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Bulkhead Design for AMD Page 3 October 27-29, 1998 <br />Acid mine drainage bulkheads have several significant unknowns <br />that potentially limit their usefulness: <br />1) What is the acceptable leakage around a tunnel <br />bulkhead, along the contact between the bulkhead and <br />the rock and through the lower permeability rock <br />immediately adjacent to the tunnel? <br />2) What are the natural flow paths for impounded acid <br />mine water that may bypass a bulkhead into the tunnel <br />downstream from the bulkhead or to the ground <br />surface? <br />3) How long will the bulkhead last? <br />9) Will unknown geologic conditions and(or) mine <br />connections prevent the mine pool from reaching the <br />planned elevation? <br />Regardless of the location of a single bulkhead, water <br />impounded upstream of the bulkhead may see the open downstream <br />portion of the tunnel as a significant low resistance path for mine <br />water discharge. The quantity of water forced back into the mine <br />workings or to discharge at the ground surface by a bulkhead versus <br />the quantity discharging into the downstream tunnel of a single <br />bulkhead will depend on the rock substance, directional fracture <br />and structure controlled permeability of the rock formation. <br />BULKHEAD DESIGN CONSIDERATIONS <br />Concrete tunnel bulkheads designed to contain acid mine <br />drainage water must be: <br />(1) long enough to prevent leakage along the contact <br />between the concrete and the rock, <br />(2) thick enough to prevent shear failure in either the <br />concrete or rock, <br />(3) either thick enough to prevent tensile failure of the <br />downstream face or contain sufficient tensile <br />reinforcement to support the tensile stress, <br />(4) deep enough to prevent hydrofracturing of the <br />formation, <br />(5) acid resistant enough to last the requisite time <br />interval and <br />(6) strong enough to resist the maximum credible <br />earthquake. <br />- 3 - <br />