1993-04-27_REVISION - M1977378 (2)

Home Browse Search

L.R. perino page 14 __ 3ril 27, 1993 anticipate a life for the American Tunnel Bulkhead exceeding 100 years. If a bulkhead is installed downstream of the American Tunnel bulkhead and on the portal side of the steel set supported fault zone the downstream face should be protected from chemical attack by the acidic water, approximately 460 gpm at a calculated pH of 2.5, entering the American Tunnel at the fault zone. In this case a similar buffering wall of crushed limestone should be erected against the downstream face of the American Tunnel bulkhead before the downstream bulkhead is constructed. The question is asked on page 8, "What, in descending order of probability of occurrence, are the likely failure modes for the bulkheads at the end of their functional life?" 1) The most probable cause of the Sunnyside Mine bulkheads ceasing to function is unacceptable leakage through fractures around the bulkheads. This mode is historically the most common bulkhead failure mode. Historically excessive leakage has been corrected by either higher pressure grouting at the rock-concrete contact to close permeable fractures or by increasing the length of the bulkhead. Leakage of corrosive mine water through the rock immediately downstream of the bulkhead face will increase slowly as the impoundment pressure increases. The bulkhead could, under this scenario, be subject to chemical attack on the bulkhead concrete exposed on the downstream face. The lower 5.9 pH of the Terry Tunnel water (Simon Hydro-Search, 1992, p 47) would probably, under this scenario, be the first bulkhead to leak at an unacceptable rate. Leakage at the immediate airside face of the Terry Tunnel bulkhead will tend to oxidize as it comes in contact with air. Damage to the airside of a bulkhead should be visible when inspected. The fact that the bulkheads were designed to carry 1.4 times the maximum possible load, i.e. the maximum Lake Emma outflow design head, and to function as one-way beams means that even if the reinforcing steel is lost the bulkheads should still function effectively as plain concrete bulkheads until considerably shortened. For example, a plain concrete Terry Tunnel bulkhead should not actually approach failure until its 8-foot length is reduced to 6.5 feet. In the case of the American Tunnel bulkhead it should not fail until its 25-foot length is actually reduced to 12 feet. 2) The next most probable mode of failure of a bulkhead to function properly would result from failure to properly construct the bulkhead. A bulkhead could cease to properly retain water if the grouting were incomplete along the rock-concrete interface. A minor leak along the contact could cause progressive erosion and(or) corrosion of the bulkhead. Such a minor leak should be detected by inspection during impoundment, and could be corrected by additional grouting. A construction deficiency that could cause premature failure of the American Tunnel bulkhead to function would be failure to properly encase the bypass pipe in concrete. This should also be detected by inspection during impoundment, and could be corrected by grouting along the bypass pipe.