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• GHMXSIopeEvaluation <br />3. The factor of safety of a 600 foot high slope was evaluated for 20,000 realizations, using strength <br />parameters randomly selected from the strength distribution. <br />4. The results are presented in a probability of stability for a given slope angle. The probability of <br />stability is the percentage of the 20,000 realizations for which the factor of safety equaled or <br />exceeded unity. <br />Two strength sets were considered: <br />1. The strength of disturbed samples of the rock material (which was for the most part recovered <br />from drilling as rubble). This material had a (logarithmic) mean friction angle of 39.7° and a <br />(logarithmic) mean cohesion of 6.6 psi. <br />2. The estimated strength of intact samples of the rock material, which was determined by adding <br />the average intact shear strength of the two intact samples of the material recovered (207 psi) to <br />the disturbed parameters, giving a (geometric) mean friction angle of 39.7° and a (geometric) <br />mean cohesion of 213.6 psi. <br />3.2.2 Results <br />Based on the statistical block analysis, an overall slope angle of 45° was determined to be likely to be <br />stable in the northeast quadrant of the GHMX. For this slope, there is a 50% probability of stability <br />• using the disturbed strength parameters, and a 95% probability of stability using the estimated intact <br />strength parameters. <br />This result was considered to generally confirm the overall strength analysis performed by the Hoek and <br />Bray method, reported above. The slightly higher stable slope angle predicted by this method indicates <br />that the factor of safety of 1.1 used in the Hoek and Bray method is conservative, in that it gives a high <br />probability of stability. A maximum overall slope of 45° will be used for design in this material. <br />3.3 Unconsolidated Material <br />The upper benches of the GHMX may encounter unconsolidated material, comprising alluvium, <br />colluvium, and/or mine backfill overburden. This material has not been tested for material parameters, <br />so the design of slopes is conducted using typical values for this material (ABC, 2004): <br />Angle of internal friction: 38° (range 34° to 42°) <br />Cohesion: 0 psf (range 0 to 575 psf) <br />The alluvial materials likely have some cohesion, based on their ability to stand at angles clearly greater <br />than their angle of repose. However, to ensure that the slopes that are designed are stable, it is proposed <br />that the inter-bench angle of these materials is designed at the expected angle of repose. In addition, a <br />modest catch bench of 10-foot width is proposed at the base of each bench. <br />Based on this design, the overall slope in unconsolidated material is expected to be approximately 36 <br />degrees. <br />• <br />Report1385E.20071126 <br />