STRENGTH CHARACTERISTICS

STRENGTH CHARACTERISTICS OF A MODELLING SILTY CLAY

by

©LI NG LIN, B.5c.

A thesissub m itted to the Schoolof Graduat eStudies inpartial fulfillme ntof therequirementsfor

the degreeof Mas t erofEnginee r ing

Fa cultyof Engineering and AppliedScien ce MemorialUrriver ait y ofNew found lan d

March 1995

St.John's Newfou nd land Can ada

1+1

Naliooal Ubra lY 01canada Acquis400nSand BibliographicServicesBranch 395WelIinglOnSlreet OIIawa.()ilarlo Kl~0N4

~~b~~~^nationalo Oirecliondesacquisitionsor des servicesbibliographiques 395.rveWel!onglon

~rxw~O<1la,io)

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DEDI CATE DTO MY MOTHER

AND MY SISTERS

Ab stract

Shear strengthis an importantsoil parameter.Many geotechnical failureshave beencausedby inadequate shear strength . In order to obtain a high strength mod elling siltyday for usein centrifugemodeltests of soil-pipeline interaction, directsheartests,shear vane testsandCODepenetration tests wereconductedto lnw 'ligate its shearstrengthbehaviour.

This thesisfirst presentssome background on the shear strength ofsoils and measurementtechniques. The detailsofthe direct shear tests, theshear vane tests, the conepenetrationt~t9and centrifugemodelling are also discussed. The silty clays usedin thisst udy were kaolin andakaolin-siltmixture;the plasticity indices were30.6%and19.5% respectively. The kaolin-silt mixture (K-S)was obtainedby mixing equalamountsofkaolinandsiltby weight.

Direct shear and shearvane tests were conducted on bothclays at different ver- ticalstresses undervariousoverconsolidation ratios (OCR's). This thesis presents these test procedu resand thetest results. Cone penetrat iontests werealsoCOil-

ductedin a centrifugeat 50gravities to verifytheresultsfrom the directshea r and shea rvane tests andtocorrelatethe cone penetrationresistance to the undrain ed shear strengthfromshearvane anddirectshear tests. CentrifugemodeUing prin- ciplesand the currentmet hodstoconvert cone tip resistancetoshearstrengthare also presented. Testresultsshowthat the shear strength ofK-S is higherthanthat ofpurekaolin. Shear strengthsof heavilyoverconsolidatedclays obtainedfrom theshear vane tests are higher than those obtain ed from the direct shear tests .A

ccrrelat ica between cone tipresistanceand undrained IMar.ttftlt;this presented.

The shearstrCDt;th or K·Sinterpreted fromCPT te5uluWA:!Iroundtolie between thedirect shear and shearnnetestresultsandshow.agood cormatiOIlinthe measur ement ofshear strength.

Acknowledgm ents

Iwould liketo acknowledge thea.ssistanc~ofmany people in mystudyfor the degreeof Masterof Engineering. Particular than ks areduemy supervisor,Dr.

J.I.Clark, forhisguidance and financial support throughoutthisprogram,and Dr.R.PhillipsandMr.M. Paulinfortheirguid ance and helpduringthe tests.

Appredalionand gratit udeare also extendedto:

(a ) Mr.D.Cameronand aU the other membersofgeotechnicalgroupat C.CORE for their help.

(b)Mr.H.Dye andother sta ffinthe Machine Shopof the Facult yofEngineering for their helpand suggest ionsin manufacturingthetestdevice.

(c)Dr.P.Morinand Mr. C.Ward of the Facult yof Engineering fortheirhelp in preparing the directshear test device.

(d) NOVA Corporationforsupplyingthe test materialsandsponsoring thetest program .

iii

Contents

Abstract Acknowledg ements Tableof Contents List of Figures Listof Tables List or Symbols 1 Introduc tio n 2 LiteratureReview

2.1 Shea.r StrengthofClays 2.2 MeasurementofShear Strength ...•.

2.2.1 DirectShear Test..•....•.•. .. 2.2.2 TriaxialTest... ... ... .• . 2.2.3 Shear Vane Test ...

2.3 Properties of Kaolin and Kaolin-BasedMixtures..•.

iii iv vii

xii

11

2.4 Cone Penetrati on Test . . . •.•••. .... .•... .. 14

... . . ... . .. 18 20 2.5 GeotechnicalCentrifuge Modelling

3 Material sTest ing 3. 1 PhysicalCha racteristics 3.2 Oedomet erConsolidation Test.

3.2.1 TestProgram .

3.2.2 TestResults .. . . ... ... . • . .

16

18

20 21 3.2.3 ResultAnalyses.

..She a rStrengthTests 4.1 DirectShear Test.. . .

. 28

32

33

4.1.1 TestApparatus . ... 33

4.1.2 TestingProcedures. ...•.•. . . •..•. 36

4.1.3 TestResultsof Kaolin . 39

4.l.4 TestResultsof Kaolin-SiltMixture .. . . .••. . 43

4.2 Shee rVaneTests .

4.2.1 Principles.• •... ••..•... 4.2.2 Tuband Vane Design.. 4.2.3 Spring Calibrat ion

45 47 49 50 4.2,4 TestingProced ures ,•. . . . .•. ..

so

4.2.5 Test Resultsof Kaolin ..

4.2.6 TestResults ofKaolin-SiltMixture . . •...

54 56

tl.2.7 EffectofRepeatedLoad ing ... . . ... .•. ... 57

$ Cone Pe netrationTesting .1 5.l PrinciplesofCentrifugeModelliog .• . . .•... •... 62 5.2 C-CORE CentrifugeCentre ...••. . . • . . . •.. ... 6-1 5.3 TestDesign.•• • ... ...• •.... .• ...•• ...• 65 5.3.1 ConePenetroll1eler•.•. .. .•. •.••. .. • • •... . . 65 5.3.2 SmallTubConePenetration Tests •..•..• .. . • •. . . 66 5.3.3 LargeTubConePenetration Tests •.•. •.. • . . . 68 5.4 Cone TipResistances•. ... •.... . . •.... . . 12 5.5 Undrained. ShearStrength(c,.).• ... . ..•. .. 76 5.5.1 SelectionofConeFactorNc•••• •••• • • ••••• •••• 76 5.5.2 Determination ofc,from CTP Data .. . . • . ... . .. 78

6 ComparisonandDiscu ssion

• •

6.1 StrengthofKaolinandKaolin-SiltMixture.•..•.•. . . ..• ••85 6.2 DirectShearandVane Results ..• • • • •.••..• •...• 88 6.3 Comparisonof CPT ResultswithDirectShearand Vane Results ..• 91 6.4 CorrelationofNefromDirect Shear andVaneTests... ... .... 93

7 SummaryandConclusion s Refer enc es

A Fact onfe rArea Correct ionin DirectShearTests B Records of ConeTip Resistancedur ing CP T C LOT Results duri ng Consolid at ion of Large Tub II

O .

100

10.

roa

110

List of Figures

2.1 StressStates and Shear StrainDistributionillDirectShearTest... 8 3.1 GrainSize Distribution Curves

3.2 AccumulatedDeformationwithTimeof Kaolin 3.3 AccumulatedDeformat ionwith TimeofK· S . 3.4.coO":CurveforKaolin

3.5

e 'O":

CurveforK·g . ..• •.,, , ..

