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Bulkhead Design for AMD Page 9 October 27-29, 1998 The available design data includes in descending order of confidence, the strength of the concrete and steel, if used, the strength of the maximum credible earthquake, the strength of the rock, the maximum possible water head and the in situ stress field. Design of a concrete bulkhead can proceed once the mine layout and maximum possible hydraulic head are known and the bulkhead location selected on the basis of known hydrologic conditions and rock properties. The bulkhead location must first be prepared by removing rock loosened during the tunnel excavation. Hydraulic Pressure Gradient The pressure gradient (P9) across a bulkhead is the hydraulic pressure, in psi, divided by the thickness of the bulkhead, in feet. Figure 1 presents the types of water-impoundment bulkheads generally used. It should be noted that the typical "taper plug", such as shown on Figure 1 is 7°. A bulkhead for a tunnel must be in intimate contact with the tunnel walls to prevent leakage along the concrete-rock interface around the plug. Bulkhead failure by leakage around the bulkhead, in the case of mine bulkheads, is more likely than failure of the bulkhead under thrust. Loofbourow in the Society of Mining Engineers (SME) Mining Engineering Handbook (1973, Sec 26.7.4) states "no indication of structural failure resulting from thrust was noted" in the case of ten bulkheads subjected by hydraulic pressures in excess of 1000 psi and which relied solely on normal rock surface irregularities, referred to as a "parallel plug" on Figure 1. High hydraulic pressure differentials across a bulkhead can be achieved by placing a long plug with a low resistance to water flow along the concrete-rock interface or by placing a short plug with high resistance to water flow along the concrete-rock interface achieved by grouting the concrete-rock contact. The Mining Engineering Handbook also recommends, in the .same section, 40 to 25 feet of plug length for each 1000 psi of hydraulic head, i.e. pressure gradients from 25 to 40 psi/ft. The recommended concrete-rock grout pressure is "a few hundred psi!'. In practice, the grouting pressure must be kept below the formation breakdown pressure to prevent hydrofracturing. This limitation is particularly important for near surface bulkheads in order to prevent opening of fractures and possible release of impounded water through the formation to the open tunnel downstream or possibly even to the ground surface. Garrett and Campbell Pitt (1961) reported the results from 26 mine bulkheads, 12 "parallel plugs", that relied solely on the irregularity of the tunnel walls, and 14 "taper plugs". However, they presented field data for 7 ungrouted bulkheads indicating acceptable leakage and pressure gradients from 18.0 to 26.2 psi/ft, averaging 21.4 psi/ft. The pressure gradient for the original - 4 -