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American Tunnel bulkheads #2 & #3 Page 2 January 15, 2001 <br /> between Station 19+96 and Station 20+20, that is on the portal <br /> side of the water bearing fractured rock zone between Station <br /> 27+00 and Station 31+00 . This fractured rock zone with its <br /> associated water inflow coincides with a surface stream course 640 <br /> feet overhead. The water impounded behind Bulkhead #2 will be <br /> forced back into the water bearing fractured rock zone, <br /> potentially partially reestablishing the groundwater regime before <br /> the American Tunnel was driven and provided a release for water <br /> stored in the fractured rock and apparently recharged from the <br /> surface water drainage system. Refilling the fractured rock zone <br /> should significantly reduce the oxidation rate for the sulfide <br /> minerals exposed on the fractured rock, which will be deprived of <br /> the oxygen entering the fractured rock zone from the tunnel and <br /> being carried by the oxygenated surface water currently descending <br /> to the tunnel along the fracture rock zone. The Bulkhead #2 <br /> location will also impound the heavy water seepage entering the <br /> American Tunnel at the Station 20+30 fault zone. <br /> The location for Bulkhead #3 was selected in the relatively <br /> dry and impermeable rock zone between Station 3+67 and Station <br /> 3+82 at a depth of 160 feet. This near portal bulkhead had to be <br /> at a depth greater than 139 feet to assure that it will not be <br /> subject to hydrofracturing at the maximum potential hydraulic head <br /> of 773 feet on the portal side of Bulkhead #2, as shown on Figure <br /> 3 . The Bulkhead #3 location will also impound the flow of water <br /> sheeting down both ribs of the American Tunnel between the Station <br /> 17+20 and Station 18+70 . This appears to be the largest single <br /> water source between the planned locations of Bulkhead #3 and <br /> Bulkhead #2 . The sheet flow on both ribs between Station 17+20 and <br /> Station 18+70 lies directly below the maximum 773-foot head <br /> location. <br /> Bulkhead #2 is designed to resist a water head of 640 feet <br /> and the horizontal acceleration from the maximum credible 6. 5 <br /> magnitude earthquake along the Ridgeway Fault, 25 miles to the <br /> north (Sullivan, et al, 1980; Weisser, 1982; Knight Piesold Co. , <br /> 1988) . The deep beam bulkhead design specifies 10-foot thick, 410 <br /> rebar on 6-inch centers at the downstream (portal side) form face, <br /> #10 rebar on 6. 5-in centers on the upstream (mine side) form face, <br /> 3.5-inch minimum concrete cover between the #10 rebar and the form <br /> faces, Type V sulfate resistant cement with pozzolan and 3, 000 psi <br /> concrete; meets all design requirements, as shown in the Appendix <br /> A calculation sheets . Pipe cut-outs in the upstream and <br /> downstream rebar cage will require 5-foot long, 45' angled, #10 <br /> corner trim bars to eliminate any possibility of concrete corner <br /> cracking when the bulkhead is fully loaded. The upstream #10 <br /> rebar cage is necessary to resist the possible acceleration of the <br /> line-of-site water between Bulkhead #2 and Bulkhead #3. <br /> Bulkhead #3 is designed to resist a water head of 773 feet <br /> and the horizontal acceleration from the maximum credible 6.5 <br />