1993-04-27_REVISION - M1977378 (2)

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1 � L.R. Perino Page 3 &rril 27, 1993 worst case Tan = 20, the cohesion of the Sunnyside Mine Latite will be at least 1150 psi. In addition, Farmer (1983, p 11) presented a probable minimum strength for basalt, a similar volcanic rock, of 14500 psi with a minimum cohesion of 2900 psi. Farmer's basalt data indicates a Tan R for his lowest strength basalt of 6.25. The implication is that the minimum cohesion for the weakest of the latite uniaxial compressive strength core samples tested of 10260 psi would be on the order of 2050 psi. The design shear strength (vc) for the concrete is 110 psi, somewhere between approximately 5 percent and 10 percent of the estimated minimum cohesion of the jointed, fractured but partially rehealed latite. It did not seem necessary to perform triaxial testing because it is not physically possible for a rock with a compression strength of at least 10000 psi to have a cohesion lower than the 110 psi design shear strength of the concrete. The resistance to shear of the rock-concrete interface, i.e. bond strength (f.) , was briefly discussed on page 14 of the March 10, 1993 report. As indicated, Merritt (1983, p 7-62) reported in his Standard Handbook for Civil Engineers that, "Typical values range from 70 to 200 psi, increasing with rock quality." Merritt stated that, "For rock with RQD > 50%, f. can be estimated as 0.05 fc or 0.05 UC, whichever is smaller, except that f. should not exceed 250 psi." The relatively massive rock present at the proposed bulkhead locations and the core recovered during drilling indicate that RQD (Rock Quality Designation) values at the proposed bulkhead locations should be closer to 90% than 50%. Merritt was referring to friction bearing rock socketed piers. The geometry of friction bearing piers approximates the geometry of a bulkhead except for the smaller diameter and greater smoothness of drilled rock sockets versus blasted rock tunnel surfaces. Therefore, a 70 psi bond strength for the rock-concrete would be conservative for design and 150 psi (0.05 times the 3000 psi concrete design strength - fe ) would be a realistic design bond strength. The potential application of the rock-concrete bond strength would be for the unrealistic assumption that no irregularities were present along the tunnel roof, ribs and floor. If it is very conservatively assumed that the tunnel walls are perfectly smooth, the tunnel cross section is circular and the rock-concrete bond strength is a minimal 70 psi the calculated bond resistance is 10,300,000 lbs to resist the 16,300,000 lbs of maximum thrust, factor of safety (FS) of 0.63. Applying the circular cross section, the perfectly smooth walls and the probable 150 psi bond strength yields a calculated bond resistance of 22,100,000 lbs, FS = 1.35. Simply changing the geometry to a more realistic square cross section results in a bond resistance of 13,100,000 lbs for the very conservative 70 psi bond strength (f.) , FS = 0.80, and 28,100,000 lbs for the probable 150 psi bond strength, FS = 1.72.