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SFCTIONEIGHT 8.1 CONCLUSIONS Subsurface Conditions: Conclusions and Recommendations The results of the geotechnical field investigation indicate the stratigraphy at the proposed Sharkey's Lake Gravel Pit consists of, from top down, a topsoil layer, a sand and grave] layer, and asiltstone/claystone bedrock unit. The bedrock unit generally grades from weathered rock at the surface to more competent rock approximately 3 feet below the bedrock surface. The bedrock unit on the floor of Sharkey's Lake Gravel Pit will control the inflow/outflow of groundwater through the bottom of the pit Proposed Perimeter Soil-Bentonite Cutoff Wall: A soil-Bentonite cutoff wall was designed to control seepage through the sidewalls of the pit. The proposed soil-Bentonite cutoff wall will be installed by the slurry trench method and will be keyed into the competent siltstone/claystone bedrock unit. The cutoff wall will be approximately 12,000 feet long and encompass an approximately 9,900,000 sq. fl. The cutoff wall will be 2.5 feet wide and extend from the ground surface to a minimum of 4 feet below the top, of the weathered bedrock and have an average depth of approximately, 38 feet below the ground surface. Seepage: The performance of the gravel pit as a water storage reservoir with the proposed soil-Bentonite cutoff wall was evaluated by performing seepage modeling and by comparing the results with the Design Standard requirements of the SEO. According to the results of the seepage modeling analysis presented in section 7.2, construction of a cutoff wall that extends a minimum of 4 ft. into the weathered siltstone/claystone bedrock will result in a conservatively estimated inflow of approximately 90 gpm which is less than the SEO's maximum allowable design flow of about 140 gpm. 8.2 RECOMMENDATIONS ~ Consfruction: Based on the results of our investigation and analyses, asoil-Bentonite cutoff wall can be constructed to meet the SEO's Design Standard seepage requirements. The cutoff wall should extend from the ground surface into the lower permeability siltstone/claystone layer an average ' vertical distance of 38 ft. (see Drawings 2, 3, and 4). The specifications for construction of the cutoff wall are presented in Appendix E. Based on the backfill mix design presented in section 6.2, we recommend that the soil-Bentonite backfill have a minimum Bentonite content of 1.5 percent by dry weight of which a maximum of 0.5 percent maybe assumed to come from the slurry. The remaining minimum of 1.0 percent must be added to the backfill as dry Bentonite. Construction QC and QA testing should be performed on accordance with the specifications. N*:\'PR~OJEGCTS\22238240 PLATTE_SAND GRAVELwSUB_OOU2.0_WORD_PROC\PLATTESANDBGRAVELGEOTECHNICALDESIGNREPORT4.O.DOGVbNOV-05\\ 8-I V~