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Bulkhead Design for AND Page 2 October 27-29, 1998 <br />(2) thick enough to prevent shear failure in either the <br />concrete or rock, <br />(3) prevent tensile failure of the downstream bulkhead <br />face, <br />(9) deep enough to prevent hydrofracturing of the <br />formation and <br />(5) acid resistant enough to last the requisite time. <br />The available design data includes, possibly in descending order of <br />confidence, the strength and corrosion resistance of the concrete <br />and steel, the strength of the rock, the maximum possible water <br />head, the magnitude of the maximum credible earthquake and the in <br />situ stress field. <br />INTRODUCTION <br />Ideally, impounding acid mine drainage behind a drainage <br />tunnel bulkhead should reestablish the pre-mining groundwater <br />regime. That hasn't happened and isn't likely to happen in the <br />future. Even partially filling old mine workings should, however, <br />be beneficial. Partial filling should raise the mine depressed <br />water table to the mine pool elevation. Partial filling of mine <br />workings should slow the rate and decrease the quantity of <br />groundwater entering mine workings. Partial filling should deprive <br />the submerged sulfide minerals of most of the oxygen necessary for <br />producing acid, decreasing the rate of acid generation. In <br />addition, partial filling should improve the quality of acid water <br />discharges from the mine. In effect, bulkheads can help but will <br />never be a complete cure for acid mine drainage. <br />Historically, and logically, mineral deposits have been <br />exploited from the top down. This has resulted in many near <br />surface access openings at the deposit outcrop. Some of the <br />surface openings interconnect and some don't. Plugging the lowest <br />draining portal may or may not significantly raise the level of the <br />mine pool. Later in the life of a mine and a mining district, the <br />deeper mine workings must be dewatered by pumping in order to <br />continue mining. In such deeper mining operations, low level <br />drainage tunnels may have been driven. Drainage tunnels have the <br />potential, if plugged, for impounding water in a large part of the <br />total mine excavation. Under no reasonable scenario, however, will <br />plugging a single mine opening raise the mining depressed water <br />table to its pre-mining level. <br />- 2 - <br />