arma-91-1115_comparison of direct shear and hollow cylinder tests on rock joints

8/12/2019 ARMA-91-1115_Comparison of Direct Shear and Hollow Cylinder Tests on Rock Joints

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RockMechamcs s a Multtd/sc/phnarycience,Roegiers ed ) 1991 Balkema, Rotterdam. ISBN906191 194XComparisonf directshear ndhollowcylinder ests nrockjointsTimothyB.Reardon, ricC.Drumm& DanLange-KombakInstituteor Geotechnology,epartmentf CivilEngineering,heUniversity f Tennessee,Knoxville, Tenn.

ABSTRACT: For the fundamentalstudy of the behavior of rock joints, and thedevelopmentf constitutive odelsor oint response, hollow ylinder pparatusHCA)hasbeen developed o overcomehe deficiencies ssociated ith the direct sheardevice.A seriesof baseline estshavebeenperformed o compare he resultsobtainedwith theHCA to thoseof the directsheardevice.The resultsndicate hat the HCA yieldsslightlylower friction angles han the direct sheardevice. However, he initial stiffness btainedin the HCA is greater, probably a result of the increased igidity of the torsionalconfiguration.1 INTRODUCTIONThe convenience f the direct sheardeviceexplains ts widespread se to determine hebehavior f rock oint strength nddeformationalesponseBrown1981;Franklin1985;Sun,et al. 1985;HutsonandDowding1990). Alongwith this convenienceomeseveralsignificantimitations.These imitationsnclude he inability o determine he principalstressesxcept t failure,nonuniform tressest the oint, andhighstress oncentrationsat the edges. n addition, he response nder argedisplacementsanonlybe assessedyreversing he directionof shear, and joint water pressure s difficult to control andmeasure.Although he directsheardevices appropriateor design nd analysis,t is notadequate or a fundamental tudyof the behaviorof rock oints or for the developmentof constitutivemodels or joint response.

Hollowcylinder pecimensavebeenutili7.edo overcomehe deficienciesssociatedwith the directsheardevice Kutter 1974;Olsson 986;Olsson1988;Power,et al. 1988;Yoshioka ndScholz1989). With the discontinuityrientedperpendicularo the axisofthe cylinder, force s appliedalong he axis. A torque s then appliedabout hat axis,thusallowing or the determination f the shearing tressesequired o deform rotate)the oint (Figure1). The historical seof thin-walled nnular pecimensf softsubjectedto an applied orque s well documentednd s gaining opularityHvorslev 939;Bishop,et al. 1971;Lade 1981;Hight, et al. 1983;Saada1988). Due to the favorable tress tatein thesedevices, imilargeometries avebeenused n the studies f othermaterialsikeintact rock (Handin, et al. 1967;Christensen, t al. 1974;Cox and Scholz1988) and thehigh emperature estingof concrete nd rock (Bazant,et al. 1981;Bazant,et al. 1986).The geometry f the hollow ylinder pparatusHCA) lends tself o an investigationfthe effectsof various tress athsand anisotropy ssociated ith manymaterials Saada1988). An additionaladvantages the continuous ontactof the discontinuityurface,which alleviates he stress oncentrationshat occur n direct shearat the leadingandtrailingedgesof the joint.

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NORMAL 8TRESS

OI$CO#TI NUITY

SHEAR STRESS

CONFININGPRESSURE

Figure 1. Schematic f the hollowcylindergeometry nd stressorientationsThe University f Tennessee-Hollowylinder pparatusUT-HCA) hasbeendevelopedto perform fundamentalnvestigationseeded or studying nd modeling ock jointbehavior Drumm 1988) Figure2). Unlike earlierHCA's used or joint testing,he UT-HCA will allow he application ndcontrolof the confining ressure pplied o both he

inner and outer cylinderwalls,and the joint waterpressure t the joint interface. Thesampleused n the UT-HCA has an insideand outsidediameterof 100mmand 150mm,respectively. n MTS biaxial oad ramewith an electro-hydraulicosed-loopystemsused o control he normalstressaxial orce)and shearstress torque). A functiongenerator ontrolshe rate of displacementndcyclicotationof the UT-HCA specimen.The normalstressand shearing tress an be calculated y the following quations(Hight et al. 1983):

