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1. INTRODUCTION <br />The Geisert Gravel Pit (Geisert) site is located in Greeley, Colorado. The entrance to the site is <br />about 700 feet west of the intersection of North 11 Avenue and H street. A soil - bentonite <br />backfilled trench ( "slurry wall ") installed from original ground surface into the underlying <br />bedrock is planned for the site. The slurry wall will enclose an area that has been partially <br />mined and will continue to be mined in the future and the slurry wall will function as a <br />groundwater barrier. The slurry wall is designed to be installed completely through the soil into <br />competent, low permeability sedimentary bedrock to significantly reduce groundwater seepage <br />into the pit, thereby meeting the seepage standards for a lined gravel pit as defined by the <br />Colorado State Engineer. Performance of the liner and bedrock will eventually be demonstrated <br />using a 90 -day liner test as defined by the "State Engineer Guidelines for Lining Criteria for <br />Gravel Pits (August 1999) ". The slurry wall is design to withstand up to 72.5 ft of differential <br />water head from outside the slurry wall to inside the pit, which could occur when the pit is <br />dewatered to bedrock. <br />The proposed slurry wall will be approximately 7,200 ft long. The slurry wall will be installed <br />to a range of minimum depths of approximately 47 to 76.5 feet, with an average trench depth <br />of 67.7 ft and will be a minimum of 42 inches wide. The slurry wall backfill will consist of a <br />low permeability soil - bentonite mixture. Lyman Henn has been retained by Hall -Irwin <br />Corporation (Hall - Irwin) to perform geotechnical investigation and design for the slurry wall. <br />Hall -Irwin will construct the slurry wall for Weld County. <br />G:\PROJECCS \111025 Geisert Gravel Pit Slurry Wall \O \O1 \Geisert Slurry Wall Design Report.doc 1 <br />