3.6 ComparisonofCuRe mits ... . 4.1 Direct ShearTestDiagram..

"

22 23 25 28 31 34 4.2 Wheatsto ne-BridgeCircuitforShearStress Measurement,.... 36 4.3 TypicalShearStress vs ShearDisplacementfrom Direct ShearTest 38 4.4 ShearStrength

o r

Kaolin fromDirect ShearTests .. . ..••. , .•U 4.5 NormalizedShe.uStrength of Kaolin from DirectShearTests •..•42 4.6 WaterContentofKaolin after Direct Shear Tests ... .•. •. 42 4.7 ShearStrengt hof K·gfromDirect ShearTests. , . . .. . . ...•. 45 4.8 Normalized Shear Strengthof K·SfromDirect Shear Tests . .• 46 4.9 WaterContentofK·SalterDirectSbe.u Tests... 46 4.10ShearVaneDiagram

vii

.. . . . .. . . .... . .... 48

4.11 The TubUsedin the VaneTests . 51 4.12NormalizedShear Streng thof Kaolin withOCR byVane. 56 4.13NormalizedShelLTStrengthofK-S with OCRby Va ne. 58 4.14 Effect of Repeated Loadingon Kaolin. 60 5.1 Correspondenceof StressBetweenPrototypeandModel 63

5.2 Cone UsedintbeTests . 66

5.3 TestDesign of LargeTubI• • . 70

5,4 Test Design of Large Tub II •.. 71

5.5 Effed of WaterTa.bleinCentrifugeTest 73 5.6 Effect of SurfaceDesiccat ionDuring Cent rifugeFlight . 74 5.7 TipResist ance ofNativeandBackfillClay. 74 5.8 TipResistanceof BackfillwithDifferentWat er Contents(withWater

Migratio n to the Nat iveClay). . . ... 75 5.9 TipResista nceof Backfill withDifferentWaterContents (without

Wat er Migrationto theNat iveClay)... 75

5.10Effectof WaterMigra~ion .. 76

5.11 Relationshipbetween Cone FactorN.andOCR 77 5.12 Tip Resistance of TestsL214and L230. . •. 80 5.13 Tip Resist ance of TestL240 and L243. . • 80 5.14Average TipResist anceof testsL214,L230, L240 and L243.. 81 5.15PorePressureduringtheconsolidationofTransectA-A Tests 81 5.16 Pore Pressureduringthe consolidation of Transect B-DTests... 82 5.17PorePressure duringtheConsolidationofTransect C·C Tests 82

viii

5.18Surface Setlleml!'atdurin!: the ConsolidationorTransect A-ATest! •83

5.19 Distribution or Tota]StteJll,Effective Streuand PorePressure.. .. 83

5.20 AverageOvercoDsolidationRatio •• • . .. .• • •.•. . •. ... . 84

5.21AverageUndr ained ShearStrengthfrom Cone Tests.•..•• • • • . 84

6.1 ComparisonorStrengthorKaolin andK·S fromDirectShearTests .86 6.2 Comparison ofStrengthofKaolinandK·S fromVaneTesll • 87 6.3 Comparison of ShearStrengthof Kaolinfrom VaneTest! ... 88

6.4 ShearStrength ofKaolinfromDirect Shear and VaneTests.. , .•. 89

6.5 Shea rStrengtho(K· SfromDirectShearand VaneTests . . . 89

6.6 Comparisono( St rengt hs{rom CPT,Direct ShearandVane Test!.. 92

6.7 NeValuesfromDirectShear and VaneTests• .••.•... . 93

6.8 Comparison ofNeValues(or K·S..• . . .•.•... .. ...94

A.l AreaChange ofSpecimendUriD&Shea.riD~.. • . • . .. . . ... . .107

D.I ConeTipResistance o( LI03andLl04•... . ...1I0 8.2 ConeTip ResistAnce o(LI05end LI07•.. • ... . . . ... . •••1I0 8.3ConeTipResistanceofL109•. . ....•. . . •.• • ..••. .III 8.4 Cone Tip Resistanceof Ll12andLIl4 111 8.5ConeTipResisla.Dceof LlI 3 andLU6 .• •• . . ••. ••••. . . •112

8.6 Cone Tip Resist ance o( LllS andLUB.. .. . . .. . .. .... . •112

8.7ConeTipResistanceof LU 9.. . .. .. . . .... . . .•...113

8.8Cone TipResistance ofL208 and L209 .• .. .. . ,.• ... ....113

8.9 Cone Tip Resistan ce ofL210andL211" •.... .... .114

8.10 Cone Tip Resist anceo(L212andL216.. . . .... . . •.• •114

B.11 Cone Tip Resistanceof L213and L217 B.l2 Cone TipResist ance of L221 andL223 . B.13Cone Tip Resistance of L224 and L225..

8.14Cone Tip Resistanceof L226 andL229..

.115 115 .._ . _...116 .116

C.2 SurfaceSettlementofTransectC-C.. . . . .

B.ISCone Tip Resistance of 1.227.. . . ..117

8.16 Cone TipResistanceofL234 and L237 .._. . .. ._ .. _.. . ..117 8.17Cone Tip Resistance of L236 and L238. . ... . . . ..118 B.18Cone TipResistan ceof L239 and L242.. .118

C.l SurfaceSettlementof TransectB-B 120

.120

List of Tables

2.1 PhysiCAlPropertiesofSpeewbite Kaolin 12

2.2 Consolidation Parameters of Speewhite Kaolin... 12

2.3 StrengthConstants

or

Spes white Kaolin . .., .. . . .. .... 13

2.4 Propertiesof SomeKaolin- BasedMixtures... . .. . . 13

3.1 Properties ofKaolin andK·S . ..•.. .. .• • •• . . . .• ••19

3.2 ParameterafOftheCa.lculationor Void Ratio...• . •.. ....•.24

3.3 Compres!lionParametersofKaolin •..•. .•• •••..•.. ...26

3.4 Ccmp ressicnParamete rsof K·S Mixture .•..•. .. •. .• •...27

U Direct Shear TestResults of Kaolin.• •.• • . . •...••..•.. 40

4.2 Direct Shea r TestResults

or

K·S _.. . . .. . .. . . .. .. H 4..3 GeometryoftheVaneUsed intheTest...•. ... ...•.50

4.4 ShearVaneTestResultsofKaolin. . ....•.• •. .• • .. .•.•55

4.5 SheafVane TestResultsofK·S

"

4.6 Shear Vane Resultsof Kaoli n under Repea ted Loading ... .•.•. 59

6.1 Strength Parametersof Kaolin and K-S. ...•..•... . ... 86

A.I FactoN forA~aCorrection ...•.. .•...•. •..•.108

"

'.

Am A, CPT c C,

D DSS

List of Symbols

centr ifugeaccelera tion centr ifugemodelacceleration prototypeacceleration coefficientofcompressibility ce nt rifugemodelarea proto typ earea.

conepenetration test coh esionofroil compression inde x swellingind ex undrained shear strength normalizedshea r strength coefficient ofconsolida tion diamet erof van eshaft va nediam e ter directshea rboxtest mea ngraia size

xli

r;

F,

I.

G.

s.;

11

11. 11.

Ip

k, k"

K.

K-S

voidratios externalforce inmodel exter nal force inprototy pe self-weightforceinmodel seU-weight force in prototype side frict ionofcone penetration test gravitationalacceleration spec ificgravity centrifuge model depth vaneheigh t

sample height inoedometertest initi alsample heightin oedometertest solids heightin oedometer teat lengt hoCdrainage path averagesample heightin cedornete r test plasticityindex

liquidityindex coefficientof permeability coefficientof eart hpressureat rest kaolin-silt mixture

xiii

L, LDT

M.

M.

N N, OCR PPT p'

p :

q,

T

model leng thdimension prototypelengt h dimen sion lineardeformationtransducer coefficient of volumecompressibility resistingmcrner],along thesurface ofcylinder resis tingmomen tattwo endsofcylinder dimension scale

conefactor overconsolidationratio porepressuretransducer mean effectivestress preconsolidationstress conetip resistance centrifugerota tio nalradius vane arearatio

friction ratio of cone pen etra tiontest vane blade thickness

time require dtoreach90% consolida tion torque appliedto vane

xiv

T.

TXE

WJ

Wp a.p

t.

t,

timefa.ctor

tria xialexte nsio nteet pore pressur e specific volume

finalwatercontent after direct shear test liquidlimit

initial watercontent plasticlimit strengthconsta nts unitweight of prototype verticalload increment changein voidratio vertical deformation

final vertic aldeformati on changein vert ica l loa d strainin model strain prototyp..

slope ofloadin g-reloading linein v:lnp' plane slope of norma lcomp ression lineinv:lnp'plane mate rialdensity

0',0'... """

,

.

^,

. .

V,",,100',,2'.Tv'O'w

TJ

. '

principal stresses totalverticalstress effectiveverticalstress effectivelateral stress stressin mo del stressin prototype shearstress shear stressatfailure effect iveangleof internalfriction angularvelocity

xvi

Chapter 1 Introduction

Thedet ermi natio n of soil strengthisan im portant aspectof geotechnicalen- gineering, The und rainedshear strengt hof clays is affectedby thesoils physical proper tiesandstressconditions(La.ddetd" 1917). Theundrainedstr engt h can beestimated fromfield tests as wellaslabora to ry tests. Asfield testsare usually expensive and time-con suming,laboratory sheertests arewidelyusedtoprovide insight into thestrengt h beha viourof days.

In 1991,theCentreforColdOceanResources Engineering(C.CORE)I'.tMemo- rial University of Newfoundlandundertook".co ntract projectentitled "Centrifuge Modelling of Laterally Loaded Pipelines".The aim of thisstudy wasto investigate theload transfer behaviourof buriedpipelinesusingcentrifuge model testsand to determinethesoil-pipelineinteraction factorsand theeffects oftrench geometry.

