_characterization of the parameters that govern the peak shear strength rock joints

8/12/2019 _Characterization of the Parameters That Govern the Peak Shear Strength Rock Joints

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Rock Mechanics n the National nterest,Elsworth,Tinucci& Heasley (eds), 2001 Swets& Zeitlinger isse, SBN 90 2651 827 7

Characterizationf theparametershatgovern hepeakshear trength frock ointsG.Grasselli & P.EggerSwissFederal nstituteof Technology,ausanne, witzerlandJ.WirthFHBB, Basel, SwitzerlandD.HopkinsLawrenceBerkeleyNationalLaboratory, erkeley,USA

ABSTRACT: In Switzerland, here s concem hat sliding along oints under damscould lead to stabilityproblems.As part of a research roject undedby the SwissFederalOffice for Water andGeology,more hanfifty constant-normal-loadirect-shearestshavebeenperformed n induced ensile racturesor seven ocktypes.Damage zonesare evident on all of the sheared urfaces. here is evidenceof both crushingandbreakingof surface sperities. amage s relativelysparse, nd the locationof the damaged ones s stronglyrelated o geometricaleatures.However, he relationshipsetween urface oughness,tress istribution, nddamageare complicated nd difficult to study, n part, because he boundary onditions oreming he me-chanical ehavior hange ontinuouslyuringshearing. ne of theprimaryobjectives f thiswork s to betterunderstandhe micromechanicalehavior f jointsundershear oads, ncluding hecreation f damage ones.This requiresunderstandinghe relationships etweenmaterialproperties, urfacegeometry,contactarea,stress istribution, nd he creation f damage uringshearing. methodologyor predicting amage uringshearing asbeendeveloped asedon analysis f mapsof the oint surfaces btained eforeandafter shear-ing usinga three-dimensionalpticalsystem. he surfacedata s analyzed o identify he areason the ointsurfacesmost ikelyto be n contact uringshearing;.e. areaswith positive lopewith respecto the shear i-rection. x)cal gradients re also aken nto accountn predicting hoseareasof the oint surfacesmost ikelyto be damaged uringshearing. he damage redicteds comparedo the damagemapped n laboratoryestspecimens.1 INTRODUCTIONShearing f rock oints n-situoccursundera varietyof boundaryconditions.However, it is possible oidentify two characteristicbehaviors.The first oc-curs under conditionswhere the joint can freely di-late, such as exist on rock slopes.This condition sduplicated n the laboratoryby maintaininga con-stantnormal oad (CNL) duringshear ests.The sec-ond characteristicehavioroccurswhen oints areconstrained, o that any dilatancy ncreaseshe nor-mal load actingon the oint; e.g., for joints n foun-dationpilesor a block n a rock mass.This conditionis simulatedn the laboratory y keeping he normalstiffness onstantCNS) duringshearing.For studyingoint behaviorunder he foundationsof dams, t is reasonableo postulate hat the highwaterpressure gainst he wall of the dam producesshearingalong fracturesunder the foundation.De-pendingon the orientations f the oint setsand theirdepth,each oint is relatively ree to dilate; normalloads are typically in the range between 0.2 and5.0 MPa. To study these conditions, t has beenjudged that the most appropriateaboratoryexperi-

mental set-up s the CNL shear test. Therefore, tostudy the frictional response f rock joints underconditions that simulate those in dam foundations, aseriesof direct-shear ests were performedon in-duced tensile fractures n seven rock types. Testswere alsoperformedon replicasof tensile ractures.In addition o shear oads nducedby water pressureactingon dams, he change n water evel that occursduring the year can causecyclic loading of jointsunder he dam.To study he effectof cyclic oading,testswere also conductedwith up to five shearcy-cles on the same sample; .e. after 5 mm of sheardisplacement,he sampleswere repositioned t theorigin andsheared gain.The obvious advantage of using natural rocksampless that it allows us to test rock typescom-monly found underground nd in foundations.Wecan thus investigate he influenceof different pa-rmeters on shear strengthand failure mechanismson sampleswith the materialproperties ncounteredin practice.The advantageof using replicas s thatthey make t possibleo independentlytudy he twoparameters hat most strongly nfluence shear be-havior:normal oad and he morphology f the oint

