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• GHMXSIopeEvaluation <br />4. BENCH DESIGN AND CONSTRUCTION <br />4.1 Rock Walls <br />The standard wall excavation method that has been developed at the Cresson Project is to mine three, <br />35-foot high faces, with the face pre-split with closely-spaced holes inclined at 79 degrees from vertical. <br />pre-split each bench face to 40 foot depth for each pass, and each third bench leave a single, 35-foot <br />wide bench. Specific details of bench design for each rock type are presented below, with backup <br />computation and illustration in Plate 10. The design for all rock zones is summarized in Plate 11. <br />4.1.1 Vern Poor Rock <br />Walls cut in very poor rock will likely ravel back to an approximately loose condition. Based on the best <br />fit to the shear test data in the very weak materials, the zero-cohesion effective stress friction angle with <br />no confining stress is 50.7°. Assuming that this slope is the average slope after raveling, there is <br />sufficient catch capacity in a 40° overall slope mined at 79° interbench slopes to hold all the ravel <br />material. Indeed, there is sufficient catch capacity to hold ravel back to 27.9°, which is approximately <br />the observed scree pile slope in the original mine (Plate 2). The raveled slope condition determines the <br />slope angle to which this material can be mined, which is estimated at 40° overall, and a 45° slope <br />between major benches. <br />• 4.1.2 Poor Rock <br />The poor rock has sufficient integrity for intact rock samples to be taken from approximately 25% of the <br />material drilled. This indicates that the slope will not entirely ravel when excavated, providing careful <br />blasting is conducted to form the wall slopes. Based on this observation, walls in this zone can be <br />excavated to approach their overall stable slope angle of 60° at 600 feet. While some raveling will take <br />place, a 50° slope provides sufficient catch to prevent bench over-filling. <br />4.1.3 Fair Rock <br />The. fair rock zone contains material that approaches the characteristics of the rock in the rest of the <br />diatreme, except that it is considerably less intact. With careful. slope blasting, the wall might be able to <br />be excavated to its overall stable angle of 60°. However, the fractured nature of the rock indicates that it <br />will not be possible to achieve this slope on the benches due to raveling, so a design slope of 55° has <br />been chosen. It would be expected that over most of the wall in this zone the catch capacity will be <br />substantially utilized. <br />4.2 Blasting <br />The GHMX mine slopes will be located in material that is significantly more fractured, and significantly <br />weaker, than the rock that has been mined in the Cripple Creek diatreme to date. In order to achieve <br />slopes steeper than 30°, the angle of repose of scree formed by this material, it will be necessary to <br />perform blasting with great care near the final walls. <br />• <br />Report 1385E.20071126 <br />