Kaolinclaywas used as themodelclay. Theundrained.shear strengthof kaolin

W/LlJe:ltimated usingthe followingempirical equat ion [Pocrocshaeb ,1991):

(1.1) wherec",istheundrai nedshear strength,

0':

^{is the}effect ive verticalstress and OCR

isthecverco nsolidetjc a ratio.However, theundrained streo &thofthe soilestimated Wlinsthe above equat ionW&Ifound to notcorrelate wellwiththe centrifusecone penetration test results.

Furthercentrifugemodellingoflatera llyloaded pipeli nesisbeingconductedat theC-COREcentrifugecentre. The effective ver ticalstress,

0':

^{can beestimated}

ulin~

O':=pNgh... (1.2)

wher ef'is the density ofthesoll(submerged den sity forthesoil belowwate rtable), Ngis theaccelerationlevelduringthe centrifugetC!lt Andh", isthe soil depth.

The one-dimension ally consolidatedundrained direct simplesheartest results of dayspresentedby Ladd and Edgers (1972)ia dl ceted that strength o(someover- consolidateddartcan be expressedusing

(1.3)

wherethevalueofmisasoilparameter,

0':"

isthe effectivevertical stressatthebe- ginningofshear,OCrepr esents overconsolidaledsoilsandNCrepresents nor mally consolidat edsoils.

Inorder topredict theundrainedshear strengthprofile,itisnecessaryto developa reliable relationshipbetweenthe undrai ned shearstrengthc"and the effectivevert icalstressO'~. Such relat ionships aretypically assumed to be ofthe form

5:-

=Q(OCR)"

v:

^{(I.' )}

wherethe parameters a and(Jshouldbe properlyestimated.

Equt.tion (1.3) and Equation(1.4) are essentiallyidentical. For normally con- solidated cleye,Equation(1.4)becomes

~=Q.o. ^(1.5)

Thepurposesof this thesis areLainvestigate the und rainedshearstrengthof the kaolin clay andto develop a higberstrength model c1a.y for usein the centrifuge model studyofsoil-pipelineinteraction.Kaolin day and a kaolin-siltmixturewere tested as the modelling silty c1a)'3.The followingchapters of this thesisintroduce the test techniq uesend the results or directsheartests, shear vane tests end cone penetrationtests for the estimation of the undrained shear strength of the kaolin and thekaolin~siltmixture.

Chapter 2

Literature Review

One ofthe purposeoCth:.sthesis is to investiga.te thestrengthbehaviourofkaoli n and a kaolin-siltmixtureusin gdirect shear tests, shea rvane testsand conepenetr a- tiontests. In order to providesomebackgrou nd relat edto thestudy, this chap ter presents a review ofdirect sheartests,shearvanetestsandcone penetr at ion tests, and introd uces theapplications of cen t rifuge modellingingeotechnicalengineering.

A rev iewofthe behaviourofkaolin-based daysis also presented.

2.1 Shear St rengt h o f Cl a ys

Shear strengthof asoil is theinternalresistance per unit area thatthesoil mass can offertores ist failureand slidin galonganypla neinside it(Das, 1985).In geotechnical enginee d ng, manyfailures ofsoilslructuresandfoundatio ns result (ro m inadequa te shearstrength.Sheustre ngthis animportan~mechanicalch aracte rist ic or soil, is Influencedbymany(actorsand maybe estima te dIromfieldand laboratory testresults.

Tbe shea rstrengt hof a so ilisdirectl yrelate dto Itsnormal stress;the relations hip

bet weenthe norma lstressandsheQ.l'stressona failur eplanecan be exp ressedas (2.1)

where, TJ isthe shearstres s at failu re,(1'is theeffectivenonnalstress,~'is the effectiveangleof int ernalfriction and c'is cohesion. Thisequationis wellknown asMohr-Cou lombfail ure criterion. For coheeicnlesematerials,suchas sand,the valueofc'is usuallyzero.

However , thestrength be haviourof soilsisverycom plicat ed.The shearstrength ofsoilsis affectedbysoilphysicalproperties(suchasmineralogyand consistency), confining pressure, stresshistoryandother fact ors.Somemai n factorsinfluencing soilshearst rengt h are119follows:

Void ratio: Voidratioisperhapsthemostimport an t pa rameteraffecting the shea rstrengt hofsa n ds. Gen erally speaking,thelowerthevoidratio, thehigh er theshearstrength(H oltzandKovacs,1981). Theparameter~'inEquation(2.1) cha nges notonlywit hsoiltyp e, hutalsowith soilvoid ratio.

Timeeffect:Creepofsoilisthetime-dep en dent st ra in wh ichdevelo psat arate controlled bythe soilviscou s resistance (Mitc hell, 1976 ) . Secondaryconsolida t ion ofsoils which contin uesalterprimaryconsolid at ionisakindofcreep. Creepofa soilcausesnotonlystrainbutalsost ress redistributionwithinthesoil(Kavazanjia.n andMitchell ,1984).

Anisotropy: The anisotropy ofsoil incl udes three aspects: the anisotropy of the soilst ructure , the etreeeesapplied to the soil and the boundaryconditio ns

(Duncan andSeed,1966 ).Duri ngone-d imensionaldtpo.itionend load ing, particlf'S 01toilsttndtobecomehoriwn t ally orie nted(Laddd 41.,1977). Thisorienlation 01particles causesinhere ntaniso t ropy01the soilsandresul ts inchanges ofst rt'tlgth a.ndothe r properttesofthesoile. DuncanandSeed(1966)haveshownthat the anisotropy ofparticleorienlationofII.kaolinitecauseslLImuch&5a10%mangein undra ined stren gt h.

St reSShistory: One01themostim portan tcharacteristics ofsoils isthlltsoil streng t his significantlyin8uencedby stres shistor y.Inma.nycases,etrceehisloryof

II.soil isrepresentedbytheoverconsolida t ion ratio(OCR).Inonedimensio n al test - ing,an overccnsolid ated soilhunotonly a lower void ratiobut also:l.higherlatera l stressthanano r m ally consolidated.soil;theundra inedstreng th of cvcrccneclidated soUishigherth anthatof normally consolidatedsoil. Th e one-dimensionallycon- solida ted undrained directsimplesbeartestresultsorclayspresentedbyLaddand Edgers{1972}indicated thatthe undrainedstreng t hofcve rccnsoli datedclaY'in- cre.ueswit hoverconsolidatiOllra t ioandeffective verticalstress(11:)atthebeginning oCshea r .

ThesheArstren gt h ofclay.isalsoaffectedby the degreeofsaturation ,sample dist ur b ance,soiltype,grainsizedhtrib ution,ra teofshe ar ing,temperatur eand otberenvironme nt al cond itions.

2.2 M easurement of Shear Strength

Shearat rcngth ofeoil can beestimatedfrom theempirical correlat ionandalec can be determined directlyby some of thelaboratory tests, As discussed above, thereare many factors which may a1fect ttc strengthofsoils. Thesbeerstrength determinedby differentlaboratory testsmay alsochange, dependingDOtonly00 thetype oftest b'llalso onthe drain ageandconsolidationcondi tio ns. Among the many teatmethods, the direct shearlestand triaxia ltest arethetwomaintypes of shear tests. The laboratory vane test is alsoa commonly usedtest,especially(Of soft days.

2.2.1 DirectShearTest

The directshear testisone of the oldeststrengt htests becauseit has beenused for more than200yearssinceCoulomb. This test relates shearstrengthat failure direc tlytonormal stress andthus canbeused to definetheMohr-Coulomb failure envelop.

Inthistest, e epcclmen container,called'ebeer bcx', is separated horizontallyinto twohalves.One-hellisfixed,whilethe otheris either pulledorpushedhorizontally, A norm al load maybe appliedtothe soilspecimen. A stress-orstrain-controlled shea rCoreecan beapplied to thespecimen.As the appliedshearforceishorizontal, thefailureplane isrestr ained tobehorizontal(Holtz and Kovacs, 1981).

There arc somelimitat ions anddisadvantagesassociated with directsheartest.

In the test, thesoilspecimen isforcedto failalongthe horizonta lsplitplane and not along theweakestplane(Das,1985). Inaddition,theshear stressdistribut ionover

~

^~,

^' .

