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SECt1ANONE Exeeutiue Summary In this study we: (i) evaluated the suitability of converting the proposed Shazkey's Lake Gravel Pit into along-term water storage reservoir meeting the Colorado Division of Water Resources, State Engineer's Office (SEO) August 1999 Guidelines for Lining Criteria for Gravel Pits; and, (ii) prepared design documents, plans and specifications, and a quality assurance program for construction of asoil-bentonite cutoff wall around the proposed reservoir to satisfy the SEO's requirements. To complete this study, we performed a field investigation to determine geotechnical conditions, conducted laboratory testing on representative samples of soil and bedrock, and completed analytical modeling and analysis of potential seepage into the proposed storage reservoir. The results of the field investigation indicate that the stratigraphy at Sharkey's Lake Gravel Pit consists of, from top down, a topsoil layer, a sand and gravel 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 relative position, thickness, extent, and permeability of these layers were modeled to evaluate seepage into the proposed storage reservoir with a perimeter soil-bentonite cutoff wall. 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. fr. The average depth to bedrock in the reservoir will be approximately 34 feet. The SEO Guidelines for Lining Criteria for Gravel Pits include a Design Standard and a Performance Standard. The Design Standazd states that "The intent of the reservoir lining design is to achieve ground water inflow (leakage rate) into the reservoir that is not greater than 0.03 ft3/day/ftz multiplied by the average vertical depth of the perimeter wall as measured from the ground surface to the pit bottom along the toe of the pit slope, plus 0.0015 ft3/day/ft2 multiplied by the azea of the bottom of the liner system or natural bedrock bounded by the perimeter wall. Appropriate geotechnical evaluations and analyses should be performed to indicate the Design Standard could be achieved with the proposed design to a reasonable probability." The SEO guidelines Performance Standard consists of a leakage rate criteria that is applied to an initial test of competency of the liner, as well as to the ongoing operation of the reservoir. The maximum leakage rate allowed by the Performance Standard is three times the Design Standard leakage rate defined above. In summary, the Design Standard is applicable during the planning and design process whereas field tests are required at completion of construction to determine if the reservoir meets the Performance Standard. Seepage modeling and analysis was conducted to demonstrate that the proposed cutoff wall concept will satisfy the Design Standard in The SEO Guidelines. Based on the approximate perimeter cutoff wall length of 12,000 feet, average pit depth of 34 feet, and approximate base area of 9,900,000 sq. ft., a maximum inflow of approximately 140 gpm is allowed according to the SEO's Design Standard. The seepage modeling and analysis done for this study (with conservative boundary conditions) resulted in an estimated inflow of approximately 90 gpm into the proposed reservoir with a perimeter soil-bentonite cutoff wall installed in accordance with the plans and specifications presented in the attached report. Therefore, it is our professional opinion that asoil-bentonite cutoff wall installed around the perimeter of the proposed gravel pit will provide the necessary N*:\PR~OJEC~TS\222382a0 PLATTE SAND GRAVELISUD 00\12.0 WORD PROCIPLATTE SANDB GRAVEL GEOTECNNICAL DESIGN REPORT 4.O.DOp75-NOV-0511 1-1 VfY.~ _ _ _ _ _ _