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Memo to Tom Schreiner 8 February 21, 2006 <br />Bulkhead Cost Estimate Permit No. M-1983-141 <br />in elevation at the surface immediately above the main bulkhead and the elevation at the secondary <br />bulkhead. Hence, the maximum hydraulic head and pressure at the proposed secondary bulkhead are <br />calculated as follows: <br />H = 7984'-7832'=152 feet <br />_ Hyw _ 152 x 62.4 <br />p 144in2 / ft2 144 = 66 psi <br />where: H =hydrostatic head (feet) <br />~„ =water density (62.4 pounds per square foot) <br />p =pressure head (psi) <br />The assumption of little communication of pressure from the main mine pool to the secondary bulkhead is <br />valid for design and cost estimating purposes because the secondary bulkhead would only be constructed <br />after the pressure in the main mine pool has equilibrated. If there were communication of pressure to the <br />adit below the main bulkhead, a second main bulkhead maybe required, and would be designed using <br />actual pressure information collected at that time. <br />Potential for Hydrofracing_at the Proposed Secondary Bulkhead <br />72 x BP <br />H= <br />where: H =height of overburden in feet <br />BP =breakdown pressure <br />y=rock density in pounds per cubic foot <br />For the proposed secondary bulkhead the required minimum overburden height to prevent hyrofracing is 28 <br />feet for the 170 pcf overburden rock densisty and the 66 psi maximum hydraulic pressure, as follows: <br />H _ 72 x 66 - 28 feet <br />170 <br />This design criterion is exceeded at the location of the proposed secondary bulkhead where the overburden <br />height is approximately 65 feet. <br />