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Supplemental Geotechnlcal Engineering Report Proposed Slurry Cut-off Walls @ St. Vrain Development Weld County, Colorado Terracon Project No. 25045300 Terracon During construction of the slurry walls, a representative of the geotechnical engineer should perform oversight to insure that work is performed in accordance with project specifications per CDNR. At a minimum, the trench slurry should be tested daily for viscosity and unit weight. Additionally, unit weight testing should be performed daily on the soil-bentonite backfill mixture to insure that it is sufficiently higher in density than the trench slurry. At least one laboratory permeability test per week should be performed on the soil-bentonite backfill mixture. The information presented in this report should be reviewed by a qualified slurry cut-off wall designer and/or contactor with input from the geotechnical engineer to develop the proper wall thickness, embedment depth, and the slurry mix required to maintain trench stability. Excavations into the on-site soils will encounter caving soils and groundwater. The contractor should be made responsible for designing and constructing stable, temporary excavations as required to maintain stability of both the excavation sides and bottom. All excavations should be sloped or shored in the interest of safety following local and federal regulations, including current OSHA excavation and trench safety standards or a slurry could be used to maintain the integrity of the trench. The soils to be penetrated by the proposed excavations may vary significantly across the site. The contractor should verify that similar conditions exist throughout the proposed area of excavation. If different subsurface conditions are encountered at the time of construction, the actual conditions should be evaluated to determine any excavation modifications necessary to maintain safe conditions. We recommend That a clay cap be provided over the cutoff wall. The cap is generally required to provide protection against erosiori, desiccation, and freeze-thaw degradation of the wall. In addition, the slurry wall should be located below any planned future construction activities (i.e. utilities, foundations, etc.) to minimize disruption and integrity loss. As an alternative, self healing bentonite plugs or "knock-outs" could also be -used to allow for penetrations and maintain lateral support. As a safety measure, it is recommended that all vehicles and soil piles be kept to a minimum lateral distance from the crest of the slope equal to no less than the slope height. The exposed slope face should be protected against the elements. Potential Borrow Sources: The following preliminary shallow potential borrow sources have been identified for slurry wall backfill: • Slurry wall trench backfill (clayey soils for soil-bentonite mixture}: 10