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Lined Pit Guide 6 September X000 <br />Key Trench and Sk~rry 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 ocher 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:/Jwater.state.co.uslpits.htm. Once an Operator <br />can document [hat the State Engineer has determined the lined pi[ meets performance standards, the <br />Division of Minerals and Geology can release the portion of the bond that covers slurry wall <br />installation. <br />As-Built D,-awings and Final Report <br />Under [he regulated construction option, Operators are required [o provide a final construction report <br />detailing the installation of the s]urry wall, descrioing 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 pcovided below. <br />