P(1) o---A3T(2) - 2r(b-a )

whereP is the normal orce,A is the joint surface rea,T is the torque,a is the innerradius, and b is the outer radius.2. BASELINE TESTING PROGRAMPrior to beginning fundamentalnvestigationf joint response, series f baselineestsare needed o compare esultsobtainedwith the UT-HCA to those rom the standarddirect shear test. Since the state of stress in the UT-HCA is different from that in thedirectshear evice, ifferencesn the strength nddeformationalesponseanbe1116


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OAD CELL

CELL AIR RELIEFUPPER SAMPLEHOLDER

RING

SPECIMENTIE ROD(8)ACRYLICTUBE

LOWERSAMP1HOLDER

PORE PRESSURE

CELLDRAIN

PORE BUSHINGPRESSURE(2) 3' DIAM.HAFT& NST,___] FLUIDONTAINMENTLOCKS l U-lIll Ill. . _MTSINEAR/ROTARCTUATOR

Figure2. Sectionhroughhe Unersi of Tennessee-HollowylinderApparatusUT-HCA)expected. The testsdescribed ere have been conductedo compare he responseobtained from the two devices.

Baselineesting asbeenperformed n smooth, ry, artificialsaw-cutoints. The rockused s a crystallineimestone ith algal aminations. wo differentgeometries avebeenused n the directsheardevice: he standard quare 00mm 100mm ampleFigure3a)anda rectangular5mmx 100mm ampleFigure3b). The attergeometry imulateshe25mmwall thickness f the UT-HCA (Figure3c) and evaluateshe effectsof samplewidth.All jointsweresawcut andmachinedn a surface rinderprior to testing.Specimensused n the directshear estswere hen"run-in" yhand o assureull contact etween hetwo specimenalves:The hollow ylinder pecimensere nstalledn the UT-HCA andcyclically hearedunder low normal stress less than 700 kPa). In both cases, heaccumulated ougewasremovedperiodically uring"run-in" o assure clean oint.

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P P P

T T

(a) (b) (c)Figure 3. Geometriesused n benchmark esting: direct shearsamples a) 100mmx100mm, b) 25mmx 100mm, nd UT-HCA sample c) I.D. = 100mm,O.D. = 150ram

After the testswithoutgougewere completed, dditional run-in"was performeduntilthe interfacewascoveredwith gouge. This results n an additionalsurfacecondition orcomparison f directshearand UT-HCA response.The condition f the joint interfaceis found o have he greatestnfluence n the overallshear trength.Substantialncreasein the shearstrengthwas oundwhengouge s accumulated. or thisreason, esultsbothwith and withoutgougeare shown.Testswith both deviceswere performedover a rangeof normal stress onsistentwiththat required or a typical nalysis f rockslopes.The UT-HCA testswereconducted itha sinusoidal isplacement f approximately .06 radians, esulting n a rate of 0.08rad/min.The directshear estswereconductedt a displacementate of 1.0cm/min, n a singledirection.Characteristichearstress s.displacementurves, bservedor both the directsheardeviceand the UT-HCA, are shown n Figure4. To permit the directcomparisonof results rom the two devices,he horizontal cale or the directshear esults orrespondsto that shownor the UT-HCA resultsi.e., 0.10 adians= 0.75cm).Thiscomparisonsbasedon the relativedisplacementt the outside ircumferencef the UT-HCA specimen.For the tests un withoutgougen the UT-HCA, a rapid ncreasen shearstress ccursovera smalldeformation ntila peakshearstresss reached,Figure4b). As deformationcontinues, he shear stressdecreases o a relatively constant esidual shear stress. Adecreasen the residual hearstrengths observed versubsequentycles, imilar o thatreported by Hudson and Dowding 1990). A similar peak and residualshear stressresponse re observedn the directsheardevice, Figure4a).For those testsrun with gouge n the UT-HCA, a peak shear stress s no longerobserved. The shear ncreases apidly with little deformationuntil slip begins,and asdeformation ontinues,he shearstress lowlyncreases,Figure4b). With reversalof thetorque,a similar,parallelcurve s observed.On continuation f the test,the hysteresiscurvereproducestself. A similarmonotonicncreasen shearstresss observed or the

direct shear ests, Figure 4a).

