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Memo to Russ Means 5 November 14, 2007 <br />Bulkhead Cost Estimate File No. M-2007-044 <br />With their close proximity, the second decline bulkhead can be constructed using the same design as the <br />main bulkhead, with the exception that a bypass pipe will not be needed, and a pressure monitoring tube <br />will be required. The criteria for the contingency of a Pack Rat Mine bulkhead is determined as follows. <br />P- HZw 2 _268x62.4=116psi <br />144in / ft 144 <br />where: H =hydrostatic head (feet) <br />~„ =water density (62.4 pounds per square foot) <br />p =pressure head (psi) <br />Potential for Hydrofracine at the Proposed Secondary Bulkhead <br />72xBp <br />H= <br />Y <br />where: H =height of overburden in feet <br />BP =breakdown pressure <br />ry=rock density in pounds per cubic foot <br />For the proposed secondary bulkhead the required minimum overburden height to prevent hyrofracing is 80 <br />feet for the 104 pcf overburden rock densisty and the 116 psi maximum hydraulic pressure, as follows: <br />H_72x116=80 feet <br />104 <br />This design criterion is exceeded for essentially any selected bulkhead location in the Pack Rat Mine. <br />Design for Hvdraulic Pressure Gradient. Secondar~Bulkhead <br />Ungrouted Bulkhead L = 116psi = 23.2 feet <br />Spsi/ ft <br />Low Pressure Grouted Bulkhead L = 116psi = 2 8 feet <br />41psi / ft <br />Clearly, with an eight fold decrease in required bulkhead length, and the difficulty in concrete pouring for <br />bulkheads greater than 10 feet in length, low pressure grouting is a necessity for the Pack Rat Mine <br />contingency bulkhead. <br />