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Hazeltine/Road Runners Res[ IllBrinkmann-Woodward Gravel Pits Lining Project -Design Technical Memorandum Denver Water August 2003 Executive Summa The objective of the project is to construct a below grade, lined water storage reservoir at the site of the Hazeltine, Road Runners Rest II, and Brinkmann-Woodward gravel pits in Adams County, Colorado, that meets the State Engineer guidelines For lining criteria for gravel pits. This report presents the basis of design and our engineering analyses design basis for construction of asoil-bentonite cutoff wall. The methodology used to develop the horizontal alignment for the soil-bentonite cutoff wall (wall) involved identifying a "safe zone" to provide an adequate factor of safety (FOS) against critical failure surfaces intersecting the installed soil-bentonite cutoff wall for steady state seepage conditions. Representative slope cross section were analyzed to identify the location of the critical failure surface for the selected FOS. The critical failure surface was used to identify the zone of the slope that would be potentially unstable. The alignment was developed by placing the soil-bentonite wall outside this zone of potential instability. Analysis of the interior slope provided the "interior offset." Similarly, analysis of the exterior slope provided the "exterior offset." The distance between the two offsets provided the "safe zone" in which the soil-bentonite cutoff wall could be installed. In general, the wall was located close to the exterior edge of the "safe zone" to leave the maximum available space along the crest and interior slope of the reservoir for working platforms and construction access. In areas where sufficient crest width was not available, stabilization fill will be placed along the interior slope to provide the necessary safe zone width. The total length of the soil-bentonite cutoff wall is approximately 14,800 feet. For the selected target seepage FOS of 1.5 against the SEO Design Standard, the soil-bentonite cutoff wall would need to extend between about 4 and 10 feet into bedrock. The weighted average penetration of the soil-bentonite cutoff wall into bedrock would be about 5.7 feet. GEI Consultants, Inc. iii c3oeo o3-ora~~n Hx.enioe oesgn TM