, large gravel, or cementedlayers, are met can be noted Normally not more than

1

10

08

06

040

Figure 1.3 Correction factors for undrained shear strength of normally consolidated clays obtained by field vane tests(after Bjerrum' 4)

20 40 60 80 100 120

Plasticity mdex (per cent)



50 blows (including the number of blows required toseat the sampler below the disturbed zone) are made inthe test If the full 300 mmpenetration below the initialseating drive is not achieved, i.e when 50 blows havebeen made before full penetration is achieved, then boththe depth at the start of the test and the depth at whichit is concluded must be given in the borehole record,suitable symbols being used to denote whether the testwas concluded within or below the initial seating driveAfter withdrawal from the borehole the tube is takenapart for examination of the contents.

In gravelly soil and rocks the open-ended sampler isreplaced by a cone end Investigations have shown ageneral similarity in N-values for the two types in soilsof the same density

The standard penetration test was originally developedin the USA as a simple device to obtain an indicationof soil density The test came into use in many coun-tries of the world and numerous correlations of the testdata with soil properties and analytical techniques weredeveloped Published mformation showed that the testtechniques varied widely m different countries Ham-mers and samplers of non-standard types were beingused and the method of controllmg the hammer drop,whether by free-fall or guided by rope and pulley, alsovaried It became evident that corrections to N-valuesproduced by non-standard techmques would be neededif the test data were to be used for correlation withvarious soil parameters as discussed below The correc-flon factors to be applied to the measured blow-counthave been summarized by Clayto&5 as follows

The principal correction is concerned with the en-ergy delivered to the sampler by the hammer and drillrods This has been standardized in terms of an energyratio (ERM) of60 per cent of the theoretical maximumThe measured blow-count (Nm) after correcting for ham-mer energy is denoted by the term N Thus

Nse = NmERM/60 (11)

A further correction is applied to allow for the energydelivered by the drill rods The Nse value is corrected toN by multiplying N'se by 075 for rod lengths of 3 m orshorter The correction factor is umty for lengths greaterthan lOm

No correction for sampler size or weight is necessaryif a British Standard or ASTM standard sampler is used

Some correlations of the SPT with soil characteristics,in particular the susceptibility of a soil to liquefactionunder earthquake conditions, require a further correc-tion to N'se to allow for the effective overburden pres-sure at the level of the test Thus for a standard sampler

j77l LV '-N" 60

S(1

15C

ZOC

Correction factor, CN0 02 04 06 08 10 12 14 16-

Dr 4060 per cent25C

Dr6080 per

cent

30C 35c

400

450

500

Figure 1.4 Correction factor to SVF N' value to allow foroverburden pressure (after Seed et a!' 6)

Values of CN derived by Seed et al 16 are shown inFig 1 4

Although the applications of the test are whollyempirical, very extensive experience of their use hasenabled a considerable knowledge of the behaviour offoundations in sands and gravels to be accumulatedRelationships have been established between N-valuesand such charactenstics as density and angle of shear-ing resistance as described in Section 2 3 2

BS 5930 gives the following relationship betweenthe SPT N-values and the relative density of a sand

N(blows/300 mmof penet ration)

Relativedensity

D, (%)

Below 4410

10303050Over 50

Very looseLooseMediumdenseDenseVery dense

80

The use of the SPT for calculating allowablebearing pressures of spread foundations is shown inSection 2 3 2 and for piled foundations in Section 74

(1 2) Stroud'7 has established relationships between the

JI /,

-8

80a

I



10

Mass shear strength= c=f1N(kNIm2)

8

\4

2

0

10

06 \-...

