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.

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CategoryUC-814I

SAND92-2333' UnlimitedRelease

PrintedApril1_94

The Effect of Sliding Velocity on the

Mechanical Response of an Artificial Joint in

Topopah Spring Member Tuff

W'dliamA. OlssonGeomechanicsDepartment

Sandia National Laboratories

ABSTRACT

A smooth artificialjoint in TopopahSpringMember tuff was sheared at constantnormalstressat velocities from 0 to 100 #m/s to determinethe velocity-dependence of shear strength. Twodifferent initial conditions were used: (l) unprimed--the joint had been shear stress-free sincelast applicationof normal stress, andbeforerenewedshear loading; and (2) primeA--thejoint hadundergonea slip historyafter applicationof normalstress, but before the currentshear loading.Observedsteady-staterateeffectswerefoundtobe about3 timeslargerthan forsome other silicaterocks. Thesedifferentinitialconditions affectedthe characterof the stress-slipcurve immediatelyafterthe onset of slip. Priming the joint causes a peak in the stress-slip response followed bya transient decay to the steady-state stress, i.e., slip weakening. Slide-hold-slide tests exhibit

• time-dependent strengthening. When the joint was subjectedto constant shear stress, no slip wasobserved; that is, joint creep did not occur. One set of rate data was collected from a surface

' submerged in tap water; the friction was higher for this surface, but the rate sensitivity was thesame as that for surfaces tested in the air-dry condition.

This report was prepared under the Yucca Mountain Project WBS number 1.2.3.2.7.1.4. The datain this report was developed subject to QA controls in QAGR S1232714; the data is not qualifiedand is not to be used for licensing.

iv

Contents

1 Introduction 1t

1.1 Review of the ConstitutiveEquations ....................... 2

2 Experimental Technique and Sample Preparation $

3 Results and Discussion 6

3.1 Theeffect of VelocityChanges atConstantNormalStress ............. 6

3.1.1 Unprimedsurfaces ............ _ ................ 7

3.1.2 Primedsurfaces .............................. 9

4 Creep 11

4.1 UnprimedSurface ................................. 11

4.2 PrimedSurface ................................... 12

$ Conclusions 13

6 References 15

List of Figures

1 Definition sketchfor constitutiveparametersa, b, and L. The imposedload-pointvelocity is such thatV, < V ............................. 3

2 Springandblock analogueto slidingof laboratory-sizedjoint. Vois the load-pointvelocity imposedby the loadframe,r is the resistingsheartraction,and V is thevelocity of the block................................. 4

3 Sampleconfigurationfor rotaryshearexperimentsonjoints............ 6

4 A stress-slipcurve showing the differentresponse for the same interface in theprimed and unprimedstates. Loops labeUedA throughD have been isolated toillustratetheconcept of priming........... ................ 8

5 The stress-slipcurves at differentsliding velocities for unprimedsurfaces. Allexperimentsstartedat the same initialplacementandat5 MPanormalstress.... 9

6 Theeffect of sliding velocities (indicatedin #m/sec) on aprimedsurface. Note thedifferencesbetween the way slip develops in these experimentswith those of anunprimedsurfaceshownin Figure1......................... 10

7 Ratesensitivities of the steady-statefrictioncoefficientforvariousinitialconditions. 1O

8 Variationof frictionin a slide-hold-slidetest. Note the stress-relaxationat eachdisplacementhold,andthe transientstrengthening................. 12

9 The effect of displacementholds of variouslengthson the strength......... 13

10 A creeptestnearthe slip stresson a primedsurface................. 14

vi

1 Introduction

The Topopah Spring Member of the Paintbrush Tuff within Yucca Mountain, Nevada, has been, selected as a potential repository of high-level nuclear wastes. The Yucca Mountain Site Charac-

terization Project (YMP) of the Office of Civilian Radioactive Waste Managememnt (OCRWM)

Program has been assigned the task of determining the suitability of the Yucca Mountain site.Among the concerns being investigated, the characterization of the mechanical properties of thehost rock has direct relevance to repository design, and to pre- and post-closure performanceassessment.

Licensing of a nuclear-waste repository by the Nuclear Regulatory Commission (NRC) requires,among other things, demonstration of the long-term usability of the underground portion of therepository. Such a demonstration involves analysis of the mechanical response of the rock mass tothe presence of underground openings and heat-producing waste over long periods of time. Thisreport presents data on the time-dependent properties of a fracture as measured in rate-steppingandcreep experiments in support of design and preformance stability issues. The test material wastaken from a block of welded, devitrified Topopah Spring tuff (Price et al., 1987).