Figure 2.1:Stress StatesandShearStrainDistributioninDirect Shea rTest theshea r surfaceisnotuniform(Wood,1990).The atreesetetee andshea rstrai n distri butionofsoil of direct shear test isshowninFigur e2.1. Duringshearing, the principalstress plane changes. Anoth er disadvantageof the directshear test isthat the porepressu re generate dduring the test cannot bemeasured (Vickers, 1983) and thereforethe drainagecond itionishard tocontrol;the onlywayto judgethetest beingdrainedor undrained is to controltherate ofshea ring(Head,1982).

2.2.2: Triaxial Test

The triaxialtest is the most widely used methodforthe determinat ionofshear st re ngt hparameters. In this test,a cylindricalsoilsampleis encasedby a thin rubbermembrane and placed inside apressurised chamber whichisusuallyfilled with water.The sample is subj ected to an isotropicstressbyprcssurising thefluid

in the chamber.To shear the.pecimeo.,aDaxialdressillnormally appliedthrough

&verticalloading ram to causeshear failure(Du, 1985). Compared with thedirect

sheartest,the triaxialtelt huthe followingadvantages:

(I)Drainage conditioncanbe controlledaccordingto theteltpurp oses;

(2) Porepressureinundrained tells cenbemonitoredAnd volumechangecb- servatk>ns in drainedtestsCaDbeconducted;

(3) Thesoilfailure canoccur on anyplane depe nding00.the etreseconditione;

(4) Back pressuremaybeappliedtoincrease the degreeofsaturation of soil samples.

2.2.3 ShearVan eTest

Thelaboratory shear VADe testis anothersimple waytode termine theundrained shearItren!th01soils. During testing,theveaeispushedintoasoil,a.nda torque is appliedto therodofthevane at aconstan trate_Theshearstrengt h ofthe8Dil ea.nbeobtainedby measuringthemaximumtorquerequiredtocauseacylindrical CaBute surface prescribed.bythevane bladeedges.The muimumtorqueisusually measuredfrom the spring rotation angle.As describedbyFJa.ate (1966), there are aeveral factorsaffecti ngthe resultsofvanetest:

(1) Vane Sha p eandSize

Themost populartypeofvaneisthe -t-blade rectangularvane with aH/ D (Height/Diameter)ratio of2,although Silvestrietal.(1993) used differentshapes in theirstud ies.Fta.ate(1966) andArmandal. (1975)indicatedthattheeffect ofthe vanesize on themeasuredshearstrengt hisinsignificant forfield vanetests.Cadling

andOdenstad(1950) also showed thatwhenanHIDratio of2ismaint ained,the vane bladediameterha.sDOef'ect onthe results. For laboratoryvanetellis ,Vey (1955) foundthatavanewithhigherHIDratiomayhave alu gedegreeofsample dist ur1"ance.Almeidaphysic.alandPury(1983) recommend avanesize ofISmm in diameter and14mminheight. Area.ratio,definedastheratio ofthecrosssectjc n uea ofthe vaneto the^Cf'OISsect ionarea of the shea redsoileylinder,isdirectly proportionaltothevane size. Vickers (1983)suggeststha~theMearatio shouldbe les, than 12%.

(2) Disturban ce dueto Insertio n

During penetrati on ofthevane, somedisturbance to thesoilsamplewouldoccur due tothevanerodand the vane itself. Thesoilcanstick tothevaneas itis pushed intothe soil tbus the&rearatio ofthe vane increasefl.Daviesd01. (1989) foundthatthedisturbanceofthesoil aroundthe vanesbaft wassymmet ricala.od extendedtoahout 2 timesthe radius oftheshaftfromits centerline.

(3) ShearVelocity

Although inASTM04648· 87.it ismentionedthattheva:.~deviceshould rota.te thetorquespring ata.conlLant ratecf 60·per minute.thereisnoItan da.rdvane rota tion rate{or thelaboratoryvane test. Extremely(.., trotationn.tes may cause theundrai nedsheerIt rengthto increase becauseofviscouseffects. Atvery low speed ,significant consolidation occurs which mayalsocausehighervalues ofshear st rengt h (Springman,1993).Perlowand Richard,(1977)studied theeffect ofshear velocity on vaneshearstrengt hand pointed out that sincetheshearvelocity atthe edgeof thevaneincreasesconsiderablywit hincreasin, vanediameter,significant

10

differencesin the shearing rate at the failuresurface exist between large and small vanes at similar rotation rates. The widerange of vane sizes and rotation rates CMresultin significantdifferences in shear velocity. Monney(1969) found that labor atory strengths obtainedat a rotationrate of90⁰per minute were nearly30%

higherthan strengthsmeasured at 1⁰per minuteonrelatively undisturbed marine clayey silts.

(4)Ot her Facto rs

Otherfactor:!euchas nonuniformityof stress distribution in the soil, shaft friction (water between the shaft and soilmay decrease the friction)and the standing time betweenvaneinsertion androt ation may also a.f£edthevane test results.

2.3 P rop erties of Kaolin and Kaolin-Based Mix- tures

Kaolin clay is widelyused as a modelling clay in fundamental studies.It has been used in centrifuge modelling tests to investigatemany practicalproblems (Spring- man, 1993).The behaviour of kaolin clay has been investigatedby many people such as Atk insonet al.(1987),Rossato dal. (1992),Lawrence(1980),Springman(1993) end Parryand Nadaraja h(1973).Table2.1 and 2.2summarized thepropertiesand compreaaibilityof Speswhite kaolinobtainedby some ofthese researchers.

As discussedpreviously, the undrained shear strengthcan be expressed^ll.II:

(2.2) The constantsQandPin aboveequation fer kaolin were obtained by some re-

11

Table 2.1: PhysicalProperties ofSpcswhite Kaolin Atkinson etaI. Rossato elal. Lawrence

(1987) (1992) (1980)

LiquidLimit % 65 63 69

PlasticLimit % 35 33 38

PlasticIndex % 30 30 31

Specific Gravit y 2.61 2.61

Clay Fraction <2Jlm % 80 82

Table2.2: Consolidation Parameters ofSpcewhltc Kaolin (afterSpringman, 1993)

o; e c,

lO-'~ml"

^Source

kP. mm²/ "

400·700 1.10 0.35 0.68 AI·Tabbaa. 1987

120-450 1.21 0.57 0.34 Dransby 1993

100-200 1.30 0.18 0.95 Ellis1993

54·91 1.54 0.25 2.87 Sharma 1993)

43·86 1.54 0.27 2.06 Springman1993)

searchers through shearvanetestscondu ctedin-flightin thecentrifugeand arc listedin table2.3.

FromTable 2.1,itcanbeseenthat the plasticityindex of kaolinisabout30%.

Thedayfraction ofkaolin is about80%whichis muchhigher thannatu ra l days and resultsina.lower stiffness a.ndweaker shear strengt h. Thisdisadvantage may be improved bymixingkaolinwith somegranular material toobtainkaolinmixtures

12

Table2.3:StrengthConstantsof Speewhite Kaolin Reference

Nunez1989) Bcltcu etai.1993 Sprin man1989)

Note: 0,{3are mean values.

0.22 0.19 0.22

0.62 0.67 0.706

Table2.4:Propertieso(SomeKaelin-BasedMixtures (after Rossatoetal.(1992) and Springman(1993))

PureKaolin KSS KRF NaturalClay

u;

~3C%i

w.

% 63 38 35.5

wp % 33 21 17

Ip % 30 17 18.6 30

G. 2.61 2.63

Cla.yFra.ction<2pm % 82 55·65 43

",fa TXC 0.197 0.244 0.233 0.308

c, a TXE 0.180 0.186 0.199

c..!tT"",or DSS 0.152 0.180 0.222

13

with higher strength . The kaolin-basedmodelling clay! have been shown to have some advantages compared to purekaolin(Rossato dal., 1992).Springman(1993) obta ineda kaolin-basedmixtureby mixing70% Speswhitekaolin with30% 180 gradesilica rockflour (KRF). Rossatodat.(1992) obtained another mixtureby combining50% Speswhite kaolin,25% finequartz sand and25% industrialquartz silt(KSS). The physical properties ofthese two materials are listed in Table2.4, where TXC is theshort form fortriaxialcompressiontest,TXE is triaxial extension testand

nss

is direct simple shear test. From table2.4,it can be seenthat the sheerstrengthof the kaolin mixtures arehigher than pure kaolin.