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surfaces. singreplicasof the same oint, the influ-ence of normal load on peak shearstrengthwas in-vestigated.Using replicas of different joints, testswere conductedunder constantnormal load to studythe nfluenceof morphology n shearbehavior.2 SURFACE MEASUREMENTS

After evaluationof severalmeasurementechniques,a systembasedon an advanced opometricsensor(ATS) was udged best or measuring urface opog-raphy. ATS (Fig. 1) is an optical measurement ys-tem developedspecially or the automotive ndustrywhere three-dimensionalmeasurementof objectsplays an integral role in reverse engineeringandquality control. In recent years, fringe-projectionsystemshave gained increasedusage n this field.These systemsuse a combinationof white lightfringeprojection,riangulation, nd phaseshifting oobtain a fast and robust calculation of dense three-dimensional ointclouds.ATS offers he advantagesof high precision,and goodrepeatability,while alsobeing ast andeasy o use.Moreover, he lightweightand small dimensions f the transport arryingcase(seeFigure 1) makes t possible o use the system nthe field where we have achieved he sameaccuracyas is obtainedunder aboratory onditions Grasselli& Egger 2000).

Figure 1. Measuringhead of the advanced opometricsensor(http://www.gom.com).3 SHEAR TESTSA seriesof experimentswere performedon naturalrock sampleswith freshly nducedoints that wereassumedo be perfectlymatedat the onsetof shear-ing. Jointswere nduced singa three-point-bendingconfiguration pplied o the rectangularock speci-mens hat were 30-cm tall with a 15-cm squarebase.To set the initial positionof the samples, he twohalveswere manuallyplacedsuch hat the oint sur-faces were matched.As the joints are fresh tensilefractures, the surfacesare well mated, making itrelativelyeasy o position he samples recisely. hejoints were sheared ver approximately mm. Forschistousock suchas gneissand serpentinite,estswere performedon joints both parallel and perpen-dicular o the planesof schistosity.Direct shear estswere performedusinga servo-

hydraulic est frame. Specimens f jointed rock arefixed into the upperand lower shearboxes and si-multaneously subjected to normal and shearingstresses. hear and normal loads are applied by hy-draulic jacks equippedwith servo-valves.The nor-mal load is transferredo the samplevia a sphericaljunction, and the horizontalmotion is guided by aprecision inear bearing,which is designed or lowfriction and a single degreeof freedom (translationonly). This insures hat the upper-sample oldercanmove during shearingwith a minimumof frictionand bendingmoment.For eachconstant ormal oadshear test (CNL) the following values were re-corded:- normal oad on the oint plane;- shear oad parallel o the oint plane;- horizontaldisplacementn the sheardirection;- verticaldisplacementdilatancy)of the uppersideof the samplewith respect o the lower (measured

at four points).High stressesalong the edges of the samplesmake them proneto failure (Hopkins 2000). To re-duce the stress concentrations at the boundaries, therock specimens re squeezednto two steel frames,one on each side of the sample.The test specimensthat are replicasof rock joints are insteadproducedwith a lateral idge around he outside dge.The experimentallyderived curvesof stressver-sus displacement Fig. 2) generally display twotypes of characteristic ehavior.The first is charac-terized by a steep rise in stress to peak shearstrength, ollowed by a rapid drop n stress nd thena gradual taperingoff toward a value that corre-sponds o the oint's residualstrength see "first cy-cle" curve n Figure 2). The second s characterizedby behavior hat is more or lessplastic,exhibitingasmall or no distinct peak in stress.The degree ofplasticity s greatly nfluencedby the joint's surface

0 i i i i i i i i0 0.5 I 1.5 2 2.5 3 3.5 4

Horizontaldisplacementmm]Figure 2. The ratio of shear load to normal load is plottedagainst horizontal displacement or six shear cycles on thesame granite test specimen.The peak-shearstressdecreaseswith eachcycle.

