In Situ Geomechanics Climax Granite, Nevada Test Site.

UCRI-53076In Situ GeomechanicsClimax Granite, Nevada Test SiteF. E. HeuzeW. C. PatrickR. V. De la CruzC. F. VossApril 1981 In , X s

DISCLAIMERThis document was prepared as an account of work sponsored by an agency of the United States Government.Neither the United States Government nor the University of California nor any of their employees, makesany warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that itsuse would not infringe privately owned rights. Reference herein to any specific commercial products, process,or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the United States Government or the University of California.The views and opinions of authors expressed herein do not necessarily state or reflect those of the UnitedStates Government thereof, and shall not be used for advertising or product endorsement purposes.Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore NationalLaboratory under Contract W-7405-Eng-48.

UCRL-53076Distribution Category UC-70In Situ GeomechanicsClimax Granite, Nevada Test SiteF. E. Heuze*W. C. Patrick*R. V. De la CruztC. F. VosstApril 1981Earth Sciences Division, Lawrence Livermore National LaboratorytDepartment of Mining Engineering, University of Wisconsin, MadisonLAWRENCE LIVERMORE LABORATORYlLUniversity of California * Livermore, California * 94550 -Available from: National Technical Information Service * U.S. Department of Commerce5285 Port Royal Road * Springfield, VA 221610 17.00 per copy * (MicroficheS3.50 )

FOREWORDThis project was supported under the Department of Energy's National WasteTerminal Storage Program and was administered through the Nevada OperationsOffice as part of the Spent Fuel Test in Climax Stock granite (SFT-C) at theNevada Test Site (NTS).Dr. F. B. Heuze directed the project and wrote this report.assisted in the field by Dr. W. C. Patrick.Be wasProfessor R. V. De la Cruz andhis assistant, Mr. C. F. Voss--both from the Department of Mining Engineeringat the University of Wisconsin, Madison--participated as consultants andperformed some of the field tests.Mr. D. Wilder contributed to the definition of the geology at SFT-C andprovided rock quality designation (RQD) values.Mr. J. Norman and Dr. N.Burkhard aided in the analysis of the shear wave records.the manuscript.Ms. D. Olson typedMs. Carol Gerich was the report's technical editor.assistance is gratefully acknowledged.iiTheir

2Spent-Fuel Test (SFT-C) Mine-by Experiment1.1.1Mine-by at SFT-C1.1.2Modeling of the Mine-by1.1.3Discussion.2.3.8.9.2.1.1The NX-Borehole Jack .2.1.2NX-Jack Tests at SFT-C.2.1.3Discussion.NX-Borehole Jack Tests.Modified NX-Borehole Jack Tests4.12.12.12.12.16.162.2.1Principle of the Test.162.2.2Modified NX-Borehole Jack Tests at SFT-C.192.2.3Discussion.Petite Sismique Tests.19.222.3.1The Petite Sismique Method .222.3.2Petite Sismique Tests at SFT-C.232.3.3Discussion.24.332.4.1Estimate Based on Rock Mass Rating (RMR).332.4.2Estimate Based on the Q-system.352.4.3Estimate Based on Models of Tunnel Relaxation.36.Other Estimates of Rock-Mass Deformability.Comparison of Modulus Estimates.3.In Situ Normal Stiffness of Climax Granite Joints4.In Situ Stresses4.1.In Situ Deformability2.5.Geologic Setting2.4.1.32.3.Purpose and Scope of Project2.2.1.22.1.Previous Stress Measurements by the USGS.36.38.41.42.4.1.1Reported Stress Values.424.1.2Discussion of Horizontal and Vertical Stresses.434.1.3Discussion of Secondary Principal Stresses.47iii.

