2008-04-15_REVISION - M1980244 (192)

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• GHMXSIopeEvaluation 2.8 Shear Strength Due to the rubblized nature of much of the core obtained from the northeast quadrant of the proposed mine, the shear strength could not be determined by intact strength testing. Accordingly, direct shear testing was used to establish the strength parameters of the bulk of the rockmass. A total of eleven drained direct shear tests were performed. Test reports are presented in Attachment 2, a summary of the results are presented in Table 3. The Mohr-Coulomb effective stress envelopes are plotted in Plate 6, and the average shear strength parameters that are derived from the 11 tests, the results are: • Effective Stress Cohesion: 19.6 psi • Effective Stress Friction: 39.9° This result suggests that there is some inherent cohesion in the rubble materials, even when (as here) it is tested in a remolded form (as distinct from undisturbed). It is noted that these values are somewhat higher than frictional characteristics of (mainly) joints that have been tested in the Cripple Creek diatreme volcanics previously: • Effective Stress Cohesion: 8.7 psi • Effective Stress Friction: 36.5° • 2.9 Groundwater The pre-mining groundwater elevation in the Cripple Creek diatreme was approximately 9,600 ft msl. The diatreme has been progressively drained by a series of tunnels that have been installed for this purpose since 1896. Current district drainage is through the Carlton Tunnel, which was driven in 1941 at an elevation of approximately 7,000 feet, and which flows by gravity to Four Mile Creek seven miles from the mining district. As a result, the entire diatremal rockmass has been drained to an elevation of approximately 7,200 ft msl, and surface mines in the diatremal material will have drained slopes. 2.10 Rock Mass Rating (RMR) The Rock Mass Rating is a system of rock mass classification that is used for the design of rock slopes (Hoek and Bray, 1989). The RMR combines the five stability-determining elements of strength, rock quality, fracture intensity, nature of discontinuities, and groundwater conditions to produce a rating of the rock mass. The CSIR rock mass rating method of Bieniawski (1976) is used here, and is set out in Plate 7. The RMR is computed for each drill hole using the RQD, strength, fracture frequency, joint condition, and groundwater condition. The results are tabulated for each borehole in Plate 7, and the RMR distribution for the GHMX is presented in Plate 8. The rock mass rating varies significantly over the minesite, from very poor rock in the northeast quadrant, to fair-good quality rock in the west. The generally low RMR values (particularly in the northeast quadrant of the mine) result from the low intact rock strength, the low RQD, and the high • fracture intensity. This combination of adverse slope stability factors makes the stability of the northeast segment, and to a lesser extent the entire mine, problematic. Report 1385E.20071126