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A laboratory characterizationof rock-jointbehavior and mechanicalproperties of the Apache Leap tuff has been undertakenby the Center for Nuclear Waste Regulatory Analyses (CNWRA). The major concern driving this study is the ability of currentlyexisting rock-jointmodels to adequatelyrepresent dynamicrock-jointbehavior. These currentrock-jointmodels were developed based on data takenunder unidirectionalpseudostaticloading conditions.Thus, the ability of the modelsto predictjoint performance under cyclic pseudostaticand dynamic loading conditions has not been tested. Further, only limited cyclic pseudostatic and dynamic loading data are available to validate these rock-joint models. Consequently,joint behavior undercyclic pseudostaticand dynamic loading is not well understood. As partof this laboratory characterizationof rock joints, rock joints were tested under cyclic pseudostatic and dynamic loads in order to obtain a better understandingof dynamic joint shear behavior and to generate a complete data set that can be used for validation of rock-jointmodels.A numberof methods are currentlyused in rock engineeringpractice to determine the joint roughness coefficient(JRC).However, these methods grossly underestimatedthe jointroughnessof the Apache Leap tuff joints. Consequently, none of the methods evaluated in this report seem capable of providing a reasonablejoint roughnesscoefficient value that can be readilyused to estimatejoint shearstrength. The results of the direct shear tests on the Apache Leap tuff joints have indicated that both under cyclic pseudostaticand dynamic loadings the shear resistance upon reverse shearing is smaller than that of forward shearing andthejoint dilationresultingfrom forwardshearing recovers duringreverse shearing. Within the range of variation of shearing velocity used in these tests, the shearing velocity effect on rock-jointbehavior seems to be minor, and no noticeable effect on the peak joint shear strength and the joint shear strength for the reverse shearing is observed.Both joint forward and reverse shearing are important phenomena of rock-joint behavior. Reverse _hearing can result from earthquake, thermal loading, or both. Failure to consider this aspect of joint _ehavior in an underground structural design and performance analysis could result in (i) an _verestimation of the stability of emplacementdrifts and emplacementboreholes and (ii) predictionof an 3correctpatternof near-field flow (includingpreferentialpathways for water and gas).

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