Deformation studies were performed on tuffaceous rocks from Yucca Mountain, Nevada Test Site to determine the strengths and coefficients of friction under confining pressures from 10 to 50 MPa at room temperature. Frictional strengths of 30{sup 0} sawcut samples increased with pressure and reached values of around 150 MPa at the higher confining pressures. However, the failure strengths of the intact samples were quite unpredictable. The coefficients of friction ranged between 0.7 and 0.9 for all specimens. These data can be used in conjunction with in situ stress measurements at Yucca Mountain, to evaluate the potential for earthquake activity in the region. 1 ref., 7 figs

Byerlee, J.

Morrow, C.

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3- I?Chapter 49FRICTIONAL SLIDING AND FRACTURE BEHAVIOR OF SOMENEVADA TEST SITE TUFFSby C. Morrow and J . ByerleeU.S.Geological SurveyMenlo Park, California CONF-840633 — 1 8DE87 0 0 8 4 6 3ABSTRACTDeformation studies were performed on tuffaceous rocks fron YuccaMountain, Nevada Test Site to determine the strengths and coeffi-cients of friction under confining pressures froa 10-50 MPa at roomtemperature. Frlctlonal strengths of 30* sawcut samples Increasedwith pressure and reached values of around ISO MPa at the higherconfining pressures. However, the failure strengths of the Intactsamples were quite unpredictable. The coefficients of frictionranged between 0.7 and 0.9 for all speciaens. These data can beused in conjunction with In situ stress aeasurements at Yucca Moun-tain, to evaluate the potential for earthquake activity In theregion.INTRODUCTIONThe tuffaceous rocks of Yucca Mountain at the Nevada Test Siteare being studied by the Nevada Nuclear Waste Storage Investigationsproject (NNWSI) as a possible host for a radioactive waste reposi-tory. An understanding of the physical properties of these tuffs,particularly the frlctlonal sliding and fracture behavior, Is neces-sary in order to deteralne whether in situ stresses or stressescaused by repository alnlng aay exceed the stresses required tocause frlctional sliding on faulted surfaces or the failure ofIntact rock.SAMPLE DESCRIPTIONThe two candidate repository rocks used In this study were the)Topopah Spring Menber of the Paintbrush Tuff and the Bullfrog Member467DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED46« ROCK MECHANICS IN PRODUCTIVITY, PROTECTIONof the Crater Flat Tuff. The Topopah Spring lies in the unsaturatedzone above a deep-lying (400-600 m) water table at Yucca Mountain,whereas the Bullfrog Member Is located below the water table. Atthe horizons of Interest, both rocks are devitrified and nonzeollt-ized, however, the Tokipah Spring is more densely welded than theBullfrog.PROCEDUREIn order to determine the coefficient of friction of the Bullfrogand Topopah Members, sliding experiments were performed on 63.5 mmlong by 25.4 mm diameter cylindrical samples containing a sawcutinclined at 30* to the cylindrical axis (Fig. 1). Confining pres-sures from 10 to 50 MPa in iO MPa steps simulated a range of burialdepths. Deformation was carried out at a strain rate of 1 0 / suntil the samples had she:tened by 10 mm In the axial direction. Tostudy the possible effects of time, these experiments were repeatedat the slower rate of 10 /s on new samples. All samples were satu-rated with distilled water. A gas accumulator maintained the porepressure at 0.5 MPa to ensure complete saturation during the courseof the experiments. All tests were performed at room temperature.Similar studies were carried out on intact specimens of the tuffsto determine the fracture strengths and coefficients of friction forsliding on faulted surfaces. These experiments were also conductedon saturated samples at strain rates of 10 and 10~°/s and confin-ing pressures from 10 to 50 MPa.RESULTSStrengthFig. 2 shows typical stress-strain (differential stress-axialdisplacement) curves for sawcut samples of the Topopah Spring memberat a strain rate of 10 7s. The maximum strength Is around 150 MPaat a confining pressure of 50 MPa. In each test, there is verylittle difference between the residual strength and the maximumstrength of the sample. These plots are similar to the results forthe Bullfrog Member, both in terms of strength levels and the sta-bility behavior. In all cases, strength Increased uniformly withconfining pressure. The samples frequently exhibited stick-slipbehavior at the higher pressures. This usually occurred as a singlelarge stress drop, rather than small multiple events. At the slowerstrain rate of 10 /s the results were nearly the same for both tuffmembers. That is, the strain rate effect was less significant thanthe differences in strength resulting from sample variability. Thiscan be seen in Fig. 3, where maximum strengths are plotted withconfining pressure for both the Bullfrog and the Topopah Spring atthe two strain rates. There was no major difference in the strengthof the tuffs at the two strain rates. Instead, the most noticeablerNEVADA TEST SITE TUFFSPORE PRESSUREINLET469O-RING SEALTUFF SAMPLESTEEL ENDPLUGFig. 