1993-04-27_REVISION - M1977378 (2)

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L.R. Perino Page 5 _ -il 27, 1993 report, and that failure in shear along these partings, caused by the applied hydraulic thrust, should be the controlling design criterion if the measured shear strengths are less than the concrete." will now be addressed. Direct shear testing of sawed and surface ground rock samples is not necessary for bulkhead design because 1) scaling of the tunnel roof, walls and floor will remove any loose pieces of rock, 2) the irregularities present in the tunnel walls will induce a normal force across any weaknesses present that parallel the direction of the tunnel walls, 3) the natural weaknesses present in the rock were tested as part of the uniaxial compression samples and 4) blast induced fractures radiate outward from the blastholes and will not provide a path for shear movement and failure, see Figure 1. Direct shear testing of sawed and surface ground rock samples is essential for designing rock slopes. Rock slope failure along natural joint sets requires the residual angle of surface friction and rock-on-rock cohesion for design because the natural joint surface portion of the potential failure surface will be much weaker than the intact rock portion (Abel, 1987) . However, the potential sliding block must move outward into the pit in order to fail by sliding into the pit. The same outward movement of the concrete bulkhead will be necessary to release a bulkhead and allow it to move down the tunnel. In the case of a bulkhead, the opposite wall of the tunnel will resist any such outward movement. The following calculation indicates the magnitude of normal force that would develop in the concrete of the American Tunnel bulkhead in order to override a 1-inch height irregularity. Elastic modulus of the 3000 psi design strength concrete (Ec) (ACI 318-89, Section 8.5) Ec = 57,000 J fc = 57,000 ,/3000 = 3,120,000 psi/(in/in) Average strain (Ec) across 13-foot wide American Tunnel bulkhead concrete Ec = [1/(13xl2) ] = 0.00641 in/in Average concrete stress across 13-foot wide American Tunnel bulkhead Ec = Qc/Ec :• ac = EcEc and Ec — Qc/Ec ac = EcEc = 0.00641(3120000) = 20000 psi