2008-04-21_REVISION - M1977300 (2)

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~Ii~R~°JG ~ E~IVIRCJI~~lENT1~L ~~RVI~E~ (._LC metal mine pools, any sulfate is adverse to concrete. Additionally, some individual samples of small pools on the mine floor showed sulfate concentrations as high as 7190 mg/I. As a result, a highly sulfate resistant concrete mix specification, using a flyash addition, was provided (see specification at Tab 7). The specified Sulfate Resistance Factor of 0.75 required the importation of a railcar of a special fly ash from the Bridger plant (see mix data at Tab 9). This highly sulfate resistant flyash was used for both the bulkhead concrete and the portland-flyash grout mix. 2.4 Design Head The design maximum head for the bulkheads was conservatively set at 120 feet. This is greater than the elevation approximately 116 foot difference between the Steve level (6600.56') and the open Charlie level (6716.23'). If the mine filled above the Steve level, water would run out the Charlie level before the design maximum head was reached. Additionally, were the mine to fill significantly above the Steve level, there is at least one adit between the Steve and the Charlie which has been backfilled with rock and soil, which would likely begin to seep before the mine pool reached the Charlie level. Per a-mail correspondence from Susan Wyman at Whetstone Associates, they have determined that the mine pool is not expected to reach the Steve level, except possibly from infiltration in very wet years. A copy of this communication is included at the back of the Structural Calculations section under Tab 3. 2 5 Hydrofracturing Potential at Maximum Head Hydrofracturing potential is the possible fracturing and failure of the rockmass above a bulkhead, as a result of the uplift force from the water head pressure exceeding the downward overburden force of the rockmass. The potential for hydrofracturing was checked using previous rock density data that Dr. John Abel, P.E. had for the Schwartzwalder mine, of 163 pounds per cubic foot (PCF). Based on the weight of water at 62.4 PCF, it was determined that a minimum height of about 46 feet of rock would need to be above each bulkhead for resistance to hydrofracturing ,assuming that the rock has zero tensile strength. A review of the cross-section mine maps provided by Cotter indicates that rock depth above the bulkhead locations are well over 100 feet, and the more recent rock test results (Tab 4) indicate that the lowest measured rock density in the immediate area of the bulkheads was 173.8 PCF, thus providing even more • resistance to uplift. 6