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14AR-17-98 15:11 Fram:CCBV GOLD MNf~ 1719666325 • T-317 P 04/10 Job-647 <br />Wall Rock Material Strengrlt <br />The steength chazacteristics of the fractured Cresson Mine rock mass was evaluated by Call and <br />Nicholas duriae the pre-mining stabiliry study using laboratory-scale, direct shear tests. Three <br />tests of each of two materials were performed, with the following results: <br />Porphyritic phonolite (3 tests) Effective sues friction angle = 37.9° <br />Volcanic breccia (3 tests) Effective stress ftiction angle = 40.6° <br />The performance of the mining faces allow an evaluation of the actual sheaz suength of fractures <br />in the Cresson rock mass. Examination of fracture surfaces has shown them to be relatively <br />"clean," that is, there is razely any gouge. Most of the fracture surfaces aze irregular. These <br />chazacteristics are expected to result in a high internal angle of friction; in other words, there <br />is a high resistance to movements along the joinu and fractures. <br />In order to evaluate the actual strength of the rock mass in the field, a detailed evaluation of the <br />performance of blocks and wedges ("structures") is the excavated faces way undertaken. This <br />survey involved the entire highwall system. Approximately 77 structures were identified in the <br />highwalls in which fractures or fault planes created potential wedges or blocks. The blocks and <br />wedges fall into one of the following two categories: <br />1. Previously removed, in which the features on which the failure had taken place were <br />identifiable, bur the block or wedge is missing or no longer in position; or <br />2. Currently stable, in which both features on which movement is possible are still <br />identifiable, and the block or wedge is still in position. <br />The information ob[ained Etas been "back-analyzed" to provide asite-specific escima[e of the <br />actual frictional characteristics of materials in the slopes. This procedure provides the equivalent <br />~ of approximately 77 large-scale sheaz tests and uses the measured in-situ shear planes <br />encountered in the Mine highwalls. The resulu of this analysis shows chat 95 percen[ of the <br />structures were bordered by joint sets dipping at greater [hart 40°. Therefore, the results of the <br />evaluation show [hat the 95 percent confidence value of the effective stress friction angle is 40°. <br />Throughout these assessments of wedgec and blocks, there did not appear to be a preference for <br />either stable blocks or missing blocks to occur with certain joint and dike sets. <br />In addition to the in-situ evaluation of suength described above, CC&V has recently performed <br />direct shear tests of the Cresson ore after crushing. This ma[erial would be expected [o be <br />inherendy less strong than the same ma[erial prior to mining, where block sizec aze lazger, and <br />where interlocking provides significant additional strength. Nonetheless, the friction angle for <br />the Cresson ore was 38 degrees and the cohesion was 1275 psf (8.8 psi) (see CC&V's January <br />9, 1996 submission which is also referenced later in [his letter). This testing further confirms <br />that the friction angle representative of the highwalls is consistent, and relatively high. <br />3 <br /> <br />