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INTRODUCTION This report summarizes construction observation results for the on-site and laboratory quality assurance testing conducted during the construction of the Stagecoach Reservoir slurry wall. Information contained within this report was collected as part of the construction related engineering services that were provided by Deere and Ault Consultants Inc. (D&A) throughout the slurry wall construction. The slurry wall was constructed by Hall-Irwin Construction under a contract with Lafarge North America, Inc. and Albert Frei & Sons. Lafazge North America, Inc. is preparing to mine gravel aggregate at the site. Upon completion of the mining operations, the site will be used as a water storage reservoir. Construction of the Stagecoach slurry wall began in early October 2005 and was completed in late December 2005. The construction consisted of lining a proposed grave] pit with approximately 7,100 feet of soil-bentonite slurry wall. The goal of the slurry wall liner is to minimize hydraulic interaction between the proposed gravel pit and the sunounding alluvium. The reduction in hydraulic interaction aids mining efficiency in two ways: 1) reducing dewatering requirements during mining operations, and 2) by reducing draw down effects on any neighboring water wells. Upon completion of the mining activities, the final goal of the slurry wall construction is to provide a primazy low permeability liner for a below grade water storage reservoir. The slurry wall was constructed using adesign/build approach. The scope of D&A's work during construction included on-site quality control and observation of the slurry wall construction. Hall- Irwin Construction supplied the labor, equipment, and materials to construct the slung wall. PROJECT DESCRIPTION The project site is located near Henderson, Colorado, and is bound by Old Brighton Road on the east, the South Platte River on the west, and is roughly 1,000 feet north of East 124th Avenue. The site is more particularly described as being located in Section 26 and 35, Township 1 South, Range 67 West of the 6th Principal Meridian, Adams County, Colorado, as shown on the vicinity map (Figure 1). Prior to slurry wall construction activities, the site was used for livestock grazing. The ground surface of the site is relatively flat; however, it has two distinct terraces. The terrace on the east side of the property is roughly 10 to 12 feet higher than the western side. This transition in elevation occurs near the center of the site. The project area is located approximately 20 miles east of the eastern flank of the Rocky Mountain Front Range. Regional geologic mapping of the area (Trimble, 1979)' indicates the near surface bedrock at the site is the Denver Formation. The Denver Formation is described as being comprised of claystone, siltstone, and conglomerate with the claystone and siltstone partly altered to montmerillonitic clay. Bedrock at the site is overlain by alluvium mapped as the Post-Piney Creek Alluvium and some Broadway Alluvium. Trimble, D. and Machette, M., United States Geological Survey, Geologic Map of the Greater Front Range Area, Front Ranee Urban Corridor, Colorado. 1979 - 1 - January 2005 Stagcmach Final Contuunion Repon.Joc •