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LiLhIlIMMIJ <br /> SARE, INC. <br /> ceoArchnfeae Engineers�carsbYc6on Manna@ conwlmnts <br /> MEMO <br /> PROJECT NAME: GP Ranches, LLC <br /> PROJECT NO.: 11.124 <br /> FROM: Darin R. Duran, P.E. <br /> DATE: March 21, 2013 <br /> SUBJECT: West Farm Pit Seepage and Rapid Drawdown/Slope Stability Analyses <br /> BACKGROUND <br /> The project site is located about one mile east of Lamar, Colorado and about 900 feet south of the <br /> Arkansas River. The proposed project will construct a gravel pit and a water storage reservoir on a <br /> relatively flat site which is currently used for agricultural purposes. Soils encountered in borings at <br /> the site generally consist of clay to clayey sand, silty sand, and gravelly sand to sandy gravel <br /> overlying shale bedrock. Before gravel mining operations start, a cutoff wall will be constructed <br /> around the perimeter of the site. The cutoff wall will be keyed into bedrock. Successful construction <br /> of the cutoff wall will allow the site to be dewatered while meeting or exceeding the State of <br /> Colorado standards for maximum allowable rates of groundwater infiltration. <br /> The proposed method of cutoff wall construction is the one-pass trencher method. This method <br /> uses a track-mounted vehicle with a cutting boom resembling a large chain saw. This method will <br /> result in a slurry wall that is approximately 18-inches thick. To our knowledge, this method has not <br /> been used within the geologic formations found at the site. <br /> ANALYSIS <br /> We assumed the reservoir will have side slopes of 3H: 1V. The top of the slope will be located 25 <br /> feet from the outside edge of the slurry wall. The 18-inch wide slurry wall will be keyed a minimum <br /> of 3 feet into competent (non-weathered) bedrock. We assumed a completely full reservoir to have <br /> a water elevation of 3598 feet. <br /> Steady state seepage of an empty reservoir was also analyzed to determine if the "Design <br /> Standard" portion of the leakage requirements contained in the State of Colorado's State Engineer <br /> Guidelines for Lining Criteria of Gravel Pits(August 1999) should be met. These guidelines contain <br /> both design standards and performance standards with the maximum allowable groundwater inflow <br /> from the perimeter and the bottom of a lined gravel pit. These guidelines are shown in Table 1. <br /> TABLE 1. State Engineer Guidelines for Maximum Allowable Inflow <br /> Location Design Standard Performance Standard <br /> Pit Perimeter 0.03 ft3/day/fr' 0.09 ft3/day/ft' <br /> Pit Bottom 0.0015 W/day/ft2 0.0030 ft3/day/ft2 <br /> 7108 South Alton Way, Building B• Centennial,Colorado 80112•www.cesareinc.com <br /> Phone 303-220-0300 • Fax 303-220-0442 <br />