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roughness nd the appliednormal oad,as well as bythe materialproperties f the rock.These changes n behavior that occur duringshearingare associatedwith the selectivedestructionof asperitieson the joint faces (Yang & Chiang2000). Understandinghe progressiveailureprocessbeyondpeak strength s essential or understandingthe behaviorof joints in situ.4 PARAMETERS THAT GOVERN THE PEAK

SHEAR STRENGTH

The shearbehaviorof rock oints dependson manydifferent factors. However, it has been possible oidentify the parametershat have the strongestnflu-enceon the peak shearstrengthof rock oints testedunder aboratory onditions:- shear direction;- joint surface eatures morphology);- material propertiesof the rock (compressive ndtensilestrength and their ratio), basic riction an-gle, and Young's modulus);- materialanisotropy;- applied oads;- initial aperturedistribution matching atio).Parameters hat describe roughnesshave to beconsideredhe most mportantgeometrical oundaryconditions controlling the shear process becausethey determine he size and spatial distributionofcontactareasduring shearing.Our experimental e-sults are consistent with those of Gentler et al.(2000) who assert"the size, shape,and spatial dis-tributionof damaged reasdependon the sheardi-rection and the degreeof stressand horizontaldis-placement.Obviously he damage ncreases s stressand displacementncrease."Our experimental estsshow that no damage appears prior to the peakstress; amageoccursprincipallyduring the soften-ing and residualphasesof shearing,and the loca-tions of contactingasperitiesat the beginning ofshearingseem to correlateclosely to the damagedareas that appear later. The common characteristicamong all thesecontactareas s that they are withoutexception ocated n the steepest ones hat face thesheardirection.The shapeof the damage onesde-pendson the local geometryof the fracturesurface,including he size andshapeof the asperities, s wellas the materialproperties f the rock.5 PROPOSED METHOD TO PREDICT

CONTACT AREA AND DAMAGE DURINGSHEARING

Only a subsetof the area in contactunder the ap-plied normal load plays a role in the shearprocess.To estimate the size and location of those contact ar-eas that are actively nvolved n the shearprocess,t

I

v

tan0*:-tanO-cosFigure 3. Planesand anglesused o measure he azimuth a')anddip angle 0) that define he surface rientation f trianglesthat make up the reconstructedoint surface.The apparentdipanglewithrespecto theshear irection,7, is defined s heangle between the horizontal plane and the intersectionbe-tween he triangle rock surface)and he sheardirectionplane.is necessaryo first specify he sheardirection.Onlythoseareas hat face the shearvector t) can provideshear resistance.

As describedn Section2, the oint surfacesweremeasuredbefore and after shearingusing the ATSscanner. he joint surfaces re reconstructedromthe three-dimensionalointclouds hat are producedby the ATS systemusinga speciallydevelopedri-angulation lgorithm. his approachesultsn a dis-cretizationof the oint surface nto contiguousrian-gles, whose geometric orientations are easy tocalculate based on the orientation of the vector nor-mal to the planeof the trianglen (seeFigure3.) Theaccuracyof the reconstructionepends n the den-sity of measurements;he more dense he measure-ments he higher he accuracy f the reconstruction.This methodof discretization f the oint surfacessparticularly dvantageousor estimatinghe areasofthe surfacesn contactduringshearing.The shear plane is determined experimentallyfrom LVDT measurementst the peripheryof thesample.The shearvector points n the directionofI PosSrvelope'>23I PosSvelope0=