Mechanical discontinuities such as faults, bedding planes, or joints are important mechanicalconstituents of most rock masses; they increase the compliance and decrease the strength of therock mass. Stress changes and slip-induced dilation can affect the contributions of mechanicaldiscontinuities to fluid permeability of the rock mass (Olsson, 1992; Olsson and Brown, 1993).Furthermore, the constitutive response of joints in silicate rocks is of a type that can lead todynamic instabilities under a range of conditions. These instabilities are referred to as "stick-slip" and manifest themselves in experiments as jerky sliding accompanied by stress drops. Thisbehavior on a larger scale is thought to be analogous to earthquakes (Brace and Byerlee, 1966). Ithas been shown (Wong, 1992) that most mine seismicity, e.g., rock bursts and bumps, is due to slipon geologic discontinuities. Further,it has been established that slip on joints in Grouse Canyonwelded tuff is accompanied by acoustic emissions (Holcomb andTeufel, 1982) and therefore it isimportant to explain this aspect of time-dependent joint behavior of tuff.

For simplicity, the term "joint" is used throughout this report, but the results apply to anyinterface in rock with vanishingly small tensile strength.

The shear resistance of a joint depends on the normal stress history across it, the sliding velocity• history, the temperature, the roughness, and thepresence of water and gouge. It is generally accepted

that normal stress is the dominant factor controlling the shear strength of a given joint and that all, other effects have second order influence. This report focuses on time- or rate-dependence. It is

important to understand that these effects, though lesser in magnitude of associated stress changes,can have important consequences. For example, it has been established that the details of therate-dependence part of the constitutive description can either enhance or suppress instabilities inslip (Gu, et al., 1984; Rice and Ruina, 1983; Ruina, 1983). As such, these rate effects have a direct

SLIP

Figure 1: Definitionsketchforconstitutiveparametersa, b,andL. Theimposedload-pointvelocityis such thatV. < V.

bearingonmineseismicity.

Constitutiveequationdevelopmentforthetime-dependentslipofinterfacesatinvariablenormalstressislimitedatthistimetoencompassingtheeffectsofvelocity,thehistoryofvelocityandthestateofthesurface.Thetheoryispurelyphenomenologicalinthatnounderlyingphysicalprocesshas been identified thatpredicts theresponse.

1.1 Review of the Constitutive Equations

The resistance to slip r is coupled to the normalstress¢ to dominantorderthroughthe coefficientof friction # accordingto the slip condition

_"-/=_< O. (I)

When the net shearstress r -ft¢ is less thanzero thereis no slip. Whenabsoluteequalityholds, therateeffects enterthe problem.Whethercreepwill occurfor r - per< 0 is notyet well-documented.Thisreportpresentsresultsof severalshort-termcreep tests, andthisissue isnow being investigatedin long-term (about 3 months)creep tests. Whenthe net shearstress on ajoint is less than zero,the rock mass responsein shearis approximatelythe sameas that of the intactrock.

There are twoclasses of frictionalconstitutivelaws: slip-dependence(oftenreferredto as slip-weakening)and rate- and state-dependence. Slip-dependencelaws embody monotonic surface

2

evolution that may be relatedto damage accumulation. These laws have the defect that there isno explicit rate-dependenceand observedrepeatedinstabilitiesare not predicted. The rate- andstate-dependencelawcan predictrepeatedbehavior,butmay notbe appropriateforthe initial stagesof slip when strongsurfaceevolutionto a steady-statetakesplace. Fracturebehaviorspansboth

• types of description. In general terms, frictionalslip has elements of bothdiscrete memory(sfip-dependence)and fadingmemory(rate-andstate-dependence)of paststress and velocity (Olsson,1987b).

The rate effects described hereinate best discussed within the context of the rate- and state-dependencemodel. Experimentalworkof Dieterich(1972, 1978, 1979)followed byRuina(1980,1983)andDieterich(1981) led to the developmentof the currentlyacceptedformulationforeffectsof sliding velocity history and time onjoint hehavior(Ruina, 1983;Rice andRuina, 1983). Riceand his coworkers(Gu et al., 1984; Rice and Gu, 1983) implementedthe constitutive equationsinto predictivestability analyses. Tullis and Weeks(1986) gave an accountof results fromrotaryshearexperimentson granitewithintheframeworkof this theory.

The constitutive model is based on observed changes in friction during and immediatelyfollowing abruptchanges in velocity. The state of the surface is included throughtime-evolvingstate variables.The basic equationsdescribingthis rate-and state-dependentbehaviorat constantnormalstressand whenthe slip conditionis satisfied(1) are:

= + b,'e,+ lnCV/V.), (2),ldt = -(V/L,)[W,+ (3)

In these equations, V is the sliding velocity, _i are state variablesthat evolve with time, p. isthe friction at an arbitraryreference velocity V,. The parametersa and bi are constants to bedetermined, and the Li are the distances overwhich the _Fdevolve. A simpleexperiment,such asshown in Figure 1, providesall the parameters.