6 \

70

10 20 30 40 50 60 70

Plasticity index

Figure 1.5 Relationship between mass shear strength, modulus of volume compressibility, plasticity index, and SPT N-values(after Stroud' 7)

N-value, undrained shear strength, modulus of volumecompressibility, and plasticity index of clays as shownin Fig 1 5 However, the adoption of the SPT fordeter-mining the shear strength and deformabihty of clay soilsis not recommended in preference to the direct methodof making laboratory tests on undisturbed samples. Thisis because the relationships which have been establishedbetween the SPT and the strength and deformability ofclays are wholly empirical, taking no account of suchfactors as time effects, anisotropy, and the fabric of thesoil When laboratory tests are made, these factors canbe taken into account in the test procedure which canbe selected in a manner appropriate to the soil charac-teristic and the type, rate, and duration of the load whichwill be applied to the soil

Various corrections are necessary to the standardpenetration test values before using them to calculateallowable bearing pressures and settlements Theseadjustments take account of looseness and fineness ofthe soil, the effects of overburden pressure, and theposition of the water table The procedure for makingsuch adjustments is described in Sections 2.3 2 and 265

Dynamic probing employs various forms of rod withor without cone or other specially enlarged ends whichare driven down mto the soil by blows of a drop hammerThe number of blows for a given distance of penetra-tion is recorded The Borros penetrometer is usedm Britain and other European countries It employs a63 kg hammer impacting a 505 mm cone at a rate of20 blows per minute The number of blows required for

Ii

fz

10 20 30 40 50 60

Plasticity mdex

Modulus of volume compressibility= = (mi/MN)

08 __



35 -0 030-

25- II20- /0'15 -

n=025N10- 3/ _5o 00 'r"l I I I I

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70SPT N (blows/300 mm)

(a)

3 6 9 12 15 18 20

SPT N (blows/300 mm)

(b)

Figure 1.6 Relationship between dynanuc CPT n and STP N(a) In sands and gravels (b) In chalk (after Cearns andMcKenzi& 8)

a penetration of 100 mm is denoted as n The torqueon the cone is measured to provide an additional meansof interpreting the data There is very little informationpublished in Britain on correlation between n-valuesand the SPT N-value or q values from static cone pen-etration tests Cearns and McKenzieii have publishedrelationships between n and the SPT N for sands,gravels, and chalk as shown in Fig 1 6 Dynamic prob-ing is a useful means of supplementing conventionalboring and zn-situ penetration tests, and is particularlyadvantageous in delineating areas of weak soils over-lying stronger strata and for locating cavities in weakrock formations

The static (Dutch) cone penetration test (CPT) isused widely in European countries and to a lesserextent in Bntain and North America for investigationsin cohesionless soils Three types of penetrometer arein general use (Fig 1 7) In all three types the cone endhas a base area of 1000 mm2 and an apex angle of 60The mantle cone shown in Fig. 1 7(a) was developedby the Delft Soil Mechamcs Laboratory Separate deter-minations of cone resistance and skin friction on thesleeve tubes, and the combined resistance of cone andtubes are obtained over stages of 200 mm (sometimes

lesser stages are adopted down to 100 mm) In thecase of the friction jacket (or Begemann) cone shownin Fig 1 7(b) the skin friction is measured over a shortcylindrical jacket mounted above the cone which canbe jacked down independently of the cone and the sleevetubes above it

The electrical cone (Fig 1 7(c)) was developed inThe Netherlands by Fugro NV With this equipmentboth cone and sleeve tubes are jacked down continu-ously and together The thrust on the cone end and on a120mm length of cyhndncal sleeve are measured sepa-rately by electrical load cells installed at the lower endof the penetrometer Signals from the load cells can betransmitted to a computer and data plotter, the latterproduces a continuous record in graphical form of thevariation in cone resistance and sleeve friction withdepth A full description of the CPT equipment, themethod of operation, and the apphcation of the testresults to foundation design has been given by Meigh i

It has been found that the cone resistance as meas-ured by the three types does not differ sigmficantly, butthere are, of course, differences in the measured sleevefriction Empirical methods have been developed where-by the type of soil can be identified by the separate andcombined end and fnctional resistances The author does

ES

75

6

I:ce.

