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~•. aS <br /> <br />MainCressonSlo eEvaluation AdrianBrown <br />Table 10. NGI Rock Mass Classification <br />Factor Discussion Value <br />RQD fracturin Varies from 70%-97%, avers es 85% 85 <br />Jn Joint set number Two 'oint sets for the most art 4 <br />Jr Joint rou hness number Rou or irre laz 'oints, undulatin 3 <br />Ja Joint alternation number Unaltered walls, surface stainin onl 1 <br />Jw Joint water reduction D 1 <br />SRF Stress reduction factor Medium stress, limited weak zones 1.5 <br />Tunnelin uali index = QD/Jn * Jr/Ja * Jw/SRF 42.5 <br />Rock classification Good/Ve ood <br /> <br />7.1.3 Hoek and Brav Rockmass Parameters <br />Hoek and Bray (1981) present a method of computing the effective geotechnical rckmass parameters for <br />jointed rock, based on the rock intact strength (as quantified by the unconfined compressive strength) and <br />a range of parameters depending on the rock type, fracture intensity, and joint condition. The parameters <br />can be obtained from detailed large scale laboratory testing of jointed rockmasses, or empirical formulae <br />can be used based on experience of such laboratory testing, applied to specific conditions. The latter <br />approach has been applied to the Main Cresson Mine conditions. <br />The Hoek and Bray table for determining parameters for the computation of the rockmass failure <br />envelope is presented in Table 11. The Hoek and Bray method requires a description of the type of rock, <br />and the type of fracture system. The following descriptions most closely describe the rockmass at the <br />Main Cresson Mine, particulazly on the East Wall: <br />CSIR Rating 69 <br />NGI Rating 42.5 <br />Rockmass quality "Good to very good; fresh to slightly weathered rock, slightly disturbed, <br />with joints at 1-3 meters" <br />Rockmass type "Fine grained polymineralic igneous crystalline rocks, andesite, diorite, <br />diabase, and rhyolite" <br />Based on this description, the pazameters selected from the table are as follows: <br />m 1.7 <br />s 0.004 <br />A 0.525 <br />B 0.698 <br />T -0.002 <br />• <br />1385D.980672 40 <br />