2.4 Cone P enetra tion T.,st

The cone penetration test (CPT)is atech nique for the measurementof soil propertiesby pushing an instrumented cone into soilsat a constant rate.The main applicationsof CPT areto determinethe soil profile and identify soilsand to evaluate soil engineering parameters. Insome cases,CPTs may be accompan iedby boringsto achieve more reliabletestresults. The CPT can provide continuous measurement of ground conditions;it also causes lessdist urbance ofsoil layersaasccietedwith boring and sampling. The CPTtechniquehas beenwidely used inresearchand engineering practice. Because of the complex changes or stress,strainand pore pressure duringthe conepenetr ation test,itie difficultto make a comprehensive theoret icalenelysie. In engineering practice,the analysis of CPTis highlyempirical (Meigh, 1987).

For a standardcone penetrometer, the cone is60~and the cross-sectlona larea is

14

usually10, 15or 20 cm3•Incentrifugemodelling,much smallercone penetrometers havebee n used (FergusonandKo , 1981;Davieset0/.,1989). The standardrate ofia-eituconepenetratio nis20±5 rom!!(Meigh,1987). Some penetromete rscan measurebothtip reeietenee(q.) and side friction(I.);otherscan only provideqe- Otherpiezoconepenetrometershavealso been developed for additionalmeasurement of porewater pressureinsoils duringcone penetration(KonradandLaw,1987;

Mayne andHoltz,1988;Sullyand Campanella,1991).

Anoth erimpor tant parameterofCPT is thefrictionratio,Rf,whichisdefined as thera t io ofsidefriction to tip resistanceand expressed as

(2.3)

and whichis veryusefulforsoil dassificatic0..

CPT datacan be used foresti ma tion oftherelat ive densityof normally ccnscl- idated sand(Jamiolkowski,1985) and overconsclideted sand(Schmertmann, 1975), sand strengt h(Du rgunogluand Mitchell, 1915) and otherparameters(Meigb,1987).

Also,extensive investiga tions beve been carriedout for determin ing the prope rties of clays using CPT,includingundrainedshear strengthoi normallyconsolidated clays [Lun neandKleven, 1981)and overconsolidatedclays(Marslan d and Quarter·

man,1982 ),and deformabilityof days(Meigh,1987). Inaddition,CPT has also been used for the estimatio nof pilebearingcapacity(Meigb, 1987), forthe control ofgroundimprovement(Juilie and Sherwood,1983) U1d for the det erminationof liquefactionpotentialof sand layers (Zhou,1980).

Cone tip resistanceq.,changes directlywithundrainedshearst rengthofclay,

1.

e".,which is usuallydetermined by in situvane test;the expression equating thetwo parameters is (Schmertmann,1975; De Ruiter, 1982)

c,,=~N,

where(7110is total overburden pressureandNe isa cone factor.

(2.4)

A range of the Ne values bas been found(Amaret ai"1975;Lunnc et al.,1976);

there is no universalNe for all clays. Tbe value orN.changes withsoil physical propertiesand overconsolidationratio.

2. 5 Geotechnical Cen t rifuge Modelling

Although many geotechnicalengineeringproblems, such asstability of slopes, earth pressure,bearingcapacity andsettlement, can be solved using theories baaed on aset of simplified assumptions, it issometimes more desirableto conduct a large scalefieldtest in order toobt ain reliable data. However,thecostandthetime required and the difficultyincontrollingthe test condition reducetheapplication valueo~fieldtests. Laboratory tests ,on thecontrary,are easytooperate and the testcondition is easier to control(Mikasaand Takada,1973). Assoilis a high non-linear materialandits mechanical propertiesdepend onthe stateor effective stress,modelsimilarity requires that any stressin the model be equaltothat in the prototype. Itis bard to finda modellingtechnique whichmay satisryself-weight stressesbetween model and prototype using thesame material. Forexample, a modelor an earthslope SOem high experiencesverydifferent self-weightetreeecs than an actualearthslope withaheightor50 m.The stress intensitydue toselr-

16

weight.of thesmellscalemodelj!muchlesethanthat. in the prototype and hence thestress-st rainbehaviourandthe patternsof deformations canbequitedifferent in thetwo sit uat ions. Inorderto solvethisp-eble m, the centrifugetechniquehas been int roduced for geotechnical applications. Incentrifuge modelling tests, the gravitati onal effectcan be modelled. Byapplyinga.centri fugalacceleration,the self-weightetreee distribut ioncan becorrected tosimulatethestressconditionsin theprototype.

Theideaofcentrifuge modellinganditspossibleuse cametobirth asearly as 1868 (Craig, 1989). Inthe 1930's,investigatorsintheUSA andthe former USSR introducedthistechnique to geotechnicalengineering (Rowe,1975). Many import ant developmentsofthe cent rifugetechnique for geotechnical problemswere madeat theUniversityof Manchest er and the Universityof C&mbrige in theUnited Kingdom in the 1960'sand70's (Rowe,1975;Schofield,1980). Currently, there aregeotechnicalcentrifugefacilitiesin theUSA.United Kingdom,France,Japan , Canada, Chinaandothercount ries.

Geotechnical centrifugemodellinghasbeen awell-recognizedresear ch technique to fulfill similarityinmodeltests forinvestigat ingmany kindsofproblemsin geotech- nicalengineering. Thecentrifuge techniquehas been used for studiesofsoilcon- solidat ion(Kimuraetal.,1984), retainingstruct ures(Schcherbina, 1988),dams andemban kments(LeeandSchofield,1988; Fengand Hu,1988),shallowand deep founda tions (Kutte retal.,1988), cone penetration tests (Ferguson andKo,1981;

Springman , 1993)and soilliquefaction potential(Hushmandetat, 1988).

17

Chapter 3

Materials Testing

Two types of remoulded soils were used in this study: kaolinclay anda kaolin-silt mixture(K·S).The formerwas obtainedbymixingSpeewhite kaolinpowderwith waterwhile thelatt er was obtainedbymixing equal amounts of Spcewhitekaolin and Sil-Co-Silsiltwith water. Thischapterintroducesthephysical c.haracteristics ofthese two clays andthe oedomeler consolidation test results.

3.1 Physical Characteristics

Inordertoclassify the kaolin and the kaolin-silt mixture, theplast iclimit,liquid limit,specificgravity and grain size distributionwere determined forbothsoils.

Plasticlimittests were done followingthe procedures described inASTM D4318- 84. Liquidlimitsweredetermined usingthe fallcone method.Thespecificgravity teats were conductedfollowingthe proceduresdescrib edby Bowles(1986). The test results aregiven in Table 3.1. Inthe grain size analysis tests, thehydrometer methodwas used and allsoilparticles passed through aa.o'{Smm sieve. Thegrain sizedistributionsof the kaolin, silt and kt-.olin-siltmixtu reareshown in Figure3.1.

18

Table 3.1:Propertiesor Kaolin andK-S

SoilType Kaolin Kaolin-SiltMixture

LiquidLimit ,Wl,: % 59,3 35.2

PlasticLimit ,Wp: % 28.7 15.7

Plas ticityIndex,Jp: %

3 °,6

19.5

SpecificGra.vity,G.: 1 2.63 2.62

MeanGrainSize,D5Q: mm 0.0005 0.0035

ClayFraction«2pm): % 73 42

'0 '

10~ 10"

GrainDlameler(mm)

0

-

0

,. _I

,- -

^{., /}

K"oll

/

.K.

. --

o

10 10

20

l

~ 60

;;:

c

!i

⁴⁰

0.

Figure 3.1:GrainSize Distribut ionCurvet!

19

From Table3.1andTable2.1,itcan be seen thatthe specific gravit y ofthe kaolin ofthisst udyis close tothose obta ined by Rceaetcetal.(1992)andLawrence (1980). Boththeliquidlimitand the plastic limit of the kaolin of thisstudy are lower thanthoselis ted on Table2.1,which maybedue tothefact that theclay frac tion ofthekaolinof this st udyis73%while thedayfract ionslistedon Table 2.1are80%and82%respecti vely. The plasticindiceson Table3.1andTable2.1 are veryclose.

The kaolinand thekaolin-siltmixt ureare inorganicclays. Accordi ngto ASTM D2487(1993),thekaolinis a fatclayofhigh plasti city, classified asCllithekaolin- siltmixtu re is a leanclay,classifiedasCL.Thekaolinwill havelowstrengt h and lowpermeability.

3.2 Oed om eter Co nsolid at ion Test

3.2.1 TestProgr am

1'0determinethe consolidat ioncharac teristics ofthetwosoils, codometcecon- solidation testingwas carried outby applyi ngverticalloadsto a laterall y confined speci men andobserving the vert ical deform ationofthespecimen with time.

