2011-03-14_REVISION - M1977493 (65)

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CM0001766 • Page 12 The crest surfacing was specified to provide a slip-resistant surface on top of the Dam that would withstand rutting by maintenance equipment. A standard poorly-graded gravel used in road surfacing should be used for the crest surface. The borrow area can not produce the estimated 400 cy of crest surfacing material without crushing equipment and an alternate source (e.g., purchased from a commercial quarry) would likely be more cost effective. Table 4 identifies the riprap and crest surfacing material quantities required as calculated in the `Summary of Construction Materials Calculation Brief provided in Appendix E. 4.9 Seepage Analyses Seepage from the Seepwater Pond through and/or underneath the Dam toward Robinson Lake was estimated by use of the computer modeling software package `FastSEEP', developed by Brigham Young University. FastSEEP incorporates the United States Army Corp of Engineers SEEP2D flow net generation routine. Appendix C provides the data used to perform the seepage analysis as well as the `Foownet Analysis of Seepage Calculation Brief. Table 5 summarizes the flownet analyses results of seepage rates based on varying Zone 1 materials and cutoff wall configurations, as well as the seepage rate influencing water level elevations in both Robinson Lake and the Seepwater Pond. These seepage rates were estimated to aid in cost-benefit evaluations, discussed in section 4.4 and 4.5, for using various options for Zone 1 and cutoff wall materials, respectively. All seepage evaluations conservatively assumed a low Robinson Lake elevation and varying high Seepwater Pond elevations. As a baseline for comparison, seepage was evaluated using a Zone 1 permeability of 10.7 cm/sec (i.e., achievable by using a bentonite-amended soil), without using a cutoff wall and by varying the Seepwater Pond elevation. Seepage estimates varied between approximately 9,000 and 11,000 gallons-per-day (gpd). For all evaluations of varying Zone 1 and S-B cutoff wall materials and/or configurations, a 10-7 centimeter-per-second (cm/sec) S-B cutoff wall was identified as extending from the top of fractured bedrock to the top of competent bedrock to minimize seepage through the fractured bedrock zone. For the evaluation of varying Zone 1 materials, seepage was modeled using the Mosquito Fault material, which has a 10-6 cm/sec permeability, in Zone 1 versus a S-B mixture with a 10-7 c.Ty.%cypmlclimutkwldamX6104mp2.doc 12/01/96 e = . ? . Environmental