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Water Storage Pond Compacted Clay Liner <br />Parcel 1, S% , SW'/4 , Section 35, T1 S, R67W <br />Adams County, Colorado <br />stability modeling are considered to be reasonably conservative, but the actual materials <br />placed should be tested by a geotechnical engineer, and the findings and <br />recommendations in this report modified, as appropriate. <br />Temporary Slopes Based on our analyses, GROUND recommends that temporary <br />slopes (the quarry wall back cut and the upper surface of the clay liner) be constructed <br />no steeper than 2:1 (horizontal : vertical). At this angle, factors of safety (assuming a <br />de-watered site during construction) of 1.4 or more were estimated. <br />The excavation back slopes at 2:1 (horizontal : vertical) will be subject to erosion during <br />significant precipitation runoff events as well as local, surficial slope. failures (sloughing <br />and shallow slumps). The contractor should limit the lateral extent of exposed back cut <br />slope open at any one time. Wider extents of open slope will entail an increased risk of <br />erosion and local slope failures, and associated increased slope maintenance during <br />construction. <br />Permanent Slopes GROUND also recommends that the permanent pond slopes (the <br />upper surface of the miscellaneous fill) be constructed an angle no steeper than 3:1 <br />(horizontal : vertical). Factors of safety of approximately 1.4 or more were estimated for <br />this geometry for an 'empty pond' condition. This factor of safety is considered <br />acceptable for a permanent slope for the proposed site and land use. <br />Again, surficial erosion and local sloughing requiring periodic maintenance should be <br />anticipated for a 3:1 (horizontal : vertical) slope angle, particularly in more granular fill <br />materials. Project plans should allow for periodic maintenance of the reservoir slopes. <br />Surface drainage should be designed to direct water away from slope faces. <br />It should be noted again that the material properties used for the `miscellaneous fill' were <br />assumed. GROUND recommends that the materials actually proposed for use as the <br />miscellaneous fill be tested, and the conclusions and recommendations herein updated, <br />once construction begins. <br />Significantly lower factors of safety (about 1.1) were estimated for a 'rapid drawdown' <br />condition, modeling the very rapid (e.g., in a few hours or less) emptying of the storage <br />pond when full. We do not anticipate that such a rapid drawdown condition could be <br />accomplished. However, if it were achieved, failures of the taller slopes might be <br />realized. <br />Job No. 09-3090 GROUND rEnginearing Consultants, Inc. Page 3