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XV <br /> the bulkhead and concrete must not be destroyed by corrosion or chemical <br /> reaction with the acid pine water during the life of the plug. <br /> The bulkhead design for the 10-ft wide by 10-ft high Friday Lowden Tunnel <br /> near Redding. CA is presented as an example bulkhead design. A plan and <br /> section of the Friday Lowden portal area is presented in Figure 1. The Friday <br /> Lowden Tunnel served as a haulage tunnel for the Mammoth Mine. A 6-ft thick <br /> lightly reinforced concrete bulkhead had already been designed and installed <br /> 2SS ft in from the portal at an overburden depth of 62 ft. This concrete <br /> bulkhead was not grouted, either at low pressure along the concrete/rock <br /> contact or at high pressure in the rock adjacent to the tunnel. This bulkhead <br /> had been tested with an hydraulic head of 212 ft. There was no leakage around <br /> this bulkhead at the 92 psi hydraulic pressure. <br /> This bulkhead was not effective because the acid mine drainage simply was <br /> diverted to the North Lower Gossan portal of the lowest mining level, <br /> approximately 212 ft above the Friday Lowden bulkhead. Therefore, it was <br /> "-� necessary to remove the existing bulkhead, design a new bulkhead for the <br /> Friday Lowden and design bulkheads for the three higher mining level <br /> portals. The design maximum hydraulic head is the head that could develop <br /> when the Mammoth Mine is filled to the level of the lowest point around the <br /> surface subsidence, cave, area on the east face of Mammoth Butte. Obviously, <br /> It would not be possible to hydraulically seal the surface subsidence <br /> fractures and the cave above the major collapsed mining slopes in the Mammoth <br /> Mine. This results in a design hydraulic head of 670 ft for the Friday-Lowden <br /> Tunnel. <br /> Bulkhead Oesiq <br /> A bulkhead for a tunnel must be in intimate contact with the tunnel walls <br /> to prevent leakage around the plug. Bulkhead failure by leakage around the <br /> bulkhead, in the case of mine bulkheads, is more likely than failure of the <br /> bulkhead under thrust. Loofbourow in the Society of Mining Engineers (SHE) <br /> Mining Engineering Handbook (1973, sec 26.7.4) states *no indication of <br /> structural failure resulting from thrust was noted' in the case of ten <br /> bulkheads subjected by hydraulic pressures in excess of 1000 psi and which <br /> relied solely on normal rock surface irregularities. High hydraulic pressure <br /> gradients can be achieved by placing a long plug with a low resistance to <br /> -3- <br />