2011-03-14_REVISION - M1977493 (65)

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CM0001764 Page 10 4.5 Cutoff Wall To minimize seepage under the Dam, a cutoff wall will be extended from the bottom of Zone 1 to the top of competent bedrock, as shown on Figure 5. The specified cutoff wall consists of a 24 inch wide bentonite slurry trench backfilled with a soil-bentonite (S-B) mixture that achieves a permeability of 10-7 cm/sec. Seepage analyses evaluations, summarized in Section 4.9, were performed on two cutoff wall material alternatives extending from the bottom of Zone 1 to the top of competent bedrock. As summarized in Section 4.9, seepage through a 24 inch wide S-B cutoff wall is approximately the same as seepage through a 20 foot wide Mosquito Fault material cutoff wall extending between the Zone 1 and fractured bedrock zones. Cost-benefit evaluations performed for the options indicated that the S-B cutoff wall could be replaced with the Mosquito Fault cutoff wall at less than one tenth the cost. The cost saving is accomplished not only from reduced subcontractor labor and materials costs for slurry wall installation but also by replacing foundation backfill with the Mosquito Fault cutoff wall, effectively providing a foundation and cutoff wall in one 1-0 operation. As with the Zone 1 material, if CMC found the Mosquito Fault material acceptable, than it would be cost effective to replace the 24 inch wide S-B cutoff wall with a 20 foot wide Mosquito Fault cutoff wall while backfilling the foundation excavation. Table 4 identifies that approximately 3,100 cy of soil (i.e., as calculated in the `Summary of Construction Materials Calculation Brief provided in Appendix E) and 250 tons of bentonite will be required to backfill the bentonite slurry trench and construct the slurry wall. 4.6 Zone 2 - Outer Core Zone 2 provides support and stability for the Dam. As indicated in Section 4.4, Zone 2 must be more permeable than Zone 1. However, Zone 2 must also include gradation specifications that are compatible (i.e., such that fines do not move between zones effectively weakening the Dam structure) with the Zone 1 gradations. The Zone 2 gradations identified in the Specifications provide a permeability of 10,5 cm/sec and were selected based on laboratory testing of borrow area soils and the filtration criteria evaluation performed in the `Zone Particle Size by Filtration Analysis' calculation brief provided in Appendix E. Table 4 identifies that approximately 11,800 cy of in-place material (as calculated in the `Summary of Construction Materials Calculation c:\nY_,cYruz\a. Mold.m?blb4l;jl-eoc 12107/96 .. ° Environments