Differentiationof the constitutiveequations,simplified to one state variable (i = I), leads tothe following definitionsof the variablesa, b, andL (Fig. 1):

= a (4)Oln V ,v

dp°°= a-b (5)

dln V

' --0 1 = (6)Oz Iv L

Thus, a is the instantaneousresponsefollowing a jump in the velocity, and a - b measuresthedifference between the currentandformer steady-statevaluesof friction.The steady-statefTiction,

k

kv,

4,------.

Figure 2: Springand block analogue to sliding of laboratory-sizedjoint. Vo is the load-pointvelocity imposed by the loadframe,r is the resistingsheartraction,and V is the velocity of theblock.

W =0, is= + - b) a(v/v.). (7)

If/_°° < #., then the response is said to be velocity weakening and a - b < 0. If #" >/_,, thenthe response is said to be velocity strengthening and a - b > 0. The type of response observed forsilicate rocks depends on the normal stress--low normal stress promotes stick-slip behavior, andhigh normal stress promotes velocity strengthening. Blanpiedet al. (1987) have found that for l_greater than about 100/_m/s, a - bbecomes positive, implying stability. Elevated temperaturesalso can promote stable response (Blanpied etal., 1991). It is not clear whether the state changes,d_F# 0, when the surfacesare in stationarycontactto begin with, that is, when the previous steadystate is actuallythe zerovelocity history(V = 0, for allprevioustime). Theevolutionof _Pis clearwhen the firstand second steady-statesare at finite velocities, butif V. = Ofor a long time, doesthe state change?

Joints are interfaces embedded in a deformable medium and as such do not necessarily slideover their whole extent simultaneously. Therefore, inhomogeneous slip may be important in thefield and in the laboratory (Olsson, 1984). Rice (1983) has shown that the inhomogeneous slipregime associated with the advancing slip zone boundary is of the order of 1 metre. For therotary shear configuration in particular, it has been shown that slippage is essentially uniform(Olsson, 1987b). Evidently, for normal laboratory sampledimensions, it is permissible to interpretlaboratory friction data in terms of a rigid block being slid over a smooth surface by a force pullingon a spring connected to the block (Fig. 2).

The force, displacement, and velocity _ at the distal end of the spring arecalled the load-pointvalues. In the load frameusedin this investigation,the torqueand angulardisplacementareappliedat one end of the loadingcolumnwhile the jointis at the other.Thus,constantdisplacementrates

are applied through the elastic loading column, and because of the finite stiffness of the columnthere may be a difference in the velocity of sliding and the applied velocity, Vo- V. This compliantload application is similar to a joint being loaded by the surrounding compliant rock and, in thissense, the laboratory experiment simulates nature. The difference in load-point velocity compared

, to joint-wall relative velocity emphasizes the importance of separating constitutive response of thejoint from the loading-system response.

The spring-block system is described by

dr

d_ = k(Vo- V) (8)

where k is the spring constant, Vo is the velocity at the load point, and V is the velocity of theblock. If the system unloading stiffness k is such that

b-a

k < ---_, (9)

then the system will be prone to instablility in the form of jerky slipping and sudden stressdrops (Dieterich, 1978). This shows how the value of the constitutive parameters b and a whencombined with the properties of the loading system are important to stability. Natural joints willhave different properties depending on geometric factors, mainly the roughness and degree ofinterlocking of adjacent joint waUs. A different problem not considered here is the effect of gougeon the behavior of joints. Because the response of a joint depends on some convolution of thebasic smooth-surface friction with its topographic properties, it is most illuminating to measure themagnitude of the effects of the variables on the smooth rock surfaces first, and then introduce thevery complicated problem of surface roughness. The focus here is on the effects of sliding velocity,the history of the velocity, and the initial state of the sliding surface on strength of smooth surfaces.

2 Experimental Technique and Sample Preparation

The results reported here were obtained in rotary shear experiments (Christensen et al., 1974;Kutter, 1974; Olsson, 1987ab, 1988ab, 1990, 1992; Xu and Frietas, 1988; Tullis and Weeks, 1986;

Weeks and Tullis, 1985; Yoshioka and Scholz, 1989). In this type of experiment, the sample iscomposed of two, short, hollow tubes of rock that are pressed together under controlled load, andthen torque is applied to cause sliding on the interface (Fig. 3). Further details may be obtainedfrom (Olsson, 1987a).