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I,O

-1.0-0 IoDIRECTHEAR/oIougeDIRECTHEAR/g,uge75 0.0 0.75DEFLECTIONcm)

1,0

b.J

o UT-HCAw/o gouge[] UT-HCAw/ gouge

-1 .O-O.1 O,0 0,1DEFLECTIONradians)Figure4. ShearStress s.DeflectionCurve or the (a) Direct ShearDevice normalstress= 801 kPa w/o gouge& 604 kPa w/gouge) and (b) Universityof Tennessee-HollowCylinderApparatusnormalstress 352 kPaw/o gouge& 1057kPaw/gouge)

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200

10OO

wn 0

2O0

100mm x 100mmw/o gougeo 100mmx 100mmw/gouge 100mm x 25mm w/o gouge_ 100mmx5ram/gouge

200 400 600 00 1000 200NORMALTRESSkPo)

1800

1600

1400

'' 200U"} 1 000

u o

400

200

i . ii '',"'' I'''l'''l''' l'''l oUT-HCAw/ogouge', UT-HCAw/gouge

200 400 600 800 1000 1200 400 1600 800NORMALTRESSkPa)

Figure . Mohr-Coulombnvelopeorthe a) DirectShearDevice nd b) UniversityfTennessee-HollowylinderApparatus

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From the results btained, seriesof Mohr-Coulomb nvelopesre developedor eachof the three geometries square, ectangular, nd circular) and for the two surfaceconditionswith and withoutgouge), Figure5). From the Mohr-Coulomb nvelopes,frictionanglesweredetermined y inear egressionndare summarizedn Table 1.The results rom the two sample izesused n the directsheardevice ndicate hat the

frictionangles re similar, uggestinghata changen samplewidthhas ittle effecton theoverallstrength. The resultsalso ndicate hat if the displacements assumed onstantacross he radius of the UT-HCA, wall thicknesshas little effect on the strength.Differencesn the observedtrength re thereforedue o the stress tate,variationof slipacross he radius,and the conditionof the joint surface. Theseresultssuggesthat thefrictionangles btained ith heUT-HCA are slightlyess han hoseobtained ith directshear.The results for both the direct shear and HCA indicate that an increase in the shearstrengths observed fter a substantialoatingof gouge s developed.This increasenstrengthwith accumulatedouges counter o previous bserved ata (Jaeger nd Cook,1976). Traditionally,he peakstrength ccursn a dean oint anddecayso the residual

valueasgouge evelops. ittle hasbeen eportedon he effects f substantialouge uild-up under a large numberof loadingcycles. Therefore, esting s required o furtherinvestigatehis effect.A comparison f the shearstress-displacementurves rom Figure4 indicates hat theUT-HCA resultsn a stiffer nitial response, othwith andwithoutgouge.This couldbea resultof the stress oncentrationsssociated ith the directsheardevice, eading o aprogressiveype of failurestartingat the edges nd progressingoward he center. Thisdifferencen initial stiffness analsobe attributed o the greatercompliance f the directshear device.

3 CONCLUSIONSWith the increased oncern ver he complex ehaviorof geo-materialsomes need ormore sophisticatedonstitutivemodels.Suchconstitutive odelscan onlybe developedwith a completeunderstandingf the fundamentalmechanics f the behavior. A welldefinedstress tateand he controlof stress athare essentialor thisunderstanding. heUT-HCA hasbeendevelopedust or suchnvestigations.owever,with the developmentof sucha research ool, t is essentialhat the resultsobtainedbe validatedwith respectto standardestmethods. he baselineest esults resentedn thispaperare an attemptto offer such a confirmation.

Table 1. Summary f frictionangles.FrictionAngle(de, ees)I Specimenithoutithize gouge gouge

Direct 100mm x 100mm 10.2 36.0ShearDevice 100ram 25ram 8.5 37.0UT-Ho low 150ram O.D.Cylinder x 8.3 33.5Apparatus 100mm.D.

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These initial results indicate that the shear strength obtained with the UT-HCAcomparesavorably ith that of the widelyuseddirectsheardevice.Thiscomparison asmade for two different oint conditions. Preliminary esults ndicate hat the initialstiffness btainedwith the UT-HCA is greater han that obtained n direct shear. Thedifferences probably resultof stress oncentrationsndcompliance ithin the directshear device.Although hese ests ndicate hat the UT-HCA canbe an important esearchool forthe investigationf rock oint behavior, urther estingwith different ockmaterialss stillrequired.

