1993-04-27_REVISION - M1977378 (2)

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L.R. Perino # Page 9 ,rril 27; 1993 Concrete design shear strength (vc) = 110 psi Minimum latite cohesion (c) = 1150 psi :. Concrete shear strength :9 10% of latite strength The concrete shear strength based bulkhead design presented is the more conservative for the higher compressive strength of the rock adjacent to the Sunnyside Mine bulkheads. However, one-way deep beam bending stress developed in the bulkheads required a longer bulkhead and controlled bulkhead design length. The transfer of thrust, either by shear or by compression no doubt explains the absence of bulkhead failures under thrust, as noted by Loofbourow. He stated in the SME Mining Engineer's Handbook (1973, Section 26.7.4) , "no indication of structural failure resulting from thrust was noted" in the case of ten bulkheads subjected to hydraulic pressures in excess of 1000 psi and which relied solely on normal rock irregularities. Garrett and Campbell-Pitt (1961, p 1290) reported the results from 23 grouted bulkheads. Twelve of the bulkheads were subjected to heads exceeding 200 psi, nine of which relied solely on the irregularities of the parallel tunnel walls. Thirteen of the plugs were tapered at 70. The highest hydraulic pressure applied to a parallel wall plug was an experimental 6800 psi test pressure. The highest hydraulic pressure applied to a tapered plug was 650 psi. only one unreinforced, plain concrete plug was considered to be structurally unsound due to visible cracking of the exposed face, apparently in tension, and replaced, but it did not fail structurally. The cracking was attributed to poor construction practice. What Garrett and Campbell-Pitt (1961, p 1292) reported as unacceptable bulkhead leakage rates, from 3.3 to 16.7 gpm, .was arbitrarily determined by individual mines for five bulkheads, two tapered and three with parallel walls, one of which was the experimental 6800 psi experimental plug. The excessive leakage was in every case through the rock adjacent to the bulkheads and not through either the concrete or along the rock-concrete contact. The indication of these studies is that tapering a bulkhead provides no obvious advantage. The South African gold mines have gone exclusively to parallel sided plugs under their hard rock conditions. World Mining magazine (1969, Mar) and A. Louw (1970, Mining Congress Journal, Mar and Apr) present data for four successful 60-foot long, parallel sided, unreinforced, 1000 psi grouted, 2500 psi design strength sand-cement concrete bulkheads, in 10 by 10-foot tunnels and subject to 1620 psi (4000 feet) of hydraulic head in the West Dreifontein Mine. The Canadian Mining Journal (1985, Dec) presents data on an initial unsuccessful 6-foot thick, ungrouted, reinforced concrete bulkhead and a final successful 50-foot long, ungrouted, reinforced concrete bulkhead, in an 8-foot high by 28-foot wide dead-end development entry in soluble potash salt at the Rocanville Mine in Saskatchewan. Each of these