The sample used in this study was sawed, then ground fiat and perpendicular to the cylinderaxis. Outside and inside diameters were 88.9 mm and 50.8 mm, respectively. The short (approx.50 ram) cylinders were cemented to metal disks with epoxy, and then reground to ensure that thejoint was parallel to the surfaces of the metal disks. The joint surfaces were then sandblasted to

Figure3: Sampleconfigurationforrotaryshearexperimentson joints.

a dull finish. Before the experiments,the joint was slid backand forthunder low normalstressof around1 MPa to "run-in" the surface. This procedurestabilizes the stress-slipresponse loopsand makes them reproducibleundernominally identical test conditions. The experimentswereperformedon clean surfaces that hadbeenrun-in. In thisreport,the terms "mean slip" and "slip"areused interchangeablyfor angle of rotationin radianstimes (Do + Di)/4, where Do and Di arethe outerandinner diameters,respectively.Theload-pointslipwas correctedto slip by subtractingr/(unloading slope) from the measured displacements. This differencein the stress-slip reponsewhen using these two measurescan be seen by comparingthe initial loading portionof the curvesin Figures4 and 5.

3 Results and Discussion

3.1 Theeffect of VelocityChangesat ConstantNormalStress

It had been foundin earlierstudiesontuff (Olsson, 1987a,1988b)that thereexist two recognizableinitial conditionsforsmooth, topographicallyuncorrelatedsurfacesunderthe same test conditions:unprimed andprimed. To create the unprimedinitial condition,the initially separatedjoint wallsarebroughttogether andnormal stressis applied;then the sheartest is performed. In the primedcondition, the joint has undergonesome slip at non-zero normalstress, but now the contact isstationary;the sheartest is performedon thisjoint. Theresponsefor thesetwo states to increasingshearstress (Fig. 4) is significantlydifferent.For the unprimedjoint, the shearstressrises steadilyto the initial slip stress (analogous to the initial yield stress in solids), then rolls overto the steady-sliding value, which remainsessentiallyconstantthenceforth. On the other hand, for the primedsurface,the shearstress rises steadilyto the initialslip stress, then breaksoverdiscontinuouslytothe steadysliding value. Often, a peak in stress is observed,followed by a rapiddecline to thesteady sliding value. The differencein responsehas been postulatedto be due to inhomogenous,

0.8 -- - . .... 0.8- = --'-"PRIMED

!o60.4 0,4

:: _<0.2_<0.2

' '500 2 4 6 00 0.5 1 1. ---2

LOAD POINTSLIP (ram) LOAD POINT SLIP (ram)

0.8 -- - . 0,8 -UNPRIMED PRIMEDvsUNPRIMED

o.e o.6 i

i °0.4 D _ 0.4 D

:::)

_<0.2_<0.2

' '50 - 0.5 1 1.5 2 O_ 015 ; 1. 2LOADPOINT SLIP (ram) LOAD POINT SLIP (mm)

Figure 4: A stress-slip curve showing the different response for the same interface in the primedand unprimed states. Loops labelled A through D have been isolated to illustrate the concept ofpriming.

locked-in stress on the interface (Olsson, 1987b). In the Introduction, it was pointed out that peaksin the stress-slip curve due to velocity weakening can lead to instabilities in slip. The type of peakshown in Figure 4 is slip weakening and thus can also lead to instabilities.

The importance of primed versus unprimed joints lies in their different responses. By way ofillustration, compare the sliding behavior of an in situ joint that has undergone some amount ofshear during its history to that of the same joint cut out from its surroundings for either in situ orlaboratory testing. The shear stiffness (defined as the slope of the stress-slip curve after the onsetof slip) and strength of the tested joint will bc less than its undisturbed counterpart, and the onsetof slip will bc more gradual and less prone to sudden slippage. Further, stability related responsethat may characterize the in situ, primed, joint will not be evident in data from the test specimcm.

In geophysical applications, the concern centers around continual episodes of sliding separatedby periods of stationary contact. Because the normal stress, in the experimental program, wasapproximately constant, the constitutive equations (2)-(10) sccm to capture most of the importantphenomena. Although zero velocity is a particular history of velocity, it is not clear whether theseequations apply to loading of unprimed surfaces. It is important to explore this point because joints,

0,8

0,7

0,8

0.5._ 0.4

0.3

0.1

O_ 0.06 0.1 0,16 O.g 0.25 0.3 0,35 0.4MEANSLIP(ram)

Figure 5: The stress-slipcurvesat differentsliding velocities for unprimedsurfaces. All experi-ments started atthe same initial placementandat 5 MPanormalstress.

unlike faults, have not undergonelargedisplacements.

