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plane normal to the tunnel. The simplest assumption is for hydrostatic stress <br /> conditions equal to the overburden stress. The assumption is generally <br /> conservative since the overburden stress must be present and the more general <br /> stress state measured is for the horizontal stresses to equal or exceed the <br /> overburden stress. Normal formation breakdown pressures encountered in oil <br /> field work range from 1.4 to 2.8 times the overburden stress. ibis indicates <br /> that the hydrostatic stress assumption, where the breakdown pressure equals <br /> two times the overburden stress, is not unreasonable. <br /> The resulting simplified breakdown pressure equation is: <br /> Sorb - overburden stress (psi) <br /> Therefore, any acid mine drainage bulkhead must be placed at a depth which <br /> will mot result is hydrofracing the rock adjacent to the tunnel, I.e. opening <br /> of the joints and fractures and injection of acid mine water into the rock <br /> mass, wound the plug or to the ground surface. <br /> The hydraulic breakdown pressure (8p)•available to-hydrofrac the rock up <br /> stream fro-the plug and adjacent to the tunnel is the maximce potential <br /> • Bead. Therefore, the overburden stress must be sufficient to prevent <br /> bydrofacing. The required overburden stress (Sovb) is: <br /> Sovb 8p/2 (22) <br /> 7M overburden pressure is the product of the depth (1) and density (Y) of the <br /> overlying rock. Since the density can be readily measured, the depths f the <br /> bulkhead must be selected to limit thi possibility of h�ydrofracing, <br /> follows: <br /> Sovb . Y(N)/144 8p/2 - (23) <br /> Y . density (PCf)• <br /> - depth (ft) <br /> • -14- . _ <br />