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W <br />A&SARE" <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 />Guide lines 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 ft'/day/ft' 0.09 ft3 /day /ft2 <br />Pit Bottom 0.0015 ft3 /day /ft2 0.0030 ft3 /day /f_2 <br />7108 South Alton Way, Building B • Centennial, Colorado 80112 • www.cesareinc.com <br />Phone 303 - 220 -0300 * Fax 303 - 220 -0442 <br />