2010-07-27_REVISION - M1977223 (12)

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i Earth Engineering Consultants, Ina. Colorado Beef Fresh Water Reservoir April 6, 2009 Page 3 The stratification boundaries indicated on the boring logs represent the approximate locations of changes in the soil and rock types; in -situ, the transition of materials may be gradual and indistinct. Observations were made while drilling and after completion of the borings to detect the presence and depth to hydrostatic groundwater. Free water was observed while drilling at depths of approximately 8 feet within the gravel mining operation and at depths on the order of 28 to 34 feet at exterior boring locations. Subsequent groundwater level readings were completed at the piezometer locations with groundwater observed at two of those locations at depths of approximately 31 to 26 feet. Fluctuations in groundwater levels can occur over time depending on variations in hydrologic conditions and other conditions not apparent at the time of this report. Zones of perched and/or trapped water can be also develop within more permeable zones in the subgrade soils at times throughout the year and are commonly encountered in more permeable soils overlying low permeability bedrock. ANALYSIS AND RECOMMENDATIONS The borings completed in the non -mined areas of the site indicate clayey overburden soils extending to depths of approximately 5 to 17 feet. Those soils show upwards of 70% passing a number 200 sieve with Atterberg limits indicating slight to moderate plasticity. In general, we anticipate the hydraulic conductivity of the near surface clayey soils to be relatively low indicating good potential for use as a low permeable barrier for construction of the water storage reservoir, The underlying sands and gravels would not be suitable for this use; however, could potentially be used within the embankments constructed as a part of the reservoir development. The clayey soils show low to moderate plasticity indicating potential for erosion if left unprotected. The design of the reservoir should take into account the potential for erosion of the low plasticity clayey soils when determining placement of those materials. Crumb testing indicates the clayey soils are not likely to be dispersive and, therefore, could be considered for use as a low permeable barrier around the reservoir. It is our understanding the elevation of the floor or the reservoir would be at site elevation 480. Current groundwater elevations in the reservoir area appear to be on the order of 470 to 472. In general, the floor of the reservoir appears to be well above the groundwater