3.1.1 Unprimed surfaces

To measuretheeffects of slidingvelocity on the responseof unprimedsurfaces,thejoint _¢asslidat a constantvelocity to a preselectedtotal slip, then the shearstress and the normal stresswerereduced to zero, and then the joint returnedto zero slip. Next the normal stress was reapplied,and the joint reloadedin shearand subsequentlyunloadedas before. Thiswas repeatedat eachdesired velocity, always startingfrom the same relative position. Between each experiment, thesamplewas cleaned of gouge with a blastof compressedair. In thisway theeffect of sliding atfourdifferentvelocities for an unprimedsurfacewas measured. The velocities werevariedbydecadesfrom 0.1 _m/s to 100 #m/s. The resultsareshown in Figure 5.

The curvesfrom experimentsatlowervelocities have a "fuzzy" aspectcaused by digitalnoise,and this is insignificant. Irrespectiveof velocity,each curvehas the same shape. They all becomenonlinear in the neighborhoodof p = 0.5 andthen rolloverto the steadyfrictionwithin 0.05mmofslip. The steady frictionis functionof the velocitywthe higher the velocity,the lower the friction.These dataare summarizedbelow as "constantrate,unprimed"in Figure 7.

Becausethejoint remainsin stationarycontactwhile beingloaded to slipatdifferentvelocities,the initial (vertical)parts of the curves also representdisplacementholdtests with differing hold

v v v , i

• tlJre0.5

OA

i°. 1,_,0.2 '

0.1 i} 0.1 02 0.3 0.4 0.5 0.6 017 0 8 0 9

MEANSLIP(ram)

Figure 6: The effect of sliding velocities (indicated in #m/see) on a primed surface. Note thedifferences between the way slip develops in these experiments with those of an unprimed surface

shown in Figure 1.

times on unprimed joints. It has been observed that friction increases during nominally stationarycontact of granite (Dieterich, 1972) and welded tuff (Teufel, 1981). Those tests were done inthe primed condition and, therfore, it is interesting to examine the possibility of time-dependentstrengthening for the unprimed surfaces as tested here. The hold times corresponding to load-pointvelocities of 0.1, 1, 10, and 100 #m/s were, respectively, 7.75, 31.3, 172, 1203 seconds. Thesehold times spanned the same range as those applied to a primed surface, as discussed in sections3.2 and 4.2. The lack of any apparent effect of different hold times, especially the appearance of

a peak, is to be contrasted with the results presented below for slide-hold-slide experiments on aprimed joint.

3.1.2 Primed surfaces

Figure 6 illustrates the effects of different sliding velocities on a primed joint. At the beginning(when the surface is unprimed) the stress increases and rolls over smoothly. At 0.1 mm of slip,the shcar strcss was reduced to zero and again increased at the same rate (1 pro/s). This time, the

• stress rises to a peak and then rapidly descends to the steady value. Each successive cycle on the

now primed surface shows a similar peak and distinct, lower, sliding stress that depends on theapplied velocity. Each vertical segment represents unloading at the load-point velocity of 1 #m/s,and loading at the new load-point velocity. Note that each loading phase is a hold-time test for

I I

0,7_ . v . vTvwvT v 1"1 . I,!n , , 1 , vlv,w w . , lw,,wl 1 ! , , wwH

0

0.7 _ to.ram rate,uq_m_o_

t "0.65

0.55

Oq 4 I J I I _ I ill , 1 I I JJlJl I I J J J Illl I J J I ItllJ I I I 11 iJ

10-2 10-1 100 l0 t I02 103

SLIPI_,',I'E(micromm_rt/_md)

Figure 7: Rate sensitivities of the steady-statefrictioncoefficientfor variousinitial conditions.

each of the differentvelocities for a primed surface. The responsenow is typical for loading ofprimed surfacer--there exists a peak in the stress-slipcurve,which decays rapidly with slip to asteady-statevalue. The steady portionis ratedependent andthese dataare includedin Figure7 as"constantrate,primed".

Datafrompreviousrate-stepping(assketched in Figure1)experiments(Olsson, 1987a, 1988a)on dry surfaces,and new data from a surfacesubmergedin room-temperaturetap water are alsoplottedin Figure7. The rate-steppingexperimentsareby their very naturecarriedout on primedsurfaces. The greaterfriction for the wet surfacecomparedto the dry surfacewas also found inan earlierstudy on a differenttuff (Teufel, 1982). It appears that the steady-state friction of anunprimed surface is greater than that for a primedsurface. An importantaspect of Figure7 isthe parallelism of the variouslines. These data give a - b m,0.014 for smooth, bare surfacesofTopopahSpringMembertuff; this is about3 times that valueobservedfor granites.