Sounding tube

Frictionjacket

Protectivemantle

Mantle Retainuig sleeve

60 cone angle

(b) (c)

Figure 1.7 Types of static cone penetrometer (a) Mantle cone,(b) Friction jacket cone, (c) Electrical cone

356mm

(a)

This edition is reproduced by permission of Pearson Educational LimitedExploration in soils 13

20

010

6

2

1

Medium I Coarse Fine I Medium I Coarse Fine I MediumSILT SAND GRAVEL

Figure 1.8 Relationship between q,/N and grain size(after Burland and Burbidg& ')

not rely on such measurements, but prefers to use thecone resistance only to obtain factors from which shearstrength and deformability of soils can be estimated(see Sections 2 3 2 and 2 6 5) with the soil identificationbemg obtained from adjacent conventional boreholes

The static cone test is a valuable method of recordingvariations in the in-situ density of loose sandy soilsor laminated sands and clays in conditions where thezn-situ density is disturbed by bonng operations, thusmaking the SPT unreliable m evaluation. Only limitedpenetration can be achieved by the cone in coarse grav-elly soils An empincal relationship between CPT andSPT results and the particle size distribution of soils isshown m Fig 1 8

The Delft Soil Mechamcs Laboratory have continuedto develop the CPT for obtaimng a number of soil char-actenstics by zn-situ testing These mclude the following

(a) The piezocone (to obtam pore-water pressuressimultaneously with cone resistance),

(b) The nuclear backscattenng probe (to measure in-situ soil density),

(c) The seismocone (to measure seismic velocity),(d) The chemoprobe (see Section 110)

Pressuremeters are not descnbed in the currentedition of BS 5930 Mair and Wood'" have given adetailed descnption of the five types listed in Table 1 2together with information on their method of installa-tion, the test procedures, the interpretation of the testresults, and their applications to foundation design

Essentially, the pressuremeter consists of a cylin-dncal rubber membrane expanded against the sides ofa borehole which is either predrilled in the case of the

002 006 02 06 20 60Particle size (mm)

Values of N are not corrected for overburden pressure

Table 1.2 Types of pressuremeter

Type Installationmethod

Mea.zurementsystem

Diwneter(mm)

Menardpressuremeter(type GB)

Lowered intopreformedhole at baseof borehole

Membraneexpanded by waterpressure Volumemeasured atsurface

32, 44,58, and74

Oyoelastmeter(type 100)


Membraneexpanded bywater pressureExpansionmeasured bydisplacementtransducer

70

Self-bonngpressuremeter(Camkometer)

Drilled intosoil byintegral unit

Membraneexpanded by gasExpansionmeasured by threestrain gaugedfeeler arms

82

Cambndgern-situ high-pressuredilatometer


Membraneexpanded by oilpressureExpansionmeasured by sixstrain gaugedfeeler arms

74

BuildingResearchEstablishmentpush-inpressuremeter

Pushed intosoil at baseof borehole,or into under-size pre-coredhole

Membraneexpanded by oilpressure Volumemeasured atsurface

78



Menard-type pressuremeters or formed by the equip-ment in the case of the Camkometer and push-in types.The expansion of the membrane is measured directlyby feeler gauges, or indirectly by measunng the volumeof water or oil required for the increased diameter TheCanikometer is preferred for use in soft clays, silts, andsands because of the difficulty in mamtaimng the stabil-ity of an open preformed hole in these soils, exceptperhaps by using bentomte or foam as the circulatingfluid. The Menard and Oyo types are suitable for firmto stiff clays and weak weathered rocks, and the push-in types for soft to firm clays and silts Pressuremeterscannot be used in gravels

The Menard pressuremeters, when correctly operated,produce a pressurevolume curve of the type shown mFig 1.9(b) and the Camkometer and push-in types apressurecavity strain curve as shown in Fig 1 9(a)

