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Memo to Tom Schreiner 2 February 21, 2006 <br />Bulkhead Cost Estimate Permit No. M-1983-141 <br />1001eve1 adit, such discharge would be a regulated point source necessitating either alit plugging or water <br />management that may include long term water treatment. If the 100-level adit were to be plugged and <br />discharge at the Cash Shaft collaz were to result, then the Cash Shaft may have to be plugged. For this <br />reason, DMG requires that the base level main bulkhead be designed for maximum potential hydraulic <br />head. This design basis has been employed at other permitted mines in Colorado, including the Sunnyside <br />Mine (Permit No. M-1977-372) and the Henderson Mine (Permit No. M-1977-342). The maximum <br />hydraulic head and pressure at the proposed main bulkhead aze calculated as follows: <br />H = 8447'-7835'= 612 feet <br />_ Hyw _ 612 x 62.4 <br />= 265 sz <br />P 144in~ l ft~ 144 P <br />where: H =hydrostatic head (feet) <br />~„ =water density (62.4 pounds per square foot) <br />p =pressure head (psi) <br />Potential for Hydrofracing_at the Pronosed Main Bulkhead <br />The bulkhead must by constructed at a depth below ground surface that will provide sufficient overburden <br />pressure to prevent hydrostatic pressure from the impounded water hydrofracing the rock surrounding the <br />bulkhead. The hydrostatic pressure at which hydrofracing will occur is the formation breakdown pressure. <br />Intentional hydrofracing of rock from within drill holes is frequently undertaken by the petroleum industry <br />for the purpose of stimulating oil well production, and as a result has been intensively studied and is well <br />understood. In oil field applications, formation breakdown pressure {BP) is a function of (1) the tensile <br />strength of the rock immediately adjacent to the drill hole, (2) the in situ stress field in the plane <br />perpendicular to the drill hole, and (3) the pore pressure present in the formation. Bredehoeft, et al (1973} <br />presented the following equation for breakdown pressure: <br />Bo =T,+(3xS„o„)-S„,~ -Pf <br />where: Bp =breakdown pressure <br />TS =tensile strength <br />S,„i„ =minimum stress normal to the drill hole <br />S,„„~ =maximum stress normal to the drill hole <br />Pf=formation pore pressure <br />all terms in psi <br />The equation can be simplified for the case of hydraulic pressure behind a bulkhead in an adit. The tensile <br />strength can be assumed to be zero because the adit wall rock is jointed and is fractured by blasting, and the <br />pore pressure in and near adit wall rock must be low and can be assumed to be zero. A simple assumption <br />is that hydrostatic stress conditions are equal to the overburden stress. This assumption is generally <br />conservative since the overburden stress must be present and the more general stress state measured is for <br />