4.2.50.50.52Measurements by Borehole Jack Fracturing.534.3.1The Jack-Fracturing Method .534.3.2Results at SFT-C.544.3.3Discussion.54. .56Undercoring Stress Measurements.Procedure and Test Results4.2.2Discussion4.34.2.1.5. In Situ Poisson's Ratio of Climax Granite6. Summary and ces.57.57.58. 59. .iv.,

IN SITU GEOMECHANICSCLIMAX GRANITE, NEVADA TEST SITEABSTRACTThe in situ modulus of the Climax granite in the Spent Fuel Test (SFT-C)area of the Nevada Test Site was estimated using six different approaches.Our best estimate of field modulus as Efcomparison of the various approaches.-26 GPa was obtained from aA best estimate of laboratory modulusacquired by comparing three different sources was E, - 70 GPa.Therefore,the modulus reduction factor for the Climax granite appears to beEf / EI 0.37.In turn, our estimate of in situ rock-mass deformabilitywas used to back-calculate in situ values for the normal stiffness of thegranite joints.Our analysis of former stress measurements by the U.S. Geological Survey(USGS) shows that the horizontal stresses in the vicinity of SFT-C varygreatly with azimuth.An unexplained feature of the stresses at SFT-C isthefact that the vertical stress appears to be only 65 to 75% of the calculatedlithostatic burden.From the three-dimensional stress ellipsoid at mid-lengthin the tunnels, assuming a plane strain condition, we were able to estimate anin situ Poisson's ratio of the rock mass as v 0.246.Two other techniqueswere applied in an attempt to measure the stresses around the SFT-C heater andcanister drifts:approach.the undercoring method and the borehole jack fracturingThe former technique appears to have given reasonable estimates oftangential stresses in the roof of the heater drifts; the latter appears togive low results for stresses in the pillars.Specific recommendations are made for future tests to further characterizethe mechanical properties of the Climax granite and the in situ stresses atSFT-C.1

1. INTRODUCTION1.1SPENT FUEL TEST (SFT-C) MINE-BY EXPERIMENT1.1.1Mine-by at SFT-CLawrence Livermore National Laboratory (LLNL) is conducting a generic testof retrievable geologic storage of nuclear spent fuel assemblies in anunderground chamber at the Nevada Test Site (NTS), Nye County, Nevada. '2This generic test is located 420 m below the surface in Climax granite.Eleven canisters of spent fuel, approximately 2.5 years out of reactor core(about 1.6 kW/canister thermal output), are now emplaced in a storage driftalong with six electrical simulator canisters.Two adjacent drifts containelectrical heaters, which will be operated to simulate the thermal field of alarge repository.The three drifts are shown in Fig. 1. Their excavation was performed inthree steps:the two heater drifts were excavated first, then the top headingof the canister drift was mined, and finally, the bench was removed.Prior tothe mine-by of the center (canister) drift, deformation and stress gages wereemplaced near two cross sections, labeled stations 2 83 and 3 45 (Fig. 2).610Existing workingsV Station3 45Hole lBI1ll\yl\North heaterdriftSaion/2 34,Station 3 49rAdi iIiZShaftI II IFIG. 1. Spent Fuel Test layout in Climax granite, NTS.2--/r-Railcar room 'drift lStation 2 87-Canister storageSouth heaterScale20 ELdriftNew construction'rV-"I1 r

o Extensometer anchor0 Convergence heado3 Vibrating wire stressmeter6.1 rm'Stress-reliefovercore holesFIG. 2.Schematic cross section of Spent Fuel Test, showing relative locationof mine-by instruments (reprinted with permission from Ramspott and Ballou 2 ).1.1.2Modeling of the Mine-byTwo calculations related to the mine-by were reported previously. '3Inneither case did the numerical models explicitly include the geologicaldiscontinuities such as joints and shears.The rock-mass modulus was chosenfrom handbook values, since no field tests had been performed to estimate it.In the first modelthe ratio of horizontal to vertical stresses variedbetween 0.8 and l.Oj in the second model 3 it ranged from 0.8 to 1.25.Thehighest value of 1.25 is close to that derived from the analysis of overcoringmeasurements conducted by the U.S. Geological Survey around the south heaterdrift.Neither model had the capability to represent dilatancy orstrain-softening of the rock mass.The stress changes and deformations calculated by LLNLcode are summarized in Table 1 and Fig. 3.calculated by Terra Tek, Inc.with the ADINAStresses and deformationswith the DIG and TWODY codes were quite3

