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Bulkhead Design for AMD Page 12 October 27-29, 1998 <br />Bulkhead Deoth Based on Hydraulic Pressure <br />Hydrofracturing, generally referred to as hydrofracing, of <br />sedimentary formations from drillholes is frequently undertaken for <br />the purpose of stimulating oil well production. Formation <br />breakdown pressure (Bp) is a function of (1) the tensile strength <br />of the rock immediately adjacent to the drillhole, (2) the in situ <br />stress field in the plane perpendicular to the drillhole and (3) <br />the pore pressure present in the formation. Bredehoeft, et al <br />(1973) presented a study of drillhole hydrofracturing of a <br />competent rock. They presented the following well known equation <br />for breakdown pressure: <br />Bp = Ts + 3S min-Sma r. <br />All terms in psi <br />Ts = <br />S min <br />S max <br />Pt = <br />- Pf (21) <br />tensile strength <br />minimum stress normal to the borehole <br />maximum stress normal to the borehole <br />formation pore pressure <br />The equation can be simplified for the case of hydraulic <br />pressure behind an acid mine drainage bulkhead in a tunnel. First, <br />the tensile strength can be assumed to be zero because the rock <br />adjacent to a tunnel is jointed and generally damaged by blasting. <br />The packed-off section of a drillhole, on the other hand, can be <br />entirely within one joint block and is not subject to blasting . <br />damage. Second, the pore pressure present near surface and <br />adjacent to a tunnel must be low and can also be assumed to be <br />zero. Finally, in the absence of in situ stress measurements it is <br />necessary to estimate the stresses in the plane normal to the <br />tunnel. The simplest assumption is for hydrostatic stress <br />conditions equal to the overburden stress. The assumption is <br />generally conservative since the overburden stress must be present <br />and the more general stress state measured is for near surface <br />horizontal stresses to equal or exceed the overburden stress. <br />Normal formation breakdown pressures encountered in shallow oil <br />field work range from 1.4 to 2.8 times the overburden stress. This <br />indicates that the hydrostatic stress assumption, where the <br />formation breakdown (hydrofracing) pressure equals two times the <br />overburden stress, is not unreasonable. <br />The resulting simplified breakdown pressure equation is: <br />Bp = 25ov6 <br />(22) <br />Sorb = overburden stress (psi) <br />Acid mine drainage bulkheads must be placed at a depth which <br />will not result in hydrofracing the rock adjacent to the tunnel, <br />- 12 - <br />