2000-08-25_REVISION - M1977378 - Laserfiche WebLink

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Terry Tunnel Bulkhead #2 Page 3 August 17, 2000 2, 256 feet of essentially dry roof and walls immediately downstream from the bulkhead. The water head behind the existing Terry Tunnel bulkhead appears to have stabilized at approximately 92 feet (40 psi) , much less than the 650 feet (282 psi pressure head) it was designed to resist from the Sunnyside Mine pool or the very conservative 469-foot design head (203 psi) for the near portal Terry Tunnel bulkhead. Given existing hydraulic conditions in the Sunnyside Mine water impounded behind the new bulkhead could reverse the pressure on the existing Terry Tunnel bulkhead and apply 377 feet of hydraulic head (163 psi pressure head) to its downstream face. The existing Terry Tunnel bulkhead is capable of resisting a reverse head. The near portal Terry Tunnel bulkhead is potentially subject to the force from the maximum credible 0 . 087 ft/sec2 earthquake acceleration of the 3500 feet line of sight water impounded between the two Terry Tunnel bulkheads . This is a significant potential load, but the design calculations in Appendix A demonstrate that earthquake acceleration does not control bulkhead design. The bulkhead design; 7-foot thick, #10 deep-beam flexural rebar on 6-inch centers at the downstream face, 3 . 5-inch minimum concrete cover between the #10 rebar and the downstream form face, #6 temperature and shrinkage bars on 12-inch centers at the upstream face, 3-inch minimum concrete cover between the #6 rebar and the upstream form face and 3, 000 psi concrete; meets all design requirements, as shown in the Appendix A calculation sheets . Pipe and inspection cut-outs in the downstream rebar cage will require 5-foot long, 450 angled, #6 corner trim bars, as shown on Figure 2, page 24 of the March 10, 1993 "Bulkhead Design for the Sunnyside Mine" report to eliminate any possibility of concrete corner cracking when the bulkhead is fully loaded. After the walls are scaled, the rock wall surfaces should be either parallel to the tunnel axis or tapered. Irregularities along the scaled rock wall should equal or exceed 0 . 15-inch in order to fully mobilize the design concrete shear strength. Eight percent (4, 200 in2) of the 51, 800 in of the tunnel perimeter along the 7-foot long bulkhead is required as latite irregularities projecting at least 0 . 15 inches into the tunnel to assure that the design concrete shear strength is achieved at the 203 psi design pressure head. Sincerely,)W, , Q j �. John F. Abel, Jr. Colorado P.E. 5642