The pressurevolume or strain curves require cali-bration and correction This is usually done by the testoperator Mair and Wood strongly recommend that theengmeer commissioning the tests should ask for theraw data and cahbration data to ensure that the correc-tions have been made properly before commencementof interpretation The shear modulus (G) of the soil isbest obtained from the slope of the unloadreload cycleafter the expansion has reached the plastic stage. Theelastic modulus is derived from the expression

E=2G(l +v),where v is the Poisson's ratio of the soil which variesas to whether undrained or drained conditions are oper-ating in the foundation design; E is sometimes referredto as the deformation modulus because the soil does notbehave elastically at any stage of the pressuremeter testHence G orE should be referred to as the pressuremetermodulus and the strain amplitude should be defined,e g. the initial tangent modulus G, or a secant modulusG, at a shear strain of 50 per cent of the peak shearstress or at a defined percentage strain

The undrained shear strength of clays and the drainedstrength of sands can be obtamed from the pressuremetertests using the methods described by Mair and WoodOnly the self-boring pressuremeter can be used toobtain the 4i'-value of sands Mair and Wood'" pointout that '-values obtained m this way can be higherthan those obtained by empirical correlations with theSPT or CPT, but are in agreement with values obtainedfrom tnaxial tests Undrained strengths denved fromthe pressuremeter are usually very much higher thanthose obtained from triaxial compression tests This ispartly due to sample disturbance m the case of triaxialtesting, but mainly due to the different method of test.Hence it is essential to know the particular test method

Expansion of membrane

Figure 1.9 Types of corrected curves for pressuremeter tests(a) Self-bonng pressuremeter (Camkometer) (b) Menardpressuremeter

when using undrained shear strength values to obtainthe ultimate bearing of clays by the methods given inSection 2 3 2 or the skin friction and base resistance ofpiles (Sections 7.77 9)

The self-boring pressuremeter provides a goodmethod of obtaining the coefficient of horizontal earthpressure at rest (K0) in terms of effective stresses Thisis possible only in clays

The flat-type dilatometer (Marchetti dilatometer) isa 95 mm wide spade-shaped probe with an expandable

IUnloadreloadcycle

Cavity strain (%)

(a)

Pla

(13)

,tic

Pseudc -elastic

Volume

(b)



metal-faced pressure cell 60 mm in diameter on oneface of the probe The device is pushed or hammeredinto the soil either directly from the surface or from thebottom of a borehole Readings to determine the gaspressure required for initial movement of the cell, andfor 10 mm movement of the cell into the soil, are takenat 200 mm intervals of depth The device has somesimilarities with the CPT equipment rather than thepressuremeter, but it causes less disturbance of the soilthan the standard cone

The cell pressure readings are interpreted empiric-ally' 12 to provide the predominant grain size, the K0value, and the dilatometer modulus (related to the

Crust of stiffweathered clay

Test plate

Loading from testplate carried whollyby stiff crust

Soft alluvial clay

Figure 1.10

Large raft foundation

deformation modulus) of the soilPlate bearing tests are made by excavating a pit to

the predetermined foundation level or other suitabledepth below ground level, and then applying a staticload to a plate set at the bottom of the pit The load isapplied in successive increments until failure of theground in shear is attained or, more usually, until thebearing pressure on the plate reaches some multiple,say two or three, of the bearing pressure proposed forthe full-scale foundations The magnitude and rate ofsettlement under each increment of load is measuredAfter the maximum load is reached the pressure onthe plate is reduced in successive decrements and therecovery of the plate is recorded at each stage of un-loading This procedure is known as the maintainedload test and is used to obtain the deformation charac-tenstics of the ground. Alternatively, the load can beapplied at a continuous and controlled rate to give apenetration of the plate of 25 mm/mm This is knownas the constant rate of penetration test and is applicableto soils where the failure of the ground in undrainedshear is required, as defined by gross settlement of theplate, or where there is no clear indication of failurewith increasing load, the ultimate bearing capacity isdefined by the load causing a settlement of 15 per centof the plate diameter

