2011-03-14_REVISION - M1977493 (65)

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CM0001767 • Page 13 cm/sec in-place permeability. In both cases, a 10-7 cm/sec S-B cutoff wall extended from the bottom of Zone 1 to the top of fractured bedrock. Use of the Mosquito Fault material in Zone 1 in-place of a S-B material increased seepage approximately 50 percent or 370 gpd (i.e., approximately 750 gpd versus 1,120 gpd) For the evaluation of varying cutoff wall configurations, seepage through a bentonite-amended soil in Zone 1 with a 10-7 cm/sec S-B cutoff wall extending from the bottom of Zone 1 to the top of fractured bedrock was estimated at approximately 750 gpd. In comparison, seepage through the Mosquito Fault material in Zone 1 and varying widths of Mosquito Fault material used in place of the 10-7 cm/sec permeability S-B cutoff wall was evaluated. If Mosquito Fault material was used in Zone 1 and a 10-foot wide Mosquito Fault cutoff wall is extended from the bottom of Zone 1 to the top of fractured bedrock, seepage would increase approximately 650 gpd (i.e., to approximately 1,400 gpd). If Mosquito Fault material was used in Zone 1 and a 20-foot wide Mosquito Fault cutoff wall. is extended from the bottom of Zone I to the top of fractured bedrock, seepage would increase approximately 350 gpd (i.e., to approximately 1,400 gpd). This later evaluation also showed that if a bentonite-amended soil were used in Zone 1, total seepage would be the same if either the 10-7 cm/sec S-B cutoff wall or the 20-foot wide Mosquito Fault cutoff wall is extended from the bottom of Zone 1 to the top of fractured bedrock. 4.10 Slope Stability Site operations require that water be contained on both sides of the Dam. While the Dam's primary purpose is to contain the Seepwater Pond, operational logistics must include a scenario where the Robinson Lake elevation is higher than the Seepwater Pond elevation, effectively requiring the Dam to contain Robinson Lake. These operational considerations required modeling of the Dam's stability against hydrostatic forces from both sides of the Dam. The purpose of the slope stability evaluation is to determine if the Dam design is stable under static and psuedostatic (i.e., dynamic earthquake loading) loading conditions. Static and dynamic loads on the Dam were evaluated using the computer program GSLOPE, developed by Mitre Software Corporation, to determine the critical failure surfaces based on the model calculations that generate a Factor of Safety (FOS). An acceptable FOS was chosen from the Rules which indicate that varying FOS's are acceptable for varying slope stability evaluations. • c:Vlynn%cypn Nclimm%jwldam%6104mp2.dm 12!03/96 Environmental