1. Sample assembly.STEEL PLUGPOLYURETHANEJACKETfeature here is that the two tuffs were quite similar and had verypredictable failure strengths.Characteristics of the stress-strain curves for Intact specimenswere also comparable between the two tuff members. The Bullfrog Isshown as a typical example in Fig. 4. The variation in failurestrength with pressure was much less consistent than that found forthe sawcut samples. For instance, the specimen at 40 MPa fracturedat a higher differential stress than the one at 50 MPa. At 20 MPa,the tuff exhibited an apparent ductile behavior, with no peak in thedifferential stress value at all. This behavior was characteristicof the specimens that developed multiple fracture planes (3 out ofthe 20 intact samples).After failure, sliding progressed smoothly In all cases, free ofstick-slip events. The residual strengths during this segment ofthe experiments were fairly well ordered according to confiningpressure and reached values of around 200 MPa at the higherpressures.If we plot failure strength against confining pressure for theIntact samples (Fig. 5), the major difference between the two tuffs470 ROCK MECHANICS IN PRODUCTIVITY, PROTECTION200a. 150sV)uia.50- r T ~rTOPOPAH SPRINGSAWCUT SAMPLESe«IO"Vsec_L"0 2 4 6 8 10AXIAL DISPLACEMENT (mm)Fig. 2. Differential stress as a function of axial displacement forsawcut samples of the Topopah Spring Member at a strainrate of 10" /s. Confining pressures In MPa are shown atthe right of each curve.becomes evident. That Is, the Topopah Spring is about 50% strongerat failure than the Bullfrog. This is not surprising as it la amuch nore densely welded rock. The characteristics at failure dis-cussed above are also clearly evident. The unpredictable failurestrength due to sample variability, particularly of the TopopahMember, overshadows any time dependent effects.FrictionCoefficients of friction for the sawcut samples can be determinedfrom Fig. 6. Here shear and normal stress data at 10 mm of slidingare plotted, and the slope of the line gives us the residual fric-tion, p. We have chosen to plot residual friction rather thaninitial or maximum values as it is more representative of a naturalfault that has undergone extensive displacement. Coefficients offriction ranged between 0.80 and 0.88 for the four groups shown.Byerlee (1978) found that for stresses below 200 MPa, almost allrocks, regardless of type (granite, limestone, etc.), were charac-terized by a frictional coefficient of around 0.65. This line is200rISO10050500X BULLFROG• BULLFROGO TOPOPAHD TOPOPAHIO-4/secIO"6/5ecBULLFROG TUFFINTACT SAMPLES€ - 10'Vsec5I10 20 30 40CONFINING PRESSURE. (MPo)504 6 8AXIAL DISPLACEMENT (mm)Fig. 4. Differential stress as a function of axialdisplacement for intact saaples of the Bull-frog Member at a strain rate of 10" /a .Confining pressures in MPa are shown at theright of each curve.Fig. 3. Maxlmua strength of a l l sawcutsamples as a function of con-fining pressure.472 ROCK MECHANICS IN PRODUCTIVITY, PROTECTIONso rtn<Za 20010010 20 30 40CONFINING PRESSURE, (MPa)50Fig. 5. Failure strength of intact samples as a function of confin-ing pressure.shown in the figure to illustrate that the frictional properties ofthe tuffs are very typical of other rock types.The results for the Intact samples were quite similar to theresults for the sawcut samples (Fig. 7). We calculated shear andnormal stresses assuming a 30* failure angle. Almost all fractureswere within a few degrees of this value. There was a greater spreadin the stress values because the fracture surfaces were not ar,regular as the sawcut surfaces. As a result, the coefficients offriction ranged between 0.7 and 0.9 for these samples. A line witha slope of 0.85 Is again shown for reference.NEVADA TEST SITE TUFFS 473IOOr RESIDUAL FRICTION AT 10mm DISPLACEMENTSAWCUT SAMPLES, AREA CORRECTIONX BULLFROG e-ICrViec• BULLFROG 10"*/tecO TOPOPAH 10"'/tecO TOPOPAH lO'VstcI I20 40 60 80 100NORMAL STRESS, CTN (MPo)Fig. 6. Shear and normal stress data for all sawcut samples after10 mm of axial displacement, corrected for the decreasingarea of contact during sliding.SUMMARY OF RESULTS1) For sawcut samples, the aaxlnua strength increased uniformlywith confining pressure. There was a greater tendency for stick-slip behavior at the higher pressures. The Bullfrog and TopopahSpring Members had very similar frlctional strength*.2) Fracture strengths of the intact samples were unpredictableand unsystematic with confining pressure particularly for theTopopah Member. Stable sliding was observed after fracture In allcases and the stress levels during this period were a function ofconfining pressure. The Topopah Spring Member was about 50Zstronger at failure than the Bullfrog.3) The differences in strength due to staple variability over-shadowed any observable effect that may have been due to strainrate.4) The coefficient of friction for the samples containing a 30*sawcut, both Bullfrog and Topopah Spring, ranged between 0.80 and0.88. The values for intact samples ranged between 0.7 and 0.9.474 ROCK MECHANICS IN PRODUCTIVITY, PROTECTION100p RESIDUAL FRICTION AT 10mm DISPLACEMENTINTACT SAMPLES O60aa.60COtoUJa:40<luIto2020 40 60 80NORMAL STRESS, ŒH (MPa)X BULLFROG «-ICr*/«c• BULLFROG IO"V$ecO TOPOPAH 10"'/«cG TOPOPAH IO~4/secJL100aV.Fig. 7. Shear and normal stress data (assuming a 30* fractureangle) for intact samples after 10 mm of axialdisplacement.These coefficients are very typical of the average value of 0.85 forother rock types (Byerlee, 1978) at pressures below 200 MPa.ACKNOWLEDGMENTSThis work was performed for the Nevada Nuclear Waste StorageInvestigations project (NNWSI), which Is funded by the U.S. Depart-ment of Energy under Interagency Agreement DE-AI08-78ET44802.REFERENCESByerlee, J., 1978, "Friction of rock»," Pure and Applied Geophysics,Vol. 116, p. 615.

Frictional sliding and fracture behavior of some Nevada test site tuffs

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Frictional sliding and fracture behavior of some Nevada test site tuffs

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