1993-04-27_REVISION - M1977378 (2)

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L.R. Perino Page 8 April 27, 1993 The 3000 psi design strength (fc) concrete cannot support 20000 psi. An irregularity with the height of 0.15 in would result in 3000 psi in radial compression across the tunnel, as follows: Cc = (0.15/[13 (12) ) )106 = 962 micro-strain Ec = 0.000962 (3120000) = 3000 psi The frictional resistance to sliding of rock on concrete would increase as the irregularity induced radial stress increased. At the design 3000 psi maximum radial stress the frictional resistance to sliding would be approximately 1950 psi. This calculation is based on Kvapil 's tabulation of friction factors (1965, p 284) for selected rock on concrete and the use of a conservative coefficient of friction of 0. 65, see Table 1. Garrett and Campbell-Pitt (1961, p 1287-1289) presented two South African bulkhead design procedures, one based on shear strength and the other on compression strength. The shear strength design method, which involves a more conservative punching shear design for bulkheads, relies on the shear strength of either the concrete or the rock, whichever is smaller. This is basically the method employed in calculating the shear strength of the rock-concrete contact for the Sunnyside Mine bulkheads. The other compressive strength design method relies on the natural rock surface irregularities to transmit the applied thrust to the adjacent rock. The compressive design strength recommended is, "the safe compressive stress of the concrete or rock, whichever is the lesser". They assume a mean irregularity angle of 450, i.e. joints and fractures producing tunnel wall irregularity are possible at any angle from parallel to normal to the tunnel axis, that half of the potential compression bearing surfaces face upstream and half downstream and that the downstream facing joint surfaces carry no compressive load. The compressive design strength method almost always produces a shorter, less conservative, bulkhead design length in the situation where the rock compression strength is greater than the concrete compression strength. This results because the design shear strength of concrete is approximately 5 to 10 percent of the minimum design rock cohesion, i.e. rock shear strength. The concrete design compression strength is normally more than 10 percent of the rock compression strength. The specific Sunnyside Mine design conditions are, as follows: Concrete design compression strength (fc) = 3000 psi Minimum latite compression strength (UC) = 10000 psi :. Concrete compression strength < 30% of latite strength