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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 />Hy,? _ 268 x 62.4 =116 si <br />P _ ^ 144in2 / ftz 144 <br />where: H = hydrostatic head (.feet) <br />,y+ = water density (62.4 pounds per square foot) <br />p = pressure head (psi) <br />Potential for Hydrofracing at the Proposed Secondary Bulkhead <br />72xBp <br />H = <br />Y <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 80 <br />feet for the 104 pcf overburden rock densi.sty and the 116 psi maximum hydraulic pressure, as follows: <br />H _ 72 x 116 = 80 feet <br />104 <br />This design. criterion is exceeded for essentially any selected bulkhead location in the Pack Rat Mine. <br />Design for Hydraulic Pressure Gradient. Secondary, Bulkhead <br />Ungrouted Bulkhead L = .__1..1€??''.t...: _ 2-1.2 feet <br />5Psi/;t <br />?si <br />_ow Pressure Grouted Bulkhead It = __ 1 ........ l6_ ............... = 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.