4 ACKNOWLEDGEMENTSThis researchhas been supportedby the National ScienceFoundationunder contractsMSM-8604873andMSS-8915675.We would ike to thankW. J. Long or hiscontributionsto the developmentf the UT-HCA, C. S. Allin for his assistance ith the directsheartests, and W. F. Kane and D. W. Sherwood for their review and comments on themanuscript.REFERENCESBazant,Z.P., J.D. Hess& S. Meiri (1981). "HighTemperature riaxial-Torsional achinefor Concrete nd Rock,"Geophysicalesearchetters, (7), 707-708.Bazant,Z.P., S. Prasannan, . Hagen,S. Meiri, R. Vaitys,R. Klima & J.D. Hess 1986)."LargeTriaxial-Torsional estingMachinewith HydrothermalControl,"Materiaux tConstructions,9(112),285-294.Bishop,A.W., G.E. Green,V.K. Garga,A. Andresen& J.D. Brown 1971). "A New RingShear Apparatusand its Application o the Measurementof Residual Strength,"Geotechnique,1(4), 273-328.Brown,E.T., ed. (1981). Rock Characterization,esting, nd Monitoring: nternationalSocietyor RockMechanics, uggestedethods, ermagon ress,Elmsford,NY, 129-137.Christensen, .J.,S.R. Swanson W.S. Brown 1974). "Torsional hearMeasurementsof the Frictionalpropertiesof WesterlyGranite,"Proc.Third Congressnt'l SocietyorRock Mech., Denver, Vol. IIA, 221-225.Cox, S.J.D.& C.H. Scholz 1988). "Rupturenitiation n ShearFractureof Rocks:AnExperimental tudy," ournalof Geophysicalesearch,3(B4), 3307-3320.Drumm,E.C. (1988). "A HollowCylinderShearDevice or UndrainedTesting f RockDiscontinuities,"inal Project Report to the National ScienceFoundation,Award #MSM-8604873.Franklin,J.A. (1985). "A Direct ShearMachine or TestingRock Joints,"GeotechnicalTesting ournal,GTJODJ, 8(1), 25-29.Handin,J., H.C. Heard & J.N. Magouirk 1967). "Effects f the IntermediatePrincipalStress n the Failureof Limestone, olomite,andGlass t Different Temperatures ndStrainRates," . Geophysicalesearch,2(2), 611-640.

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Hight,D.W., A. Gens& M.J. Symes 1983). "Thedevelopment f a New Hollow CylinderApparatus for Investigating he Effects of Principal Stress Rotation in Soils,"Geotechnique,3(4), 355-383.Hvorslev,M.J. (1939). "Torsion hearTestsandTheir Place n the Determination f theShear Resistanceof Soils,"Proc.,Amer. Soc. Test.Marls., Vol. 39, 999-1022.Hutson,R.W. & C.H. Dowding 1990). "Joint sperityDegradation uringCyclicShear,"lnt. J. RockMech. Min. Sci.& Geornech. bstr.,27(2), 109-119.Jaeger,J.C. & N.G.W. Cook (1979). Fundamentals f Rock Mechanics. ChapmanandHall, London.Kutter,H.K. (1974). "Rotary hearTesting f RockJoints," roc., rd CongressntL SocietyRock Mech., Denver, 254-262.Lade,P.V. (1981). "Torsion hearApparatusor SoilTesting," aboratory hearStrengthof Soil,ASTM STP 740, R. N. Young and F. C. Townsend,Eds., 145-169.Olsson,W.A. (1986). "Rock ointCompliance tudies,"andiaReportSAND86-0177-UC-70, 101.Olsson,W.A. (1988). "TheEffectsof NormalStressHistoryon RockFriction," roc.,29thU.S.Syrnp. ockMech.,Minneapolis, 11-117.Power,W.L., T.E. Tullis & J.D. Weeks 1988). "Roughnessnd Wear During BrittleFaulting," ournalof Geophysicalesearch,3(B12), 15,268-15,278.Saada,A.S. (1988). "Hollow Cylinder TorsionalDevices: Their Advantages ndLimitations," dvancedTriaxial Testing f Soil and Rock, ASTM STP 977, Robert T.

Donaghe,RonaldC. Chaney, ndMarshalL. Silver,Eds.,AmericanSocietyor TestingMaterials,Philadelphia, 66-795.Sun, Z., C. Gerrard & O. Stephansson1985). "Rock Joint ComplianceTests forCompressionnd ShearLoads,"nt. J. RockMech.Min. Sc Geornech.bstr., 2(4),197-213.Yoshioka,N. & C.H. Scholz 1989). "ElasticProperties f Contacting urfacesUnderNormal and Shear Loads 2. Comparison f TheoryWith Experiment,"ournalofGeophysicalesearch, 4 B 12), 17,691-17,700.

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arma-91-1115_comparison of direct shear and hollow cylinder tests on rock joints

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