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Memo to Russ Means 3 November 14, 2007 Bulkhead Cost Estimate File No. M-2007-044 The overburden pressure is the product of the height and the density of the rock overlying the bulkhead. Caterpillar (1993) provides a density for shale of 104 pounds per cubic foot. The minimum height of overburden cover for the bulkhead to prevent hydrofracing can be calculated as follows: __ Y x H __ By S°b 144in Z l ft z 2 where: ry=rock density in pounds per cubic foot H = height of overburden in feet Solving for H yields: 72 x By H= Y For the proposed main bulkhead the required minimum overburden height to prevent hyrofracing is 44 feet for the 104 pcf overburden rock densisty and the 64psi maximum hydraulic pressure, as follows: H _ 72 x 64 = 44 feet 104 This design criterion is exceeded at the location of the proposed main bulkhead where the overburden height is approximately 470 feet. Design for Hvdraulic Pressure Gradient, Main Bulkhead The pressure gradient across a bulkhead is the hydraulic pressure, in feet, divided by the length of the bulkhead. Garrett and Campbell Pitt (1961) present a graph indicating an ungrouted plug will withstand a pressure gradient of 21 psi/ft at a safety factor of one. They recommend a minimum safety factor of four in good rock, yielding a recommended maximum pressure gradient of just over 5 psi/ft. Garrett and Campbell Pitt (1961) further indicated that low-pressure grouting of the bulkhead concrete/rock contact would permit pressure gradients of 165 psi/ft without leakage. Applying the safety factor of four produces a design pressure gradient of 41 psi/ft. Using these criteria allows the following calculations of bulkhead length for the pressure gradient component of the design: Ungrouted Bulkhead L = 64psi =12.8 feet 5 psi / ft Low Pressure Grouted Bulkhead L = 64psi =1.6feet 41 psi / ft