2011-03-14_REVISION - M1977493 (65)

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CM0001763 • Page 9 s • 4.4 Zone 1- Inner Core Zone 1 will consist of a low-permeability material that will minimize seepage through the Dam. The permeability of the Zone 1 material is the critical design criteria to control seepage through the Dam. A permeability of 10-7 cm/sec was designed to limit seepage. This permeability will be achieved by amending on-site borrow soils (i.e., other than from the Mosquito Fault Borrow Area) with bentonite. The design evaluation indicates that a six percent bentonite addition (i.e., requiring approximately 750 tons of bentonite) is a conservative percentage of bentonite required to reduce the on-site borrow soil's permeability to 10-7 cm/sec. Table 4 indicates that approximately 6,700 in-place cy (i.e., as calculated in the `Summary of Construction Materials Calculation Brief provided in Appendix E) will be required to construct the Zone 1 inner core. The quantity required to produce the Zone 1 material gradation was checked against the quantity available from the borrow areas as detailed in the `Soil Quantity Required and Available by Particle Size' calculation brief provided in Appendix E. The Zone 1 material gradations identified in the Specifications were developed in the `Zone Particle Size by Filtration Analysis' and `Soil Quantity Required and Available by Particle Size' calculation briefs provided in Appendix E. The gradation evaluations for the various Dam zones are specified to be less permeable in the Zone 1 inner core and progressively more permeable moving from Zone 1 and through Zone 2 (i.e., the outer core), Zone 3 (i.e.', the rockfill) and the riprap face. The purpose of these varying zone gradations is to minimize Dam stability impacts in the event of rapid drawdown conditions. A cost-benefit analysis was performed to determine the difference in cost and seepage rates between using the Mosquito Fault material with a permeability of 10-6 cm/sec in Zone 1 versus the bentonite-amended soil. Hauling and stockpiling of Mosquito Fault material using CMC labor and hauling equipment and renting excavation equipment would cost approximately 75 percent less than purchasing and mixing bentonite with the screened borrow area soil. If CMC finds the seepage increase (i.e., as summarized in Section 4.9) using Mosquito Fault material and the Mosquito Fault soil's chemical composition to be acceptable, than the Mosquito Fault soils would be a cost-effective alternative to the screened bentonite-amended soil for Zone 1. c.lflynnkypmslclimu\noldam%104rry2.doc 17/03/96 nvironmental