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Memo to Russ Means 2 November 14, 2007 <br />Bulkhead Cost Estimate File Na. M-2007-044 <br />where: ~I =hydrostatic head (feet) <br />y,~ =water density (62.4 pounds per square foot) <br />p =pressure head (psi} <br />Potential for Hydrofracin~ at the Proposed 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 /> <br />where: B~ =breakdown pressure <br />TS =tensile strength <br />S,„;,, =minimum stress normal to the drill hole <br />5,,,,,x =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 on a mine bulkhead. The tensile strength <br />can be assumed to be zero because the adit wall. rock is jointed and is fractured by blasting, and the pore <br />pressure in and near adit wall rock must be low and can he assumed to be zero. A simple assurnptian is that <br />hydrostatic stress conditions are equal to the overburden stress. This assumption is generally conservative <br />since the overburden stress must be present a~~.d the mare general. stress state rrzeasured is for the hori.rontal. <br />stresses to equal or exceed the overburden stress. ;~brrnal fc~rn-aation breakdovn pressures encountered i.n. <br />oil t~elcl work ra~~ge horn 1.4 to 2.S tirr~es tl}e overburden stress, indic_atin.g that the hydrostatic stress <br />assumption whore the breakdown. pressure equals tvvo times the overburden stress is not unreasc}rRabl.e. 'I`bis <br />analysis yields the following simplified breakclo~~~n equation: <br />B' <br />Soh = 2 <br />where: Snv =overburden stress i.n psi <br />