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3 November 14, 2007 <br />yiemo tQ Russ Means File No. M-2007-044 <br />Bulkhead Cast Estimate <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 shale of 104 pounds per cubic foot: The minimum height of <br />overburden cover for the bulkhead to prevent hydrofracing can be calculated as follows: <br />_ yxll __ BP <br />S°b 144in' l ft` 2 <br />where: y=rock density in pounds per cubic foot <br />H = height of overburden in feet <br />Solving for H yields: <br />72 x Bn <br />H= <br />Y <br />For the proposed main bulkhead the required minimum overburden height to prevent hyrofracing is 44 feet <br />for the 104 pcf overburden rock densisty and the 64 psi maximum hydraulic pressure, as follows: <br />72x64 <br />H ` 104 - - 44 feet <br />This design criterion is exceeded at the location of the proposed main bulkhead where the overburden <br />height is approximately 470 feet.. <br />Desi n far H draulic .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 (19G1) 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 o.f four i.n <br />goad rack, yielding a recommended maxi:am. pressure gradi.erst of just over psi/ft. Garrett anal Campbell <br />Pitt (19G.1.) further indicated that low-pressure groutiog ofthe bulkhead concrete/rock contact would permit <br />pressure gradie~-~ts of 1~~ psi.Ift without leakage. Applying the safety factor of four prod€~ces a design. <br />pressure gradient af41 psi/f1. lasing these criteria allows the fallor~ing calculations o.f;oulkh.ead length for <br />the pressure gradient component of the design: <br />64 psi <br />Ungrauted Bul.k.head L = - =1.2.$ feet <br />Spsil~ <br />G4 psi = l .z> ~rE,t <br />[.;o~v Pressure Grouted Bulkhead. ~ M 41. psi 1 fr. f <br />