Triaxial Testing

LaboratoryTeststoDetermineShearStrengthofSoils

GeotechnicalEngineeringII(ENGI6723)

PresentedbyRodneyP.McAffee,Ph.D.,P.Eng.

LaboratoryTeststoDetermineShearStrengthofSoils

•  LectureTopicso Briefoverviewofdirectsheartesto DeterminesoilshearstrengthparametersfromtriaxialtesNng:• Unconsolidated–Undrained• Consolidated–Undrained

o Triaxialtestsetupandbehaviouro UseofresultsinengineeringpracNceo Examplesoftriaxialtestresults

ShearStrength

•  Shearstrengthofsoilsisrequiredtosolveproblemsofstabilityo Bearingcapacity,earthpressures,slopestability,etc.

•  ShearstrengthisafuncNonofeffecNvenormalstress

LaboratoryTeststoDetermineShearStrengthofSoils

•  DirectShearTest

•  TriaxialTestso Porewaterpressuremeasuremento TesNngunderbackpressure

•  TypesofTriaxialTestso Unconsolidated–Undrainedo Consolidated–Undrainedo Consolidated–Drained

DirectShearTest

DirectShearTest

•  Disadvantageso Failureplaneisforcedtobehorizontalo Cannotcontroldrainageo StressconcentraNonsatthesampleboundarieso UncontrolledrotaNonofprincipalplanesandstresses

•  Advantageso Testininexpensiveo Fastandsimpleo Easytoprepareforcohesionlesssamples

TriaxialTesNng

•  CasagrandedevelopedtriaxialtesNnginthe1930s•  MorecomplicatedthandirectsheartesNng

o ButmoreversaNle

•  Drainagecanbecontrolled•  NorotaNonofprincipalstresses

o Somesmallshearstressesdoactontheboundaries

•  StressconcentraNonsarelimited•  Failureplanecanoccuranywhere•  Stresspathstofailurecanbecontrolled

TriaxialTestApparatusandassumedStressCondiNons

•  Loadframewithsteppermotordrive

•  Triaxialcell•  Dataacquisitonsystem

•  Electronicmeasurementtransducerso Load,displacement,pressure,andvolumechange

•  ComputerSoZwarefortriaxialtesNng•  Controlpanelandde‐airedwater

controlsystem

•  ControlPaneltoregulatepressureandflowsduringtesNng

DrainagePathsinTriaxialTesNng

•  The3permissibledrainagepathsare:

Q-Test (for “quick” test)

S-test (for “slow” test)

Consolidated–Undrained(CU)TestBehaviour

•  Sampleisfirstconsolidatedunderdesiredstresses

•  AZerconsolidaNoncomplete,drainagevalvesclosed•  Typically,porewaterpressuresaremeasured

o CalculatetotalandeffecNvestresses•  Excessporewaterpressure(Δu)caneither:

o Increase(+ive):specimencontractsorconsolidateso Decrease(‐ive):specimenexpandsorswells

•  Axialstressincreasedincrementallyoratconstantrateofstrain

BackPressureduringTesNng

•  Toensure100%saturaNon(necessarytogiveaccurateporewaterpressures),abackpressureisappliedtotheporewater

•  CellpressurealsoincreasedbysameamounttomaintainthesameeffecNveconsolidaNonstresses

Typical Stress-Strain Curves for CU Tests

Principal (effective) stress ratio is a simple way to normalize the stress behaviour with respect to σ’3 during the test

•  Withporewaterpressuresmeasured,wecancalculatebothtotalandeffecNvestressesatfailure

•  Typically,anumberoftestsoverarangeofstressesiscarriedout

•  NCclayspecimensdevelopposiNveporewaterpressures(σ’= σ – Δu)

•  OCclaystendtoexpandduringshearcausingdecreasedporewaterpressures(σ’= σ –(-Δu))

•  TesNngonsaturatedsandwithmeasurementofporewaterpressure

•  MohrenvelopesintermsofbothtotalandeffecNvestressesforconsolidatedundrainedtestsonnormallyconsolidatedclaysamples

•  MohrenvelopesintermsofbothtotalandeffecNvestressesforconsolidatedundrainedtestsonover‐consolidatedclaysamples

UseofCUStrengthinEngineeringPracNce

•  SoilsarefullyconsolidatedandatequilibriumwiththeexisNngstresssystem

•  Then,addiNonalstressesareappliedquickly

•  ExampleTestforNCClay:

o Consolidatedunderastressof150kPao Thenshearedundrainedinaxialcompression

o Principalstressdifferenceatfailure=100kPao Inducedporewaterpressureatfailure=88kPa

c’ and cT are assumed to be zero

Unconsolidated–Undrained(UU)TestBehaviour

•  Sampleisplacedinthetriaxialcellwiththedrainagevalvesclosed

•  NoconsolidaNonoccurswhenconfiningpressureisapplied

•  Usually,porewaterpressuresarenotmeasured•  Sampleisloadedtofailurein10to20minutes

o CalledQ‐test(for“quick)•  Testisatotalstresstestandityieldsthestrengthintermsoftotalstresses

