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Lined Pit Guide 6 September 2000 <br /> Key Trench and Slurry Wall Cap <br /> During excavation, soundings should be obtained to determine the elevations of the top of the key layer, <br /> the bottom of the excavation, and the bottom of the trench prior to backfilling. If sediments in excess of <br /> 2 inches have accumulated it is necessary to clean the trench bottom by airlift pumps or excavation <br /> equipment to remove the sand and sediment that has settled. The trench bottom should be cleaned, as a <br /> minimum, at the beginning of each shift. Soundings should be obtained approximately every 20 feet. <br /> Many of the leakage issues with slurry walls can be traced back to inadequate keying in the S-B backfill <br /> to an impermeable layer at the base of the wall. The design documents should include a description of <br /> the key inlayer and describe how trench cuttings or other observations will be used to assure that the <br /> slurry wall will be adequately keyed. At most.gravel pits, slurry walls are keyed into shale underlying <br /> the sand and-gravel. If the shale is weathered, fractured, or contains sand lenses, it may be necessary to <br /> provide a deeper key trench. This should be addressed in the design. <br /> The specifications should call for temporary slurry wall protection in the form of a noncompacted soil <br /> cover placed within one day over each backfilled 100 foot reach. The temporary cover is removed after <br /> settlement(approximately 2 weeks) and replaced with a compacted clay cover over the completed <br /> slurry wall. <br /> Post Construction Testing <br /> Typically,post construction testing will be conducted in accordance with the "State Engineer <br /> ,Guidelines for Lining Criteria for Gravel Pits," August 1999, section 3.1. This document is available <br /> from the Office of the State Engineer or online at http://water.state.co.us/pits.htm. Once an Operator <br /> -can document that-thu-STaTe gineer has determined the iined pit meets peff ormance standards, the <br /> Division of Minerals and Geology can release the portion of the bond that covers slurry wall <br /> installation. <br /> As-Built Drawings and Final Report <br /> Under the regulated construction option, Operators are required to provide a final construction report <br /> detailing the installation of the slurry wall, describing any problems that occurred, and listing the results <br /> of testing that was conducted under the approved quality assurance/quality control plan. An example <br /> table of contents for the final report is provided below. <br />