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