2013-07-26_REVISION - M2008078 (4)

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■ ■ -.��■ ■ ■ ■rte SARE ■■ ►��_� MEMO PROJECT NAME: GP Ranches, LLC PROJECT NO.: 11.124 FROM: Darin R. Duran, P.E. DATE: March 21, 2013 SUBJECT: West Farm Pit Seepage and Rapid Drawdown /Slope Stability Analyses BACKGROUND The project site is located about one mile east of Lamar, Colorado and about 900 feet south of the Arkansas River. The proposed project will construct a gravel pit and a water storage reservoir on a relatively flat site which is currently used for agricultural purposes. Soils encountered in borings at the site generally consist of clay to clayey sand, silty sand, and gravelly sand to sandy gravel overlying shale bedrock. Before gravel mining operations start, a cutoff wall will be constructed around the perimeter of the site. The cutoff wall will be keyed into bedrock. Successful construction of the cutoff wall will allow the site to be dewatered while meeting or exceeding the State of Colorado standards for maximum allowable rates of groundwater infiltration. The proposed method of cutoff wall construction is the one -pass trencher method. This method uses a track - mounted vehicle with a cutting boom resembling a large chain saw. This method will result in a slurry wall that is approximately 18- inches thick. To our knowledge, this method has not been used within the geologic formations found at the site. ANALYSIS We assumed the reservoir will have side slopes of 3H: 1V. The top of the slope will be located 25 feet from the outside edge of the slurry wall. The 18 -inch wide slurry wall will be keyed a minimum of 3 feet into competent (non - weathered) bedrock. We assumed a completely full reservoir to have a water elevation of 3598 feet. Steady state seepage of an empty reservoir was also analyzed to determine if the "Design Standard" portion of the leakage requirements contained in the State of Colorado's State Engineer Guidelines for Lining Criteria of Gravel Pits (August 1999) should be met. These guidelines contain both design standards and performance standards with the maximum allowable groundwater inflow from the perimeter and the bottom of a lined gravel pit. These guidelines are shown in Table 1. TABLE 1. State Engineer Guidelines for Maximum Allowable Inflow Location Design Standard Performance Standard Pit Perimeter 0.03 ft3 /day /ftz 0.09 ft3 /day /ftz Pit Bottom 0.0015 ft3 /day /ft2 0.0030 ft/day/ft' 7108 South Alton Way, Building B • Centennial, Colorado 80112 • www.cesareinc.com Phone 303 -220 -0300 Fax 303 - 220 -0442