Another aspect of time-dependentjoint slip is shown in Figure 8. Here the remote velocityVo is held at zero for differentlengths of time after some sliding (Fig. 9). Between each test16 = 5/_m/s. The strengtheningduringdisplacementholds was suggested (Dieterich, 1972) to berelatedto growth of asperityareaduringstationarycontact. Ruina(1983), however,assertsthat thestrengthening is a result of small amountsof slip thattakeplace while the load point is stationary,ratherthan thetime duringwhich the joint walls arein stationarycontact. Withregard to equation(3), for 16 = 0, no evolution of state (strengthening)is predicted to occur. Dieterich (1981) useda different formulationfor q' that does predict static strengthening. However, the data presentedhere suggest that in the absenceof nonzeroload-point shear stress (unprimed),no stress peaks

10

• O.M

• m0'_

.. 0J

lo.]

LOADPO_TSUPO_m)

Figure 8: Variation of friction in a slide-hold-slide test. Note the stress-relaxation at each displace-ment hold, and the transient strengthening.

reflecting time-dependent strengthening develop. If Ruina's assertion is correct, then there must bcsome very small amount of slip occurring in the primed, stationary phases. Evidently, the amountof slip must bc smaller than the slip resolution of the equipment used here (Fig. 8).

Both the static portions of the rate tests on an unprimed surface (Fig. 5) and on the primedsurface (Fig. 6) are accompanied by an increasing shear stress. Thus, one might expect somemicroscopic slippage, which by Ruina's argument should lead to peaked curves resulting fromstrengthening. The lack of stress peak development in the unprimed surface condition, indicatesthat there is something more to it. The main difference is that the rate tests on the primed surfaceare preceded by a steady-state sliding phase that is not terminated by a momentary removal of thenormal stress. Thus, it appears that when the normal stress is allowed to go to zero, the memory ofthat past slip history, which is reflected in _F= O,is erased.

4 Creep

Creep is defined just as it is for solid materials: deformation (in this instance slip) at constant stress.Creep tests were done on the surface in both initial states, unprimed and primed.

• 4.1 Unprimed Surface

A number of short-term (minutes to hours) creep tests at various stress levels were done on theunprimed surface. The load-point displacement was increased until the correspondingshear stress

ll

' II

/

i 0.if/

i0 200 400 600 800 1000 1200 1400

HOLD_ (me)

Figure9: The effect of displacementholds of variouslengthson the strength.

increasedto a preselectedlevel where thecontrol mode was switched to load; i.e., the load pointwas maintainedatconstantload. No slip was recordedin anyof thesetests.

4.2 Primed Surface

Severalcreep tests conducted on a primedsurfaceareshownin Figure 10. The small plot at thetopof the figure shows the entiretest record.Startingin theunprimedconditionat _r= 5 MPa, thejoint was slid at a constantvelocity to about0.14 mm,wherethedisplacementwas put in holdandthe controlmode switchedto load. Whenthe displacementwas stopped,the stress relaxed,nearlyinstantaneously,to a littleless than 0.6_r.Thus, thisrepresentsaconstantstress test atabout95% ofthe slidingstress. No slip was observedin 2.5 hours,atwhichtime the stress was raisedto 0.617¢(about99%of the slip stress). After somethingless than an hour, no creepwas observed, so thestress was raised to 0.623_, or greaterthanthe stressrequiredforsliding underdynamicconditionsimmediatelyprecedingthe creepexperiments.Again,nocreep was observed. Finally,the controlmodewas switchedbacktodisplacementand was increasedat the formerratecausing the stress tojump to 0.670_ beforeslip reinitiatedandthe stress rapidlyresumedits formervalue. Evidently,microscopicslippagethat occurs in associationwith thestress relaxation,leads to a strengthening,a > 0, thatactuallypreventsclassical creep. In fact, at/_ = .623, one whouldexpect, based onexperiencewith creep of solids, acceleratingcreep.

12

0.8 / ' ' '/

0.6

_1.04

0.70

0.670 .._

10.6O 0.596Y _

0.55 i i i i iO. 1 0.12 0.13 0.14 0.15 0.16 0.17

SLIP (ram)

Figure 10: A creep test near the slip stress on a primed surface.

13

5 Conclusions

Rotary shear experiments at constant normal stress have been run on a smooth, artificial jointin Topopah Spring Member tuff. In general, this rock behaves much like other silicate rocksin its sensitivity to velocity changes and displacement history, but this program has highlightedsome new and relatively unexplored behavior. Two initial states, unprimed and primed exhibitqualitatively and quantitatively different responses. The resultthat there are two limiting initialconditions, primed and unprimed, indicates that inferring rock mass response from in situ orlaboratory properties could lead to errors without substantially improved understanding of theinitial conditions of real joints. It is important to differentiate between the continued deformationof a joint that has been relieved of in situ stress from one that has not been.

