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PrK <br /> R � oae Undrround St <br /> verP7to 2Q CO ?r vCtu� -.Y,p III IIIIIIIIIIIII III <br /> sss <br /> TUMMEL OKKNEADS FOR ACID MINE DRAINAM ' <br /> Donald S. Einarson <br /> Chief Design Engineer <br /> American Mine Services, Inc. <br /> and <br /> John F. Abel. Jr. <br /> Professor of Mining Engineering <br /> ' Colorado School of Mines <br /> Abstract <br /> Concrete tunnel bulkheads designed to contain acid mine drainage water <br /> west be: (1) long enough to prevent leakage along the contact between the <br /> concrete and the rock. (2) thick enough to prevent shear failure in either the <br /> concrete or rock, (3) either thick enough to prevent tensile failure of the <br /> downstream face or contain sufficient tensile reinforcement to support the <br /> tensile stress, (4) deep enough to prevent hydrofracturing of the.formation <br /> and' (S) acid resistant enough to last the requisite time interval. ' The <br /> available design data includes in descending order of confidence, the strength <br /> of the concrete and steel, if used, the strength of the rock, the water head <br /> and the is situ stress-field. <br /> The cost of rehabilitating an old mine tunnel dictates that bulkheads be <br /> placed as close to.the portal as possible. The only factor preventing the <br /> placement of a bulkhead in the first competent rock inside the portal is the <br /> necessity to prevent hydrofracturing of the formation. If the hydraulic <br /> pressure behind a bulkhead exceeds the compressive hoop stresses in the rock <br /> adjacent to the bulkhead, acid mine drainage water will probably be lost to - <br /> the,joint system.- <br /> The reinforced concrete bulkhead`designed for the 10 by 10-ft Friday <br /> Lowden Tunnel is California calls for a 14-ft long bulkhead with #8 rebar on <br /> J 9-in. centers-each way. The owner stipulated a safety factor of 2 be applied <br /> to the design. As such, additional concrete thickness is required. The - <br />