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% ~% <br />• Main CressonSlopeEvaluation AdrianBrown <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 drained flows which have averaged 3,000 AFY since. These tunnels were <br />designed to reduce water pressures and mine inflows to underground mines in the district, and they were <br />successful. <br />As part of the Main Cresson Mine routine inspection of the mine, no water emerges from the mine walls, <br />and there is no evidence of water-induced instability. Winter observations are particulazly valuable in <br />monitoring, as influent water freezes into "glaciers" on the walls if there is seepage. No such <br />• accumulations have been observed in the mine. <br />To check the water conditions in the east wall (which will be the highest in the mine), two 750 foot long <br />holes were drilled dipping at an angle of 45° into the mine wall. These are shown as wells CR-969 and <br />CR-967 on Plate 3. Both wells were sleeved with PVC pipe to maintain hole integrity, and any water <br />allowed to equilibrate. After water levels had stabilized, the wells were measured for water level. At that <br />time, the northerly hole (CR-967) was dry, while the southerly hole (CR-969) encountered <br />approximately 70 feet of water in the bottom of the hole3. This indicates that there is essentially no water <br />pressure in the east wall. <br />• 3 This water may be perched, based on the observation that this water was reported as having been encountered somewhat above the point <br />where it finally stabilized in the drill hole. <br />7 385D.980612 11 <br />