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bulkhead is not known. If it is mostly fresh water, per the operations plan, then degradation is not <br /> an issue. If the water becomes more acidic, due to contact with the existing mine pool, then <br /> degradation could be a problem. If water testing reveals sulfate concentrations above 150 ppm, <br /> we suggest conducting a mortar bar test(ASTM C1038)using the mine water and standard <br /> Type-I cement to evaluate the reactivity. <br /> The most likely failure mode of a bulkhead is seepage and piping. Excessive seepage past the <br /> plug occurs when the higher upstream head finds fractures in the downstream rock mass or <br /> concrete-rock interface and bypasses the bulkhead. The worst case would be where the gradient <br /> and seepage are high enough to wash out material in joints or shears, leading to a piping failure. <br /> Seepage and piping can be avoided by placing the bulkhead in good ground, contact grouting the <br /> concrete-rock interface, and ring grouting the rock mass prior to installing the bulkhead. This <br /> requires a detailed geological investigation with a particular focus on joints, shears, and faults <br /> upstream and downstream of the bulkhead that could become water conduits. If seepage is <br /> excessive, it can often be reduced by pressure grouting the affected joints. Seepage and piping <br /> are a direct function of the pressure gradient across the bulkhead. <br /> The design of most mine bulkheads in the United States is based on relationships developed by <br /> Garrett and Campbell-Pitt(1961) and adapted by Chekan(1985). The relationships came from <br /> full scale testing in South Africa where an experimental bulkhead was constructed in quartzite <br /> inside the deep West Driefontein Mine. The researchers installed an un-grouted bulkhead and <br /> pressurized the space behind it until water leakage around the bulkhead became excessive. They <br /> repeated the experiment each time after grouting the contact between the concrete and rock, <br /> grouting the rock mass, and chemical grouting the rock mass. They then calculated the pressure <br /> gradient(pressure at leakage/length of bulkhead) for each case. Finally, they recommended <br /> applying a Factor of Safety between 4 and 10 to these gradients. Their results are summarized on <br /> the table below: <br /> Allowable Pressure Gradient Oft of Bulkhead Length)per Garrett and Campbell-Pitt <br /> Bulkhead Grouting Factor of Safety <br /> 1 4 10 <br /> None 10 2.5 1 <br /> Contact Grouting Only 228 57 23 <br /> Rock Mass Grouting 400 100 40 <br /> RM Chemical Grouting 887 222 89 <br /> The three-foot-thick Times Mine Bulkhead was designed and constructed without grouting, <br /> leading to a maximum allowable pressure gradient of 10 psi/ft. Pressure gradients and associated <br /> factors of safety for each loading scenario are summarized below. For the proposed operational <br /> and current conditions,the pressure gradient is acceptable. For the design and maximum <br /> operational conditions, the pressure gradient is somewhat unconservative (3.9 and 3.1 vs 4). For <br /> the maximum loading condition, the pressure gradient factor of safety is below 1,hence <br /> excessive leakage should be expected. <br /> - 5 - <br />