Typical Stress-Strain Curves for UU Tests

a) Remolded and some compacted clays b) Medium sensitive undisturbed clay c)  Highly sensitive undisturbed clay

Note: max. Δσ occurs at relatively low strains

Typical Mohr failure envelopes for UU Tests

100% saturated clay

Partially saturated clay

Increased cell pressure compresses any air voids

All samples have same water content and void ratio – therefore the same undrained shear strength

•  ChangeinbothstrengthanddeformaNonaZerremoldinganundisturbedsampleofNCclay

•  φu=0whenresultsareplogedwithrespecttototalstress

UseofUUStrengthinEngineeringPracNce

•  EngineeringloadingisassumedtotakeplacesorapidlythatΔu cannotdissipateorforconsolidaNontooccur

•  Changeintotalstressdoesnotaffecttheinsituundrainedshearstrength

•  Scope•  Terminology•  Significanceofuse•  Apparatus

•  TestSpecimens

•  Procedure•  CalculaNon•  Report

ASTMD2850:Unconsolidated‐Undrained

•  Terminology

o Failureisdefinedasthemaximumprincipalstressdifferenceorthatmeasuredat15%axialstrain

•  TestSpecimens

o Minimumdiameterof3.3cmo HeighttodiameterraNobetween2and2.5

o Procedurestoprepareundisturbedandcompactedsamples

•  Procedureo Axialstrain:1%/minforplasNcand0.3%/minforbrigleo Testshouldlastapproximately15to20minutes

ASTMD2850:Unconsolidated‐Undrained

•  CalculaNonso Axialstrain,ε=ΔH/H0

o Averagecross‐secNonalarea,A=A0/(1‐ε)

o Principalstressdifference,σ1–σ3=P/Ao CorrecNonequaNonsfor:

•  Ifallaroundpressurechangesspecimenlength

• CorrecNonforsNffnessoftherubbermembrane•  Report

o IndexproperNesofmaterialbeingtestedo IniNalH0,Diam.,γd,voidraNo,w.c.,saturaNon,etc.

o Rateofaxialstrain,strainandstressesatfailureo Stress–straincurveandfailuresketch

ASTMD4767:Consolidated‐Undrained

•  Terminology

o Failureisdefinedasthemaximumprincipalstressdifferenceorthatmeasuredat15%axialstrain,or

o Maximumstressobliquity,σ’1/σ’3•  TestSpecimens

o SameasforUUtest

•  Procedureo SaturaNonproceduretoensure:PorePressureParameter,B>0.95(B=Δu/Δσ3)

o ConsolidaNonprocedurestoensurespecimenreachesequilibriuminadrainedstateattheeffecNveconsolidaNonstressrequired

ASTMD4767:Consolidated‐Undrained

•  Procedureo AxialloadingtoproduceequalizaNonofporewaterpressuresthroughoutthesampleatfailure

o Assumingfailurewilloccurat4%axialstrain

• Rateofstrain=4%/(10xt50)o Detailsformeasuringporewaterpressures

•  CalculaNonso EquaNonsforheightandareaaZerconsolidaNon(Hc&Ac)

o Axialstrain,ε=ΔH/Hc

o Averagecross‐secNonalarea,A=Ac/(1‐ε)o Principalstressdifference,σ1–σ3=P/A

ASTMD4767:Consolidated‐Undrained

•  CalculaNonso CalculateeffecNvestressesbasedonΔumeasuredo CorrecNonequaNonsfor:

• CorrecNonforfilterpaperstrips• CorrecNonforsNffnessoftherubbermembrane

•  Reporto IndexproperNesmeasuredformaterialbeingtestedo EffecNveconsolidaNonstress,t50o Hc,Ac,Diam.,γd,voidraNo,w.c.,saturaNonaZerconsolidaNon

o Rateofaxialstrain,strainandstressesatfailureo Stress–straincurveandfailuresketch

Interpreting Scatter in Test Results

•  USArmyCorpsofEngineersProcedure:DrawstrengthenvelopeinaposiNonsuchthatdatafromtwo‐thirdsofthetestslieabovethefailureenvelope

•  PrimaryReferences:o A.W.BishopandD.J.Henkel,1962.TheTriaxialTesto R.D.HoltzandW.D.Kovacs,1981.Introduc:ontoGeotechnicalEngineering

o USArmyCorpsofEngineers:EngineeringandDesignManual:SlopeStability,AppendixDShearStrengthCharacteris:cs(EM1110‐2‐1902,Oct31,2003)

o ASTMD2850andD4767StandardTestMethods

•  LectureTopics–  Briefoverviewofdirectsheartest–  Determineshearstrength

parametersfromtriaxialtesNng:•  Unconsolidated–Undrained•  Consolidated–Undrained

–  Triaxialtestsetupandbehaviour– UseofresultsinpracNce–  Examplesoftriaxialtestresults

Summary/QuesNons