Although such tests appear to answer all the require-ments of foundation design, the method is subject toserious limitations and in certain cases the informationgiven by the tests can be wildly misleading In the firstplace it is essential to have the bearing plate of a sizewhich will take account of the effects of fissures orother discontinuities in the soil or rock A 300 mm plateis the minimum size which should be used which issuitable for obtaining the undrained shear strength ofstiff fissured clays If deformation charactenstics arerequired from these soils, a 750 mm plate should beprovided in conjunction with the maintained load pro-cedure It is essential to make the plate tests in soil orrock of the same charactenstics, as will be stressed by

the full-scale foundation Misleading information willbe given if, for example, the tests are made in the stiffcrust of weathered clay overlying a soft clay as illus-trated in Fig 110 A 1000 mm plate is generally theeconomic limit, since a 1000 mm plate loaded say to800 kN/m2 will require some 63 t of kentledge, whichis expensive to hire including the costs of transport andhandling The cost of a single plate bearing test with a300600 mm plate with 50 t of kentledge is three ormore times the cost of a 12 m deep borehole (in softground) complete with in-situ and laboratory testing.A single plate bearing test on a site is, in any case, farfrom sufficient since the ground is generally variable inits characteristics both in depth and laterally At leastthree tests, and preferably more, are required to obtainrepresentative results

Economies in plate bearing tests on rock can be madeby jacking against cable or rod anchorages grouted intodrill-holes in the rock, instead of using kentledge Singleanchors have been used successfully The anchor cable,which is not bonded to the rock over its upper part, ispassed through a hole drilled in the centre of the testplate A test of this type can be made at the bottom of aborehole

The level of the water table has an important effecton the bearing capacity and settlement of sands Thus aplate bearing test made some distance above the watertable may indicate much more favourable results thanwill be given by the large full-scale foundation whichtransmits stresses to the ground below the water tableThe plate bearing test gives no information wherebythe magnitude and rate of long-term consohdation settle-ment in clays may be calculated

In spite of these drawbacks, the plate bearing test can-not be ruled out as a means of site investigation, sincem certain circumstances it can give information whichcannot be readily obtained by other means For example,the bearing capacity and deformation characteristics of

I Loading fromI large foundationI transmitted to

underlying soft clay /

..... --

Figure 1.11 Rig for plate loading test made in a borehole (after Marsland' 13)

certain types of rocks such as broken shales or variablyweathered matenals cannot be assessed from zn-situpressuremeter tests or laboratory tests due to difficult-ies m sampling In these ground conditions it is neces-sary to sink a number of tnal pits down to or belowfoundation level The pits are carefully examined andthree or four are selected in which the ground appears tobe more heavily weathered than the average, and platebearing tests are made in these selected pits Alterna-tively it may be desirable to select the pits in the weakestand strongest ground so that a range of deformationmoduli can be obtained to assess likely differentialsettlement. The largest practicable size of plate shouldbe used and it is advisable to dig or probe below platelevel on completion of the test to find out if there areany voids or hard masses of matenal present whichmight affect the results

Plate bearing tests made in fill materials consistingof bnck or stone rubble are of doubtful value becauseof the large particle size and wide variation in densityof such material However, meaningful results can beobtained from tests made in filled ground consisting ofsands, gravels, colliery waste, or boiler ash

The procedure for making plate bearing tests is fullydescribed in BS 5930 A typical arrangement for a testusing a set-up developed by the Building ResearchEstablishment' is shown in Fig 111. This rig is suit-able either for a test in an open pit or at the bottom ofa large-diameter borehole drilled using the equipmentdescnbed in Section 8 14 1

A present-day set-up would most likely substitute aload-cell for the proving nng and a displacement trans-ducer for the settlement gauge These instruments would

be computer-controlled to give the specified rate ofsettlement and to provide a read-out of the data innumerical and graphical form