(-0.39) (-0.40)-0.34 -0.12(-0.43)%(-0.43,ga) Station 2 83(-0.41)-0,34).43)).65/ ;(-0.42)-0.32(-0.44), - 1.04-6.1 m -n-3.4 m(b) Station 3 45FIG. 3. Comparison of measured and (predicted) displacements during mine-byat SFT-C (reprinted with permission from Ramspottl).4

TABLE 1.Comparison of measured and previously calculatedvertical stress changes, as a result of the mine-by (data fromRamspott1).Dist. fromStress-Stationcanistermeter No.(Fig.drift (m)1)V5KareadingCalculatedbstress change(MPa)(MPa)VSM-12 801.0-9.7 5.1VSM-22 803.0-1.3 2.3VSM-33 021.0-7.0 5.1aVibrating wire stressmeter (VSM) calculations assumed arock modulus of 61 GPa.Minus sign indicates a decrease incompression.-Rock modulus assumed at 61 GPa.Horizontal-to-verticalstress ratio taken as 0.8.similar.This was to be expected, considering the similarity of the input.The results of modeling can be summarized as:*The models did not show a reduction in vertical stresses in thepillars during mine-by of the canister drift, as reported from the field."'*The models did not show a horizontal contraction of the pillars duringmine-by, as reported from the field. '3*The relative movements of anchors for the extensometers at 340 to thehorizontal were several times larger than predicted by the models at stations3 45 and 2 83.*The relative movements of anchors from the extensometers at 50to thehorizontal were in slightly better agreement with the predicted values at bothstations.In general, observations and calculations came closer to each other as onemoved away from the drifts.1.1.3DiscussionThe decrease in vertical stress at vibrating stressmeters VSM-1 and VSM-3can be explained from the softening of the rock in the skin of the pillars, as5

a result of blasting and stress relief that open up fractures.Also, thedecrease at VSM-2 in the core of the pillar is not unreasonable; our newanalysis of the mine-byindicates that localized pillar unloading can takeplace, depending upon the geometry of the joints and shears, even when theaverage vertical pillar stress increases.The reported horizontal contraction of the pillars, however, is morepuzzling.In a previous reportthe horizontal shrinkage of the pillars wasexplained by saying that stress arching took place over the caverns; however,the arching hypothesis was not substantiated.Arching would mean that thepillars unloaded through some load redistribution on the abutments, but thelack of stress gages in the abutments prevents confirmation of this.Whatevertruly happened, the horizontal shortening of the pillars could only take placethrough one of three modes (Fig.*Unloading before peak.*Unloading postpeak.*Offloading in4):a so-called Class IIbehavior,only because Class Ibehavior gives a strain increase.There isno published evidence that the Class IIservo-controlled machines inoffloading observed withlaboratory compressive tests can exist inthefield, where vertical loading is passive due to the overlying rock.As for the unloading assumption, a simple calculation shows the strangeconclusion to which it leads.The average pillar shrinkage at four locations(Fig. 3) is at least equal to(1.96 1.42 2.66 1.80)/4 1.86 mmFor pillars with an average width of 5.5 m, this corresponds to a horizontalunloading strain recovery:AChor 1.86 * 10/ 5.5With a Poisson's ratio v 0.246, this implies a vertical strain recovery:AvertAchor / 0.246 1.4 * 106.

Unloading-by\StrainClass I strain-softening1/ IIC. .// 1/ UnloadingStrainClass 11 strain-softeningFIG. 4. Unloading vo offloading stress-strain responsg.7