The innerdiameter of theoedometer used in thi stestwas61.8mm. The specimenWASteste d undertwo-way drainageconditions. Vertical loadswereapplied to the specimenbydead weightsthrougha lO'l.dingframe. The loadincrementratio (6(1,, /(1,,)was keptto1 (exceptthefirst toad incremen t step)accord ing toASTM D2435-90.Eachloadincrementwas appliedfor approximatel y 24 hou rs.Vertical deformat ionof thespecimenwas monitored usingadial gauge.

20

The kaolin lIJId K·S slurrieswere prepared at initial water contents of approxi- mately 100% And 70% respectively and were storedfor 24 hours for the purpose of better saturation. After the specimen was spooned into the oedometer ring, the initialheight and water content of the specimen were measured.The loading frame which gave a 14 kPa vertical stress was applied on the specimen through a loading plate and a ball-bearing. Thedial gauge weaset up and water was added all around the specimen.The timer was started andthe readings of the dial gauge were taken at approximate times of 0.1,0.25, 0.5, 1, 2, 4, 8, 15, 30 min, and 1,2,4,8,24h after it was loaded.The next stress increment was then applied withthe same procedure.

The tota.! vertical pressure was increased to 25,50, 100, 200, 400,800 and 1200 kPa in steps. After the specimen was consolidated under 1200 kPa, the specimen was unloadedand swelling was monitored. Theunloading was made in steps withthe unloading incrementequal or less than 2nOkPa. The durationof each unloading increment was from 4 to 6 hours for the completion of soil swelling.The final water content was measured when the unloading process had been completed.

3.2.2 TestResu lt s

For the twocleys,the initial water content,_wo,and specimen height,Ho,were measured before the tests were conducted.The observed accumulated deformation of the two specimens are shown in Figure 3.2and Figure 3.3.The initial void ratio, eo,and the height of solids,H.,can be calculated using (assuming 100% saturatioo)

(3.1)

21

38

E

~36E

..

x

3 .

32

30

10¹ lQ²

Time.temln)

Figure3.2:Accumula.tedDeformationwithTimeof Kaolin

22

10 10

10 10

TIme,I (min) 10

4

.-.

2

•• . ~

••

:---i

0

•

•

--..:...

50

•

~

^--.:

10

i--'-

C-- ••

,

^~ -

•• ••

-

20

.

^:-'-

4

.

_~O

2 •••

• •••

12

••• • _•••

0.,

, , , , .

3 10

Figure 3.3:Accumulated Deformation withTimeof K-S

23

Table3

.

2'

.

Paramet ersfortheCalculationof VoidRatio Soil Type

(';:,) G.

(:;.) ^'. (:~)

Pure Kaolin 91.3 2.63 54 2.401 15.9

K-S lofixture 67.6 2.62 5.\ l.771 19.8

H_.-_-~_{(1+ ,.)} ^(3.2)

whereG.is the specificgravity ofsoil. Theresults are shown in Table3.2.

According toASTM02435-90, thevoid rat ioof thespecimen ofanyheightcan be calculatedusingthefollowingequation:

(3.3) whereHis thesamplefinal height ateach load increment.

The coefficient of compressibility,all,the coe fficientof volumecompressibility, m..,andcompression index,C~,are importan t parametersused toeeumetcsoil compression.The definitionsof theseparameter s arc

m,,=

1:

^lIel'and

C~

⁼

109(t1~2ela~1)

(3.') (3.5) (3.6) whereel and e2 are the voidratios atthe beginning andtheend of theconsolidation, .6.eis the changeofthe void ratio, O'~Iand0'~2are the corresponding effective

2.

10² 10' EffectiveVeri/calSt,elS(kpa)

fO'

Figure3.4;e-O'~Curve for Kaolin

verticalpressures atCIandea,and.6.0"~is thechange ofvertical effective stress.

The coefficient ofconsolidat ionc"can bedetermined using O.S48T"ii'

c"=- t-",- (3.7)

wheretooobt ainedusing thesquare root of time method(Bowles, 1986)is thetime requiredto reach90% consolidat ion.Tv is the time factor (at consolidation degree of90%,T"is 0.8·18),and11isthe lengt hof thedrainagepathwhichis equalto half ortheav('rage sampleheight.Theresults ofc....alt.C~andm, aregiveninTable 3.3 and 3.4, inwhich,fJis the averagesample heightat any loading step. From theconsolidation testresults,e-pcurvesof bothsoils areshown inFigure 3.4and Figure 3.5.

25

Table 3.3:CompressionParametersof Kaolin

Average Ik';:'.\

H

~:, 1 ";;:'1,' " ,

^e

_I

_Il/;;P.\ ^C,

_(1/~lP.\

(mm)

0....14 48.1839000 0.0547 0.7328 2.035 55.100

14....25 41.183 3480 0.1033 0.1490 1.593 12.732 0.5435 4.77 25-50 38.6781980 0.1602 0.1665 U36 6.661 0.5532 2.64 50....100 36.161 1110 0.2497 0.1506 1.278 3.012 0.5003 1.28 100-200 33.915 778 0.313'. 0.1323 1.136 1.323 0.4394 0.80 200-400 31.903 540 0.3996 0.1212 1.009 0.606 0.4028 0,29 400....800 30.080 290 0.6614 0.1083 0.895 0.271 0.3599 0.14

800-1200 28.722 280 0.7030 0.0628 0,809 0.157 0.3566 0.09

26

Table 3.4: CompressionParametersofKHSMixture

Average Ik';:.) 7i

;~

^{(0:'/5) d, e}

'.

^{C, m,}

[mm] 1110"') (I/ MP. ) (I/MP.)

0...14 49.163 7260 0.0706 58.818 1.477 44.2243

14-25 42.3532340 0.1625 9.605 1.134 8.3801 0.3577 3.84 25-50 40.395 1215 0.2847 9.926 1.036 3.9703 0.3297 1.90 50...100 38.452 735 0.4265 9.654 0.938 1.9307 0.3207 0.97 100....200 36.603 540 0.5260 8.983 0.834 0.8983 0.2984 0.48 200_400 34.875 420 0.8139 8.429 0.747 0.4215 0.2800 0.24 400-80 0 33.318 290 0.8115 7.255 0.679 0.1814 0.2410 0.11 800_1200 32.292 276 0.9009 3.089 0.627 0.0772 0.1754 0.05

27

1.2r;;-,--,-- - - ,

. .

a:

"

~ O.8

10' 10^J

EIl ':lCliv8 Verlic alStr ess(kpa)

Figure3.5:e.O'~Curve fotf\-S 3.2.3 ResultAnal yses

In criticalsta le theory,theromprcsslonandswellinglin~'1IarcMNIU HCt Itobe straight in(n(p' )." space with~lopcs.'\and.,. respectively. Here II'istilemean effectiveprincipalstress andv isthe specificvolume (t'=lte).The valueofp'carl becalculated usin,;

(3.8) where

0':

^{is effect}iveverticalstrenand

1 <.

isthecoefficientofeartli pressurelotrest whichcanbe expressed3.!1(Mayne and Kulhewy ,)982)

J(.=(1-sin¢/)(OC.'l)"no/l (3.9)

where¢'istheeffective angle of intcrn lllfrictionandOCRis ovcrccnsclidaticn rat io.

For kaolin. 6'=23-(AI-Tabbaa,1987) andlI~ingEq'I,·.tion (3.8)thedatain 28

Table 3.3 resultsin.\=0.187and1(=0,051. The average values of kaolin obtained byAI·Tabbaa.(1987)are .\=0.187and1'1:=0.028.

Wood(1990)introduces the estimations of A and '" using

AI:l:l:2.3C~ (3.10)

(3.11) where Cc is thecompression index andC.is the swellingin dex in one-dimentionaUy consolidationtests.Accordingto thedat a.shown in Figure3.4and Figure3,5,the average values ofC~=0.453and C.=0,0926of the kaolin yield.\=0,197and1(=0,0402 while thevaluesofC.=0,294 andC.=0.0361of the K·S mixture yield>'=0.128and 11:=0.0157.

Usingthe data in Table3.3and Table3.4,therelationship betweenCuand void ratio,e, of the kaolinCAIlbe expressedas

(3.12) and forK·S mixture,

(3.13) whereCuisin m²/s. The resultsof thisstudy and theresult of kaolintested by King(1993)areshown in Figure3.6.Itcanbe seen that{orkaolin, theCuval ues of the two studiesare close at highvoidrat io levels. FromTable3.3,Table 3.4 and Figure3,6,it can be seen that for boththe kaolinand K-S mixture,the c., values ofthis studyincrease much slower with vertical st ress whenthe verticalstressis greaterthan400 kPa, The resultsare in accordancewit h the factthatc.,reaches

29

a plateau of 0.35 x 10-¹m'/$when vertical stress is between 400 kPa and 700 kPa (AI·T.bb...1987).

