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Memo to Tom Schreiner 3 February 21, 2006 <br />Bulkhead Cost Estimate Permit No. M-1983-141 <br />the horizontal stresses to equal or exceed the overburden stress. Normal formation breakdown pressures <br />encountered in oil field work range from 1.4 to 2.8 times the overburden stress, indicating that the <br />hydrostatic stress assumption where the breakdown pressure equals two times the overburden stress is not <br />unreasonable. This analysis yields the following simplified breakdown equation: <br />B <br />Sob 2 <br />where: S°b =overburden stress in psi <br />The overburden pressure is the product of the height and the density of the rock overlying the bulkhead. <br />Caterpillar (1993) provides a density for granite of 170 pounds per cubic foot. The minimum height of <br />overburden cover for the bulkhead to prevent hydrofracing can be calculated as follows: <br />__ Y x H __ Ba <br />Sob 144in2 / ft 2 2 <br />where: y=rock density in pounds per cubic foot <br />H =height of overburden in feet <br />Solving for H yields: <br />72xBp <br />H= <br />For the proposed main bulkhead the required minimum overburden height to prevent hyrofracing is 112 feet <br />for the 170 pcf overburden rock densisty and the 265 psi maximum hydraulic pressure, as follows: <br />H _ 72 x 265 =112 feet <br />170 <br />This design criterion is f;xceeded at the location of the proposed main bulkhead where the overburden <br />height is approximately 149 feet. <br />Design for Hydraulic Pressure Gradient, Main Bulkhead <br />The pressure gradient across a bulkhead is the hydraulic pressure, in feet, divided by the length of the <br />bulkhead. Garrett and Campbell Pitt (1961) present a graph indicating an ungrouted plug will withstand a <br />pressure gradient of 21 psi/ft at a safety factor of one. They recommend a minimum safety factor of four in <br />good rock, yielding a recommended maximum pressure gradient of just over 5 psi/ft. Garrett and Campbell <br />Pitt (1961) further indicated that low-pressure grouting of the bulkhead concrete/rock contact would permit <br />pressure gradients of 165 psi/ft without leakage. Applying the safety factor of four produces a design <br />