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Supplemental Geotechnical Engineering Report Proposed Slurry Cut-off Walls @ St. Vrain Development Weld County, Colorado Terracon Project No. 25045300 Terracon ranging from about 6 to 24 feet below existing site grade when checked a minimum of one day after drilling. Groundwater conditions encountered for the current study during drilling varied from about 7 to 18 feet below existing site grade. These observations represent groundwater conditions at the time of the field exploration, and may not be indicative of other times or at other locations. Groundwater levels can be expected to fluctuate with varying seasonal and weather conditions. Based upon review of USGS maps ("Hillier, et al, 1979), regional groundwater beneath the project area predominates in colluvial, landslide or windblown materials or in fractured, weathered, consolidated, sedimentary bedrock located at a depth near ground surface. Seasonal variations in groundwater conditions are expected since the aquifer materials may not be perennially saturated. Groundwater is generally encountered at depths ranging from 5 to 20 feet below ground surface; depth to seasonal groundwater is generally 10 feet or less. Zones of perched and/or trapped groundwater may also occur at times in the subsurface soils overlying bedrock, on top of the. bedrock surface or within permeable fractures in the bedrock materials. The location and amount of perched water is dependent upon several factors including hydrologic conditions, type of site development, irrigation demands on or adjacent to the site, fluctuations in water features, proximity to St. Vrain River, seasonal and weather conditions. Fluctuations in groundwater levels can best be determined by implementation of a groundwater-monitoring plan. Such a plan would include installation of groundwater- monitoring wells and periodic measurement of groundwater levels over a sufficient period of time. The possibility of groundwater fluctuations should be considered when developing design and construction plans for the project. ENGINEERING ANALYSES AND RECOMMENDATIONS Scurry Wall Cut-Off Design and Construction Considerations: The site appears suitable for the proposed construction. Relatively shallow groundwater conditions encountered within the majority of the borings along with caving soils conditions and the presence of gravel and cobble sized materials will likely require particular attention in the design and construction. Hillier, Donald E.; and Schneider, Paul A., Jr., 1979, Depth to Water Table (1976-1977) In the Boulder-Fort Co//!ns- OreeleyArea, Front Range Urban Conidoi Colorado, United States Geological Survey, Map I-8551. 6