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• WHEXSIopeEvaluation Adrian6rown <br />4. WATER <br />Water can destabilize mine walls due to a reduction in effective stress on potentially sliding surfaces, <br />and due to an increase of driving pressure on the potential failure mass (Brown, 1981). The combination <br />of these two effects is that mine wall stability is a strong function of groundwater conditions. <br />The entire Cripple Creek mining district, including the Main Cresson Mine, is underdrained by historical <br />workings, which has been historically dewatered by a system of drainage tunnels. The drainage tunnels <br />include the Moffatt (at approximately 9,000 feet elevation), the Roosevelt (at approximately 8,000 feet <br />elevation), and the Carlton (at approximately 7,000 feet elevation). The tunnels have been removing <br />water from most of the mines in the district (either by direct connection, or by drainage via permeability <br />in the rock) since before the turn of the century. The principal drainage tunnel, the Carlton, was <br />completed in 1941, and has had an average flow of 3,000 AFY since. These tunnels were designed to <br />reduce water pressures and mine inflows to underground mines in the district, and they were successful <br />in dewatering the diatremal rocks, appazently over most if not all of the district. <br />To check the water conditions in the vicinity of the WHEX project, water levels were measured in all <br />holes that were completed as part of this program. In addition, logs for all air-drilled holes were <br />reviewed to establish the water conditions in the rockmass. The results of this evaluation are as follows: <br />• 1. Granitic Rocks. The granitic rocks appear to be of low permeability, and thus may not drain to <br />the drainage tunnel system in the diatreme. Water was encountered in a number of the wells <br />tested in the granite, notably GT03-O1 and GT03-02. The water fills at most the bottom 11% of <br />the hole in each case, suggesting that there are either pockets of water encountered, or that the <br />entire rockmass is saturated, and that the equilibrium level in the well reflects the inflow and <br />outflow balance in the well. The latter interpretation is most conservative for slope analysis, and <br />will be used here. <br />2. Volcanic Rocks. All wells drilled in the vicinity of the proposed WHEX project in volcanic rocks <br />are dry, with two exceptions. Well GR-477, located on the western end of the wall, encountered <br />approximately 20-30 gpm water at a depth of 280 feet, rising to approximately 50 gpm when the <br />hole was abandoned at 410 feet. The water level stabilized in the hole at approximately 50 feet <br />below ground surface. Well GR-418, 500 feet east, encountered approximately 30-40 gpm of <br />water at 720 feet, causing the abandonment of the hole at 770 feet. All other holes in the area <br />were either reported as being drilled dry, or no mention was made of water being encountered <br />during drilling. Based on these findings, water pressure may be a factor for the north wall, with <br />some locations perhaps experiencing full hydrostatic pressure for some periods of the year. <br />• <br />Report 1385L.200309017 <br />