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