In the steady-state friction regime, the slip behavior of this rock is well-described by the rate-and state-dependent constitutive law. Fractures in a steady-state exhibit velocity weakening atthe low normal stresses used here, and b - a _ 0.014. This type of constitutive reponse, incombination with the relatively low modulus of the rock mass compared with intact rock, andthe well-known stress concentrations that occur in the vicinity of underground openings, suggeststhat dynamic instabilities may manifest themselves in the form of acoustic emissions or, possibly,higher amplitude events. These results indicate that acoustic monitoring of the repository may beuseful from a safety standpoint.

Short-term creep tests in both the primed and unprimed states show no time-dependent slip.In the primed state, the shear stress to reinitiate slip after a nominal constant slip period, exceedsthe immediately preceding steady-state value. Thus, some mechanism, either time-dependentstrengthening of static friction, or friction increases due to microscopic slip as suggested by therate- and state-dependent frcition law, is preventing slip at constant stress.

The effect of submerging the joint in water shows that the friction is higher for wet joints thandry ones, but that there is little difference in the trend or magnitude of the rate effect. Presumably,this results from the fact that the largest changes due to moisture occur in the first fractions ofa percent of water content, and thus experiments run in the so-called laboratory air-dry (but stillin ambient humidity) condition already have the necessary water content to behave similarly tosaturated samples. Thus, conclusions based on air-dry samples will still pertain to joints in situ.

6 References

Blanpied, M. L., T. E. Tullis, J. D. Weeks, Frictional behavior of granite at low and high slidingvelocities, Geophys. Res. Let., 14, 554-557, 1987. (NNA.900918.0009)

14

Blanpied, M. L., D. A. Lockner and J.D. Byerlee, Fault stability inferred from granite sliding exper-iments at hydrothermal condtions, Geophy. Res. Let., 18, 609-612, 1991. (NNA.930330.0073)

Brace, W. E, and J. D. Byerlee, Stick-slip as mechanism for earthquakes, Science, 153, 990-992,1966. (NNA.930330.0074)

Christensen, R. J., S. R. Swanson, and W. S. Brown, Torsional shear measurements of the frictionalproperites of Westerly granite, in Advances in Rock Mechanics, Proc. 3rd Cong. Intern. Soc.Rock Mech., 221-225, 1974. (NNA.900919.0172)

Dieterich, J. H., Time-dependent friction in rocks, J. Geophys. Res, 77, 3690-3697, 1972.(HQS.880517.1159)

Dieterich, J. H., Time-dependent friction and the mechanics of stick-slip, Pageoph, 116, 790-806,1978. (HQS.880517.1161)

Dieterich, J. H., Modeling of rock friction: 1. Experimental results and constitutive equations, J.Geophys. Res., 84,2161-2168, 1979. (NNA.900918.0016)

Dieterich, J. H., Constitutive properites of faults with simulated gouge, in Mechanical Behaviorof Crustal Rocks: The Handin Volume, Geophys. Monogr. Ser., 24, edited by N. L. Carter, M.Friedman, J. M. Logan, D. W. Streams, 103-120, 1981. (NNA.900919.0173)

Dieterich, J. H., and G. Conrad, Effect of humidity on time- and velocity-dependent friction inrocks, J. Geophysical Res., 89, 4196-4202, 1981. (NNA.900918.0017)

Gu, J.-C., J. R. Rice, A. L. Ruina, and S. T. Tse, Slip motion and stability of a single degree offreedom elastic system with rate and state dependent friction, J. Mech. Phys. Solids, 32,167-196, 1984. (NNA.900918.0018)

Holcomb, D. J., and L. W. Teufel, Acoustic emissions during deformation of intact and jointedwelded tuff, Sandia National Laboratories Tech. Rep., SAND82-1003, Albuquerque, NM, 1982.(SRX.820817.0119)

Kutter, H. K., Rotary shear testing of rock joints, in Proc. 3rd Cong. of the Intern. Soc. for RockMech., National Academy of Sciences, Washington, D.C., 254-262, 1974. (NNA.900919.0174)

Marone, C., C. B. Raleigh, and C. H. Scholz, Frictional behavior and constitutive modeling of' simulated fault gouge, J. Geophys. Res., 95, 7000-7025, 1990. (NNA.930330.0075)