30

2 .' 1 .'

Void ratio, .

.

_{... ..... .. ..}

... ...

_.

_{.. ...}

_.

_.. ... . ...

..

... ... ... ...

..

... ...

...

....

0 Pr...nt "'"dr.K;~;~

,

^-

- -

Presents.tudy, K·S

.

^{..\ 'v •}

t

I

<,

^....

-,~

''''.0

^"

., i: " ,~

.. .. ..^.

...

... .

..^..... ... ..^.

...

.. .. .. ....<>-..

....

..

....

.. ...

...

..

.

^-

....

... ..

..

.... ... ...

....

. .... ...

.

...

.... ....

...

1D

, ,' . . .

Figure3.6:Comparisonorc"RemIts

3.

Chapter 4

Shear Strength Tests

As described previously, there are many test methods{or measuringtheshear strength of soils. The shear strength is influenced by manyfactors, suchlUIsoil type, confining stress, soil density andstress history. In orderto determine the shear strength parametersQ andfJof Equation (1.4), a series ofconsolidated undrained directshear and shear vane testswere conducted on boththe kaolin and the kaolin-silt mixtureat different effective normalstress and OCR levels. This chapterwill present the test techniques and results of the directshear tests and shear vane tests on the two soils.

Inthe direct sheer and shear vane tests,thesoilepecimenawere normallyone- dimensionally consolidated under total stress increments similar to that described in Chapter3. During consolidation, vertical deformation was monitored.When 90%

of primary consolldeflonWallcomplete,the nextloading incrementwasapplied. The overconsolidation ratio (OCR) o(a specimen is the ratioof tbe maximumpreconsol- idation stressa:.to the currenteffective vertical stress0':2'that is,OCR=a:dO':2' In both direct shear and shear vane testa, specimens with OCR values of I, 2, 4,6,

32

8wereusedto investiga te thestrengthbehaviour or the soils.

4.1 Direct Shear Test

The directsheartests werecarriedoutforthedeterm inatio norshear strength or bothkaolin andK·S.The sp ecimens were consolidated and shea redinshearbox underOCR valuesorI, 2,4 , 6and8.Thissectionintroduces the dirCl:t sheartest appar a tus,testprocedu resand the test results,

4.1.1 Test Apparatus

Thedirectshea rtestapparatu s usedinthisstudy is tha tor the Soils Laboratory orthe Facultyor Engineeringan dAppliedScience atMemo rialUniversity ofNew- foundl and . Thediagramof thisapparat usis showninFjgure4.1,The components ofthis shelU'devi ce aredescribe d below.

Driving unit:A motoris usedto apply a horizont a lstrain-controlledshear displa cementto thespec imen. In the directsheartest, thedrivenspeedofthe horizo n taldisp lacement is usual ly from0.5to2 nun/ min (Bowles,1986)_Inthis study, ashear ing rate of 1.26mm/min was usedwhich is themaximumshearing rate of thisdevice.

Shearbox carr iage : The carriageis usedto mounttheshea.rbox andisfilled withwaterdu ri ngthetest. Thiscar riagemoves horizont allywhenthedriving motorisworkin g to supp lytheshear forceto thespecimen.

Shearbox: Thisbox isthe specimen containerconsist ing of two halveswhich canbe fixedtogeth erbymeans ortwo clamping screws. Twolift ing screwsenable

33

=

~

~

'"

_So

~ is

;; ~

J l ..

~

~

34

the upperhalf to be lifted slightly to reduce the frictionforce at the surfaceof the two halves. Theinside diameter oftheshearbox is 61.8 mm.

Loadhanger: It is madewitha.spherical seating andha.ll bearing forapplying the normalpressurethroughaloading plat e to the specimen,

Load ingring: Strain gaugesareused for measuring thehorizonta l shear force;

the details areto be introduced later in thissection.

Dialgaug e: When necessary,a dial gaugeisusedforthe meas urementof vert icaldeformation duringconsolidatingand shearing.

X-V plotter :Itis usedto recordtheshear load appliedto the specimen.

Therearetwoways of using aloadingring to measurethe shear stressapplied to the specimen.A dialgauge can be used to measurethe loadingrillgdeformation causedby the shear force and this deformationcan beconvertedtoload using the calibrat ioncurveoftheloadring. Tbe other methodisto use strai ngauges to measureload applied to theloadingring, as shown in Figure 4.1.

Inthis study,four electricalresistance straingauges weregluedtothe loading ring.Thestrain gauges were of foiltype; the gaugeresistancewas 120ohms.When a loadis applied to theloadingringasshown in Figure4.1,the twostraingaugeson theoutsidesurface of the ringa.reextendedand the othertwo gauges on the inside surfaceare compressed. These {ourgauges were connected togethertoform afull Wheatstone-bridge circuit,as showninFig";e 4,2,

Thisarrangementofstraingaugeshas two advantages. The strai nsensed by eachof the {our gauges is addedtoget herhence theaccuracy of load measurementis

35

x·y^Plollu

3,4 : COlipren ioD Cl ugel

Figure 4.2:Whcatercne-B ridgcCircuitforShear St ressMeasurement increased. Theotheradvantageisthat thiscircuitcan provide tem peratur e compen- sat ion.With a fuJIWheatstone-hridge circuit , themeasurement resultswill not be influencedbythe changes ofenvironmental temperatureorby theexciting electrical current through thegauges. This circuit was excitedbya10volt directcurrent powersupplyand the measurementwas takenby an X· Yplotter. Theloading ring

WMcalibratedusingdead weights.

4.1.2 TestingProcedures

The overall height ofthe originalehcarbox was 50mm; the upperpart wa.s25 mm andthelowerhalf was also25 mm. Accord ing to thecompressiontestresults of the kaolinpresented in Chapte r3,more than 40% compression wouldoccur under

36

a. 1000kPa. verticalload. Inorderto obtain athicker specimen,anupper hall extene lc ngivinga total60 nun inheightwasmanufactured and used iothetests.

Thetwo halvesoftheehearbcx were fixed togetherby the dampiogscrew"and placedin the carriage. The soil slurry was placed into the ehearbox usinga tea spoon. Two porousstoneswereused at the top and bottomend ofthespecimen.

Theinitial heightof thespecimenWMmeasur edimmediately after the slur ry had been poured.

Theload platewas thenplaced on thespecimen.Theload hangerwasgently put onto theload platethrougha ball-bear ing andthe verticaldialgaugewas set. Water wasadded all aroundthe specimen. Afterputting on the load banger,thetimer was started and vertic,1dialgaugereadings weretakenattimeinte rvalsaccording to ASTMD2435-90.When90%consolidationat a load stepwasachieved,the next load incrementwasapplied withthesameprocedure.Theself-weight of the hanger was thefirst load stepof 14 kPa. Theload incrementwasapplied accordingto ASTM 02435·90. Thedegreeof consolidationrequiredW~90%but100%was requiredforthelast loadincrement .To obtainan overconsolidatedspecimen,the specimen was unloadedto the required st ress level,_O"~,alter consolidationatthe maximumload increment,O"~I' Whenthespecimen wasloaded orunloadedto therequired stresslevelthe clampingscrewswere removed and two liftiogscrews were driven. After thescrews contactedthelowerhalf,afurther half-turnrotation separa ted the two halves,Thesetwo lifting screwswere removedbeforeshearload was applied.

37

lO' r--~---~--~---~--,

NormanConsolidated A'SO ii

i

/ --~=='-""""'''''''''''--_-''-!.'''­

20 i

i i

t ~

⁶⁰

'"1 : ..

0

'"

so

2 3

Shu tDIspla cement(mm)

Figure4.3:TypicalShear StressVIShearDisplacement fromDirectShearTest Specimenswereshared atarateof 1.26mm per minute.The speed oftheplotter recordingpaperW&lI1 mmper second. The shearin gof thespecimenWa5stopped if."peakvalue of,beatloadoccur red,otherwisethe .pecimenwushear ed formore than200 secondsin orderthi.thehorizontalshuI'displacement wu 1/15ofthe specimendiameter. Arterthedrivingmachine

o r

theshcarboxWASstopped, the shean:depecimeo was immediatelyremovedand thew,.tercontentof thespecimen was measured.

