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water flow or by placing a short plug with high resistance to toter flow. The <br /> shear strength of the concrete and the rock gust be sufficient to prevent <br /> shear failure across the design length of plug and along the concrete and rock <br /> interface. The depth of the bulkhead ant be sufficient to prevent <br /> hydrofracing of the formation. Finally, the plug must either be sufficiently <br /> long to accept the initial chemical attack of the acid mine drainage or it <br /> must resist the chemical attack of the acid mine drainage. <br /> Bulkhead Pressure Gradient <br /> The pressure gradient (Pg) across a bulkhead is the hydraulic pressure, <br /> ie psi. divided by the thickness of the bulkhead, in ft. The pressure <br /> gradient for the original 6-ft thick bulkhead in the Friday Lowden Tunnel when <br /> subjected to the measured 212 ft of head was 15.3 psi/ft, calculated as <br /> fol lows: <br /> Pg - 212(62.4)/144(6) - 1S.3 psi/ft (1) <br /> This original bulkhead was neither grouted along the concrete/rock contact nor <br /> .in the rock adjacent to the bulkhead. This measured value serves as a site <br /> specific indication of minimum hydraulic resistance along an ungrouted <br /> cmwete/rock contact. <br /> Garrett and Campbell Pitt (1961) reported the results from 26 mine <br /> bulkheads, more than half of which relied solely on the irregularity of the <br /> tunnel walls. The majority of the bulkheads were neither hitched, tapered nor <br /> reinforced. They reported extensive and obvious leakage along one ungrouted <br /> bulkhead in quartzite at a pressure gradient of 9.3 psi/ft. flowerer, they <br /> presented a graph Aich indicates that an ungrouted plug should be able to <br /> withstand a pressure gradient of approximately 21 psi/ft.-at a factor of safety <br /> of one. They also recommend a minimum factor of safety of 4 in good rockp <br /> yielding a reco�nsWed maximum pressure gradient of just over S . <br /> Garrett and Campbell Pitt indicated that-low-pressure grouting ng o of the <br /> concretelrock contact would permit pressure gradients of 165 psi/ft without <br /> obvious leakage. Applying a factor of safety of four produces a design <br /> pressure gradient of over 41 psi/ft. The indicated benefit from grouting the <br /> concrete/rock interface is an eightfold decrease in bulkhead length. <br /> -4- <br />