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I, I. INTRODUCTION The 25th Avenue Gravel Pit Storage Site (Site) is located in Weld County, Colorado, along the Cache la Poudre River. The Site is located approximately four miles north of U.S. Highway 34, between 25th and 35th Avenues, The Site is owned and operated by the City of ' Greeley (City). The proposed slurry wall alignment encloses two adjoining gravel pit ponds in a gravel mining area located in the river floodplain. Land use to the south, west, and east of the Site consists of active gravel mining operations. Land use to the north of [he alignment is agricultural. The completed slurry wall will function as a low permeability liner enclosing the gravel pit area. The slurry wall is required to be installed into unweathered, low permeability sedimentary bedrock. The slurry wall, keyed into the bedrock, is designed to significantly reduce groundwater seepage into the pit, thereby meeting the design standards for a lined ' gravel pit as defined by the Colorado State Engineer. In meeting these standards, the City's current water augmentation responsibilities for the gravel pit, due to evaporative loss of exposed groundwater, will not be required. Demonstration of the performance of the liner and bedrock is done using a 90-day liner test as defined by the "State Engineer Guidelines for ' Lining Criteria for Gravel Pits" published in August 1999. Upon installation of the liner, the grave] pit will serve primarily as "sealed" water storage for ' the City. When available, water will be diverted via an inlet structure from the 35th Avenue Stormwater Ditch located on the eastern edge of the Site and stored in reservoir. When required, water will be pumped from the gravel pit and discharged into the same ditch. Discharge water will return to the Cache la Poudre River, allowing the City to meet augmentation responsibilities for non-potable water supplies obtained from tributary wells and agricultural return flow obligations associated with the City's use of agricultural ditch shares ' for potable use. The City may elect to mine the remaining aggregate deposit within the pit area using dry mining methods. Currently the pit is mined to approximately 25 to 35 feet (ft) deep. The final mining depth could be up to 60 ft deep. The slurry wall is design to withstand up to 50 feet of water head differential from outside the slurry wall to inside the pit, which could occur when the pit is completely dewatered to bedrock during mining operations or when being used for water storage. Included with this report is the design plan set related to the slurry wall design (Drawings 101 through 104). This drawing set consists of the following: Drawin No. Descri lion 101 Cover Sheet ( eneral Iocation ma and Site location ma } 102 Site Layout and Drawing Key Sheet (general locations of slurry wall, construction limits, ro ert boundaries, cla borrow areas, access route); 103 Slurry Wall Plan and Profile (wall alignment, wall depth, and working latform) 104 Slurry Wall, Working Platform, and Site Reclamation Sections and Details.