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identification nddistribution f triangles otentiallydamaged during shearingare shown in a three-dimensionalmap Fig. 4).The predicted amage onesare compared o ac-tual damaged reasas determinedrom imagesofthe oint surfaces ostshearing.Degradation f thejoint surfacess apparent s crushedgaugematerialthat appearswhite in the images.Measurements fthe joint surface aken before and after shear estsare analyzed o createa map of areasdamaged ur-ing shearing. he methodologys illustratedn thefollowing section.6 APPLICATION OF DAMAGE PREDICTION

METHODOLOGYThe methodologyescribedn the previous ectionfor predicting damage on joint surfaces duringshearingwas applied o a sample C2) of Magnyyellow limestone (Encrinitic limestone, Jurassic,Bajocian tratum). he mechanical roperties f thelimestoneare listed in Table 1. As describedabove,a tensile racturewas nduced n the sample.The testspecimen as subsequentlyheared ndera constantnormal oadof I MPa over a tangential isplacementof 5 mm. After shearing,he lower half of the sam-ple was maged sing he opticalsystem escribednSection (Fig. 5). Damage s evidenton the surfaceas crushed owderymaterialand small rock frag-ments.Thesedamaged reasappear s white pixelson the mage n Figure5.Since the joint surfacewas measured efore andafter shearing, t was possible o determineandquantify he differencesn the two images.The re-sultingmapsshowing reasdamaged uringshear-ing are shown n Figures -7.To test the damagepredictionmethodology ut-lined in the previoussection, he initial surface,measuredwith the ATS opticalsystemdescribednSection2, was reconstructed.ollowing he proce-dure detailed n Section5, the triangularelementswith positive slopewith respect o the sheardirec-tion were determined, nd the dip angle associatedwith eachof those lements ascalculated. amagewas predicted o occur on those elementswith dipanglegreater han 20 degrees. he map of predicteddamage ones s shown n Figure 8. Comparison fthe map n Figure 8 to the imageof the oint surfaceafter shearing Fig. 5) shows hat the map of pre-dicted damage locationsagrees well with the ob-serveddamage.Table 1. Mechanicalpropertiesof the limestone estspecimen 2.

pm a a,/a, E0 OPa MPa GPa deg2.19 21 2.5 8.4 14.85 36

7 CONCLUSIONSThe result of an extensiveseriesof laboratoryex-periments, erformedon severaldifferent oints androck types, shows hat the mechanical ehavioroffractures ndershearstresss strongly elated o thegeometry of the fracture surfaces.Understandingandbeingable to predict he creationof damageonjoint surfaces uringshearings an important tep nunderstandingow joint and rock properties ffectpeak shearstrength.The proposedmethod or pre-dicting the locationof damage ones s shown oproduce esults hat agreewell with observed am-ageon laboratory amples. he laboratorymeasure-ments are being used to validate and improve thepredictionmodel. n turn, thesedata are beingusedto help understandhe relationshipsetweenmate-rial properties, oint geometry, and peak shearstrength.8 ACKNOWLEDGMENTSThe work describedherein was performedat theRock Mechanics aboratory f EPFL with fundingprovidedby the SwissFederalOffice for Water andGeology.

Gentier,S. S., Riss,J., Archambault, ., Flamand,R. & Hop-kins,D. L. 2000. Influence f fracture eometry n shearedbehavior nternational Journal of Rock MechanicsandMining Sciences, 7: 161-174.Grasselli,G. & Egger,P. 2000. 3D surfacecharacterizationorthe prediction f the shearstrength f rough oint Eurock2000,: 281-286.Hopkins,D. L. 2000. The implicationof joint deformationnanalyzing he properties nd behaviorof fractured ockmasses, nderground xcavations, nd faults nternationalJournal of RockMechanics nd Mining Sciences, 7: 175-202.

Yang, Z. Y. & Chiang,D. Y. 2000. An experimental tudyonthe progressive hearbehaviorof rock joints with tooth-shaped speritiesnternational ournalof RockMechanicsand MiningSciences, 7: 1247-1259.

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_characterization of the parameters that govern the peak shear strength rock joints

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