1993-04-27_REVISION - M1977378 (2)

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L.R. Perino _ Page 12 _►ril 27, 1993 deep concrete beam where the lower-fiber tension exceeds the tensile strength of the concrete and all the tensile load is supported by the reinforcement. This is, of course, not a shear failure. I am at a loss to devise a realistic method to quantify the potential for failure through the semi-infinite rock mass outside the Sunnyside Mine bulkheads. Bulkhead design must consider the potential for failure which can only occur where possible through the finite dimensions of the bulkhead and the tunnel cross section at the bulkhead location. 9. Diversion Pipe Flow Data Flow through the bypass pipe will be critical during construction of the bulkhead and for a short period thereafter. Given the predicted slow rate of head buildup in the mine pool the American Tunnel bulkhead bypass pipe must function for the construction period plus three days. Garrett and Campbell-Pitt (1961, p 1290) describe applying full load, 828 psi, to the plug on 47 Level of the Free State Geduld Mine 72 hours after completion, as they state, "in the face of a dire emergency". They understate, "In the light of the short period for which the concrete was allowed to set, the stresses were significant." The sand-cement concrete plugs in the West Dreifontein Mine were closed 7 days after bulkhead completion, with a 28-day concrete design strength of 2500 psi (World Mining. 1969, p 22, 23) . In this case the mine was under the operational constraint of having only two additional days of unused underground water storage capacity, despite pumping at the rate of 58500 gpm. 11. Ultimate Life of the Bulkheads Garrett and Campbell-Pitt (1961, p 1292) provide a listing of seven bulkheads that are still functioning. Table 2 is a modified version of their table with additions from World Mining (1969) , Canadian Mining Journal (1985) , Brown (1992) and Einarson and Abel (1990) . Of special note is the West Dreifontein bulkhead which impounded acid mine water, pH = 3.8 (Louw, 1970, p 100) with 2500 psi design compressive strength sand-cement concrete. The acidity of the water was such that milk of lime had to be added to neutralize the mine water to protect the pump impellers. Given even this limited history for bulkheads it seems reasonable to predict a minimum bulkhead life of 50 years. Additional bulkhead life should be achieved for the American Tunnel bulkhead because of the essentially neutral pH of the Sunnyside Mine water passing the location (Simon Hydro-Search, 1992, p 43) , the Type V sulfate resistant cement, the pozzolan added to the concrete mix, the limestone pH buffering placed immediately upstream of the bulkhead and absence of any recharge mechanism for oxygenated water in the submerged stopes to facilitate acid generation and the restriction to movement of that water to the face of the bulkhead. It appears reasonable to