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Hazeltine/Road Runners Rest II/Brinkmann-Woodward Gravel Pits Lining Project- Design Technical Memorandum Denver Water August 2003 The area of the perimeter liner used in the calculation of allowable leakage is calculated by ' multiplying the length and height of the liner. For asoil-bentonite cutoff wall, the length of the liner is the length of the centerline of the completed cutoff wall. The height of the liner is the ' difference in elevation between the ground surface above the cutoff wall and the ground surface at the pit bottom near the toe of the pit slopes. The depth of liner penetration below the bottom of the pit is not considered in the calculation of the allowable leakage rate. ' We calculated the allowable leakage rate based on the following considerations: t 1. The liner will be constructed around the west, north, and east sides of the combined Hazeltine, Road Runners Rest II, and Brinkmann-Woodward Reservoir. 2. The existing berm between Hazeltine and Howe-Haller Pits was not included in the calculation of the SEO allowable leakage rate or in the computation of expected ' seepage into the Hazeltine, Road Runners Rest tI, and Brinkmann-Woodward Reservoir. ' 3. The bottom of the reservoir would be at the top of bedrock. Based on data from the 34 borings drilled around the perimeter of the Hazeltine, Road Runners Rest II, and ' Brinkmann-Woodward Pits, the top of bedrock ranges from 11 to 59 feet below ground surface. The weighted average distance from the ground surface to the top of bedrock is about 35.7 feet (ground surface to pit bottom). 4. The length of the soil-bentonite cutoff wall was estimated by a generalized base line around the perimeter of the reservoir. The length of this generalized base line was about 14,800 lineal feet. The bottom area of the pit used in the leakage calculations was estimated by plan area. The bottom area used in the calculations for a combined single pit was 8,843,300 ftz. 5. Based on this rate, we calculated the SEO maximum allowable total Design Standard leakage rate into the Hazeltine, Road Runners Rest II, and Brinkmann-Woodward Reservoir is 156 gallons per minute (gpm) and the total Performance Standard leakage rate is 468 gpm. The design of a lining system should include a FOS against leakage to account for: I ) variations in subsurface conditions, 2) uncertainties associated with interpreting limited data (i.e., estimating bedrock hydraulic conductivities between widely spaced borings), and 3) the economic risk of failing to meet SEO Design and Performance Standards for a lined water storage facility. The selection of an appropriate design factor of safety and the various factors governing its selection are presented in Section 4.7. GEI Consultants, Inc. 16 o: oso o:.oi-n ~n.~~uee~~e o.,~.~ra