. Olsson, W. A., A dislocation model of the stress history dependence of frictional slip, J. Geophys.Res., 89, 9271-9280, 1984. (NNA.900403.0279)

Olsson, W. A., The effects of changes in normal stress on rock friction, in Constitutive Laws for

Engineering Materials: Theory and Applications, ed. C. S. Desai, E. Krempl, P. D. Kiousis, T.Kundu, p. 1059-1066, Elsevier, New York, 1987a. (NNA.910207.0028)

15

Olsson, W. A., Rock Joint Compliance Studies, Sandia National Laboratories Tech. Rep.,SAND86-0177, Albuquerque, NM, 1987b. (NNA.891019.0291)

Olsson, W. A., Compliance and Strength of Artificial Joints in Topopah Spring Tuff, Sandia Na-tional Laboratories Tech. Rep., SAND88-0660, Albuquerque, NM, 1988. (NNA.881202.0205)

Olsson, W. A., The effects of normal stress history on rock friction, in Key Questions in RockMechanics: Proceedings of the 29th U.S. Symp. Rock Mechanics, eds. P. A. CundaU, R. L.Sterling, and A. M. Starfield, 111-117, Balkema, Brookfield, 1988. (NNA.900403.0382)

Olsson, W. A., The effects of shear and normal stress paths on rock friction, in Rock Joints:Proceedings of the International Symposium on Rock Joints, eds. N. Barton and O. Stephansson,475-479, Balkema, Brookfield, 1990. (NNA.900426.0053)

Olsson, W. A., The effect of slip on the flow of fluid through a fracture, Geophys. Res. Let., 19,541-543, 1992. (NNA.930330.0076)

Olsson, W. A., and S. R. Brown, Hydromechanical response of a fracture undergoing compressionand shear, accepted for publication in Proc. 34 th U. S. Symposium on Rock Mechanics, 1993.(NNA.940301.0034)

Price, R. H., J. R. Connolly, and K. Keil, Petrologic and mechanical properties of outcrop samplesof the welded, devitrified Topopah Spring Member of the Paintbrush tuff, SAND86-1131, SandiaNational Laboratories, Albuquerque, NM, 1987. (HQS.880517.1704)

Rice, J. R., Constitutive relations for fault slip and earthquake instabilities, PAGEOPH, 121,443-475, 1983. (NNA.930330.0077)

Rice, J. R., and J.-C. Gu, Earthquake -aftereffects and triggered seismic phenomena, PAGEOPH,121, 187-219, 1983. (NNA.930330.0078)

Rice, J. R. and A. L. Ruina, Stability of steady frictional slipping, J. Appl. Mech., 50, 343-349,1983. (NNA.930414.0032)

Ruina, A., Friction laws and instabilities: a quasistatic analysis of some dry frictional behavior,Report No. 21 (Ph.D. Thesis), Division of Engineering, Brown University, Providence, R. I.,1980. (NNA.940301.0035)

Ruina, A., Slip instability and state variable friction laws, J. Geophys. Res., 88, 10,359-10,370,1983. (NNA.930330.0079)

Teufel, L. W., Frictional properties of jointed welded tuff, Sandia National Laboratories Tech.Rep., SAND81-0212, Albuquerque, NM, 1982. (HQS.880517.2356)

Tullis, T. E., and J. D. Weeks, Constitutive behavior and stability of frictional siiding of granite,PAGEOPH, 124, 383-414, 1986. (NNA.930330.0080)

16

Weeks, J. D., and T. E. Tullis, Frictional sliding of dolomite: a variation in constitutive behavior,J. Geophys. Res., 90, 7821-7826, 198:5. (NNA.930330.0081)

Wong, I. G., Recent developments in rockburst and mine seismicity research, in Rock Mechanics,. Proc. 33rd U. S. Rock Mech. Syrup., eds. J. R. Tillerson and W. R. Wawersik, 1103-1112,

Balkema, Rotterdam, 1992. (NNA.930414.0033)

Xu, S. and M. H. de Frietas, Use of a rotary shear box for testing the shear strength of rock joints,Geotechnique, 38, 301-309, 1988. (NN A.900918.0012)

Yoshioka, N., and C. H. Scholz, Elastic properties of contacting surfaces under normal and shearloads 2. Comparison of theory with experiment, J. Geophys. Res., 94, 17691-17700, 1989.(NNA.900403.0393)

17

APPENDIX

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18

YUCCA MOUNTAIN SITE CHARACTERIZATION PROJECT

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o

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SAND92-2333

. The number in the lower right-hand corneris an accession number used for Office of

Civilian Radioactive Waste Managementpurposes only. It should not be usedwhen ordering this publication.

NNA.940209.0001

I