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BULKHEAD EVALUATION <br /> Based statements by CMC in the permit documents, the bulkhead consists of a three-foot-thick <br /> concrete structure with some steel reinforcing where the concrete was cast directly against clean <br /> rock but not grouted. While no documentation of the structure exists, the design described is <br /> typical for underground mines and can be assumed to be correct. There are five loading <br /> conditions (water levels behind bulkhead invert) that were evaluated: proposed typical <br /> operational water level (4.5 ft), current water level (15.2 ft), design water level (17.6 ft), <br /> maximum proposed operational level (22.1 ft), and maximum possible water level where the <br /> mine pool overflows through the Wynona Shaft (102.6 ft). <br /> When designing or evaluating mine bulkheads, one must consider the possible failure modes. <br /> These include hydraulic jacking of the surrounding rock mass, shear failure around the plug, <br /> structural failure of the plug, long term disintegration (chemical decomposition) of the concrete, <br /> and excessive seepage or piping past the plug (Lang, 1999; Abel, 1998). <br /> Hydraulic jacking occurs when the water pressure behind the bulkhead is higher than the <br /> confining pressure of the ground in the area. The hydraulic jacking causes joints in the rock <br /> mass to open up, allowing more flow through them. This can be avoided by locating the <br /> bulkhead deep underground where the confining pressure (weight of rock above and related <br /> horizontal stresses) is high enough to resist the water pressure. <br /> The Times Mine Bulkhead has approximately 26 feet of cover. Based in its location near the <br /> inside road edge, most of the cover is likely rock. Assuming 20 feet of rock cover with a density <br /> of 165 pcf results in an overburden pressure of 3300 psf(23 psi). Abel (1998) suggests that under <br /> typical circumstances, the pressure required to hydraulically jack rock is twice the overburden <br /> pressure. Hence, the Times Mine bulkhead could be subjected to 6600 psf(46 psi) of water <br /> pressure from the mine pool without hydraulically jacking. As shown below, hydraulic jacking <br /> becomes a likely failure mode only under the maximum possible mine pool level. <br /> Hydra lic Jacking <br /> Condition Head(ft) Pressure(psi) Allowable Pressure(psi) Factor of Safety <br /> Operation 4.5 1.9 46 24.2 <br /> Current 15.2 6.6 46 7.0 <br /> Design 17.6 7.6 46 6.1 <br /> Max Op 22.1 9.6 46 4.8 <br /> Maximum 102.6 45 46 1.0 <br /> Perimeter shear failure occurs when the bulkhead moves along the concrete/rock interface or <br /> adjacent rock due to water pressure from the mine pool. This failure can be avoided by locating <br /> the bulkhead in good ground, roughening the surface at the rock/concrete interface, keying into <br /> the rock, and grouting the interface. <br /> The shear capacity of the Times Mine Bulkhead can be calculated by multiplying the shear <br /> strength of the concrete by the total area that the concrete is in contact with the wall rock. Unless <br /> - 3 - <br />