The shear rate was constant inall the tests . When anovcrconsolidated sample was sheared,therewas a peakshearforce(Figure4.3),whichwas used inthe calculatio nofIIhear8~tength. Itwasnotedthat fotnormallyconsolidated specimens, therewuno apparentpeakshear force valueasshown in Figure1.3.In thisfigure, Al SOwasanormallyconsolidatedspedmen shearedunderan effective

38

vert icalstressof150kPa. ElSOwas an overconsclldated specimenwithOCR value of 8 and sheered under an effectiveverticalstress of150 kPa.IntheCMewhenthere isnopeak shear force, the shear forceat failurewas considered tobe at a horizonta l deformationof1115 of theorlginaleamplediameter, which was4.12nun. Asthe shear ratewas set at 1.26 mm/min,thisshearforceWlL'taken 200secondsalter starting. Because when the samplewas sheared, the horizontalerose-sectionalarea.

of thespecimen wasreduced, an area correctionforthe calculationof specimen shear strengt hmust be made.Thedetails ofarea correction arc presentedin Appendix A.

4.1.3 TestResultsof Kaolin

In order to evaluatethe strengthbehaviourof kaolin,21 directSh('Mtests with variousstressand OCR levels were conducted. Thetest results are shownin Table 4.1, wheree..is theundrainedshearst rengthafter the area correctionwas applied,

IUJis thefinal water content ofthe specimen. The initial water contentof thekaolin was100%.

Usingthe data of Table4.1, the relations hipbetween normal stress and shear strengt hof thespecimenswith differentOCRvalueis shown in Figure4.4 where linear regression lines have beenfitted to the data of each OCR.Itcanbe seen that at each OCRlevel,the shear strengthis proportional to the normal stressat shearing. Underthesame norml\!stress, shear strength increases withincreasing OCR.Thetestresultsindicate thattheundrainedstrength ofthe kaolin is related to stress levelandstress history. Fora normallyconsolidated specimen,the slope of the OCR =Iline inFigure 4.4 gives the valueof0'. Figure4.5 shows thatthe

39

Table4.1:DirectShearTestResultsofKaolin

OCR Te l _(kPa)

" ..

_(kPa)

^".,

_(kPa)

..

^c,.lu:~ _(%)^wI

ASO SO.O 50.0 12.0 0.240 St.7

AIOO 100.0 100.0 22.5 0.225 4.8.0

1 AlSO

tso.o

ISO.O 3S.0 0.233 41.3

A200 200.0 200.0 46.3 0.232 4.3.6

A250 250.0 250.0 54.9 0.220 4.1.9 BSO 100.0 SO.O 15.6· 0.312 4.8.3

8100 200.0 100.0 33.6· 0.336

~

2 8150 300.0 150.0 50.1 0.334- 42.0

8200 400.0 200.0 64.8· 0.324. 4.0.7 8250 500.0 2.50.0 79.S· 0.318 38.8 CSO 200,0 50.0 20.6· 0.412 4.5.9

CWO 400.0 100.0 40.2· 00402 4.1.0

4 CISO 600.0 ISO.O 67.7· 0.4.51 4.0,0

200 600.0 200.0 89.0· 0.4.45 38,4

C2S0 1000.0 250.0 105.5· 0.4.22 37,4 DSO 300.0 SO.O 27.7· 0.664 14.1

6 DlOO 600.0 100,0 53.4.· 0.534 40,4

D1SO 900.0 1S0.0 74.5· 0.4.97 39.1

ESO 4.00.0 50.0 29.1· 0.682 4I.l

8 EIOO 800.0 100,0 58.0· 0.580 38.8

siso

1200.0 1S0.0 81.8· 0.545 36.6 Note:Data with· are obtained usingthe peakvalues oftheshearforces.

40

150,--~-~---~-, OCR_

o

floa

iJ .

'" 50 -e

~

:>

50 100 150 200 250 300

EffectiveVerticalStressduringshast lng (kPa) Figure4.4:ShCJ'ltStrengthofKaolinfrom Direct Shear Tests normalized9trengthc;./q~2is dependenton OCR end independentof normalstress stale.

Thelast column of Table4.1shows the watercontent ofspecimens aftershear- ing. Itcan be seenthat the water contentdecreaseswithbothnormalstressand overconsolidationratio,ThC5Cresultsareshown in Figure 4.6.

Theundra inedshear :ilrenglhparametersQandPin Equation (1.4)can be determinedby linearregressionanalysis. Mathemat ically,Equation(1.4)can be rewrittenas

1'9(!:;-)=1'9(0 )+P(OCR ).

«,

41

(4.1)

0.8 Equation4.2

0.6

0.4

0.2

o

. 11:, ..

^{50 kPa}

lIE 100

o 150

x 200

250

2 4 6 8 10

Overcon solidationRatio,OCR oo~--':--~----:---:---:'·

Figu re4.5:NormalizedShearStrengthofKaolinfrom Direct Shear Tests

60 r--~---'

3°0 SO 100 150 200 250 300

EUecllv&Vertical SIre n duringshearing(kPal Figure4.6:Wa.ter Contentof Kaolin alterDirectShear TI'..ats

42

Using the data in Table 4.1,t:le followingvalues canbe obtained:

109(0):=-0.6234; Q:=0.238;

and

P=O.44l.

Therefore, theundrainedshearstrengthofthekaolin basthe followingexpression:

::r=0.238(OCR) o.4ol1

- ,

^('.2)

wherec..is the undrainedshearst rength,u~isthe effective verticalatreee atthe beginningofshearing, and OCRisthe overconsolidation ratio.Thiscurvehas been plottedto the data of Figure 4.5.

4.1.4 TestResults of Ka olin-SiltMixtur e

To investigatet~o<istrengthof the kaolin-siltmixtur e,20 direct sheartestswere carriedoutunder various vert icalst ress and OCRlevels.The initialwatercontent forthe K-S specimen was 70%. The testresultsare giveninTable4.2,in which, area correctionhas been applied.

The relationship between undrained shear strengthand effectiveverticalstressis shownin Figure 4.7 wherelinear regressionlineshave been fittedto the data ofeach OCR.Therelationship between normalized shear strength and overconsolidation ratio isshown in Figure4.8.Figure4.9 showsthe finalwater contentsafter direct shear testing.

These tcstresultsindicate that the undrained shearstrength ofK-Sis also a functionof effectivenormalstress and OCR.Theuodrained shear strengthpareme-

.3

Table4.2:DirectShearTest ResultsofK·S

OCR T.,t

(~~I .P. :~lJ) (.;.)

c..1"':~ ^(l';)WJ

I A50 50.0 50.0 16.1 0.322 33.2

AIOO 100.0 100.0 32.1 0.321 31.9

AlSO 150.0 150.0 48.7 0.325 30.5

A200 200.0 200.0 64.5 0.323 29.1

A250 250.0 250,0 82,3 0.329 28.7

2 8100 200.0 100.0 45.8 0.458 29.2

81050 300.0 150,0 68,7 0.458 28.2

B200 400.0 200,0 90,1 0.4M 27.7

8250 500.0 250.0 112.6 0.450 27.0

4 C50 200,0 50.0 26.2 0.524 29.9

CIOO 400.0 100.0 53.8 0.538 2·{.9

Cl5 0 600.0 150.0 79.2 0.528 26,4

200 800.0 200.0 104.6 0.523 25.8

6 050 300.0 50.0 30.9· 0.618 28.3

DIOO 600.0 100.0 62.3· 0.623 26.5

0150 900.0 150.0 95.7· 0.638 25.0

0200 1200.0 200.0 127.0· 0.635 24.3

8 EOO 400.0 SO.O 38.5" 0.770 27.5

EIOO 800.0 100.0 70.6· 0.708 25.8

EI50 1200.0 150.0 103.3' 0.689 24.5

Note:Datawith·are obtainedusingthe peak values of theebear forces.The valuesof e,. of AIOO,AlSO and A200are the mean values ofthree tests.

44

OCR.

50 100 150 200 250 300

Effective VerllcalStressduringshearing(kPa)

.

~

e ::>

'SOr----~----~_;_-~-___,

.

~

<!.

,

o

1

¹⁰⁰

~

j

'" so

Figure 4.7: Shear Strength of K·S(rom Direct Shear Tests tere a andIJcan be determined using the same method as for kaolin. The following relat ionship between normalized shear strength and OCR can be obtainedforthe mixture:

5;

=O.330{OCR)0,37'O.

-.

^(4.3)

Thiscurve has been plottedto the data of Figure4.8.

4.2 Shear Vane Tests

Laboratoryshear vane testing has been provento be an effective method of determiningthe"he1\f strength ofclays. Inthis studYlvane tests werecarried out on sped men. in aspeciallydesigneddrcularsample tub using a rectangular vanewith 4 blades. The vane test device was manufacturedby the Wykeham-

45