2011-03-14_REVISION - M1977493 (43)

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CM0001786 Mr. B. Romig 2 January 22, 1997 Robinson Lake Water Elevation: The Robinson Lake winter and spring water elevations will have a significant effect on the cost of sludge handling, dewatering and the reusable percentage of the foundation excavation material. As discussed in Section 4.3 Robinson Lake high elevations [greater than 10,875 feet mean sea level (MSL)] will increase the moisture content of the sludge and foundation material which will make the foundation material unsuitable for dam construction resulting in an associated increase in excavation and hauling costs. Robinson Lake high elevations will also require dewatering of the downstream side of the construction area, increasing dewatering costs included in TEC's October 3, 1996 engineer's cost estimate to construct the Dam. Construction costs may be lower by keeping the Robinson Lake reservoir elevation as low as the weather and storage capacity requirement allow. Seepage Control Alternatives: As discussed in sections 4.1.1 and 4.9, the Mosquito Fault material was determined to have the necessary geotechnical characteristics to be used in the seepage control zones of the Dam construction (i.e., Zone 1 and cut off wall) with a minimal increase in seepage. The remote location of the Mosquito Fault material required a cost analysis be performed to determine if the material could be a cost-effective replacement for bentonite-amended soil in Zone 1 and cutoff wall construction as discussed in sections 4.4 and 4.5, respectively. The cost analysis indicated that the Mosquito Fault material would be a cost-effective alternative for use in Zone 1 and cutoff wall construction if CMC determined the chemical characteristics of the material are acceptable. For Zone 1 construction, the Mosquito Fault material hauling cost to the Dam site was estimated at approximately $15,000 using CMC trucks and labor. To purchase bentonite and mix with on-site soils to create a six percent bentonite-to-soil mixture would cost approximately $80,000. For cutoff wall construction to the top of fractured bedrock, the Mosquito Fault material hauling cost was estimated at approximately $41,000 using CMC trucks and labor. The bentonite-soil cutoff wall installation cost was estimated at approximately $180,000. The expense of bedrock excavation precludes the use of Mosquito Fault material in a slurry wall extending from the bottom of fractured bedrock to the top of competent bedrock. A Mosquito Fault cutoff wall in place of a slurry wall would require a substantially greater bedrock excavation quantity as compared to bentonite-soil slurry cutoff wall. Borrow Soil Quantities: Borrow soil quantities presented in Section 4.1.2 were developed from 1990 aerial survey data and assumptions were made to estimate the quantities excavated after the aerial survey. Areas should be resurveyed after existing construction (i.e., No. 1 Dam Pump Station and Pipeline) is complete to determine the actual quantity of borrow soils available for the Dam construction. • • • U:?.SPELLMAN?m]MAX.MOL\CLOAAX\DESSUMLT.Dl1 122/97 ???? ?? Environmental __==