1.5 Exploration in rocks

1.5.1 General requirements

Investigations into rock formations for foundation en-gineering purposes are concerned first with the allow-able bearing pressures for spread foundations or workingloads on piles, and second with the conditions whichare likely to be met if excavations have to taken intothe rock strata for deep foundations The engineer musttherefore have information on the depth of any weath-enng of the rock, the presence of any shattered zones orfaults susceptible to movement, the possibility of theoccurrence of deep drift-filled clefts, buned glacial val-leys, swallow holes, or concealed cavities, and thequantity of water hkely to be pumped from excavationsMuch of this information can be obtained in a generalway by advice from a geologist from personal knowl-edge of local conditions and the study of publishedmaps and memoirs Indeed the advice of a geologist inconnection with the siting of any important project onrock formations is very necessary An essential part of ex-ploration of rock masses to aid the interpretation ofborehole data and field and laboratory tests is a detailedstudy of the spacing, thickness, and orientation of jointsin the rock mass, together with a study of the composi-tion and consistency of any weathered rock or othermatenal infilling the joints if detailed observations ofjoint characteristics are made at locations of pressure-



Locking keys

..- Proving ring

_.-Hydraulic jack.Jacking frameSettlement gauge.,. Reference beam-fl Mfflfl loading]Li til frame

Borehole casing.

:J IRConcreteblock

Ireaderjbeams111E1- Reference

iiitt column cii1ITU. Loading I I

i:ht columntiil I Centralizing (4\tII i/fins UI

!fi1 Loading plate15 mm of plaster

UTiL=

Group of boredtension piles


Exploration in rocks 17

meter or plate bearing tests, then the test results will beapplicable to other parts of the site where similar jointingconditions exist in the rock formation

Weathering or other disturbances of the surface of arock stratum are likely to necessitate varying founda-tion levels on a site, and it is often difficult if notimpossible to assess from the results of boreholes adefimte foundation level for a structure For example,some types of rock such as marl or limestone soften asa result of seepage of water down fissures forming zonesof weakened rock of soft clayey consistency surroundingstrong unweathered matenal If a few boreholes onlyare put down in these conditions they may encounteronly unweathered rock, giving a false picture of thetrue site conditions Conversely, boreholes striking onlysoftened weathered rock might suggest the presence ofa deep stratum of soft clay overlying hard rock, whereassuch soft matenal might only exist in comparativelynarrow fissures. Glacial action can have caused deepand irregular disturbance of the surface of bedrock, forexample the breaking-up and bodily movement of shales,or the tilting of large blocks of massively bedded rock

The surface of friable, and therefore erodible, rocksmay be intersected by narrow dnft-filled valleys or cleftswhich again may be undiscovered by borings or trialpits, although a geologist would anticipate their occur-rence These conditions may require major redesign orrelocation of foundations when the actual bedrock sur-face is revealed at the construction stage of the project

There are three methods in general use for subsur-face exploration in rocks These are

(1) Test pits(2) Drilled shafts(3) Rotary core drilling

1.5.2 Test pits

Test pits are the most satisfactory means of assessingfoundation conditions in rock, since the exposed bed-rock surface can be closely inspected The dip of thestrata can be measured and it is often possible to assessthe extent of weathering in layers or fissures Thestrength of the rock and its ease of excavation can bedetermined by thai with a pick or compressed-air toolsIf necessary, blocks or cylinders of the rock can be cutfor laboratory tests However, test pits are only econ-omical when bedrock lies fairly close (say within 3 m)of the ground surface They should be used instead ofboreholes when rock level is shallower than 2 m belowground level, but for depths between 2 and 3 m a fewpits can be dug to supplement the evidence given byboreholes

1.5.3 Drilled shafts

Where rock lie

Foundation Design & Construction by M.J.tomlinson

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