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a <br />• Embankment Recommendations <br />The following recommendations are for the proposed sedimentation <br />embankment with a height of 20 feet or less. We understand from Colowyo that the <br />embankment does not meet the criteria of 30 CFR 77.216(a) (i 989) and is located <br />where failure will not be expected to cause loss of life or serious property damage. <br />Therefore, a minimum factor of safety of 1.3 for a normal pool with steady state <br />seepage is required according to state regulations (Reference 1). <br />Based upon USBR (Reference 2) recommendations for homogeneous small <br />earth dams we recommend an upstream slope of 3:1 (H:~ and a downstream slope <br />of 2.5:1. We also recommend a keyway trench at the base of the embankment with <br />a minimum width of 8 feet and a minimum depth of 6 feet. The keyway should be <br />continuous below the dam, extending into and up the abutments. The upstream face <br />and spillway should be protected from erosion as required. A conceptual detail of <br />the recommended embankment is shown on Fig. 2. <br />We analyzed the stability of the upstream and downstream slopes using <br />stability charts from Reference 3. The fully saturated condition (Chart 5) was used <br />to model normal pool steady state seepage conditions. Using the clay strength <br />measured from the previous investigation (~ = 15°, C = 500), a minimum factor of <br />safety of 1.8 was calculated. This is significantly over the required minimum of 1.3. <br />• Based upon the information from boring TH-7, the soils should provide an <br />adequate foundation for the anticipated embankment height. The bearing capacity <br />is more than adequate. The native clay soils will be suitable material for construction <br />of keyway and embankment fills. The potential settlement due to compression of <br />the foundation and embankment is estimated to be 2 to 4 inches. <br />We recommend topsoil and/or soft or wet soils be stripped from below <br />embankment areas. Keyway and embankment fill should be placed in thin lifts (8 to <br />12 inch loose lift thickness), moisture conditioned to within 2 percent of optimum <br />moisture and compacted to at least 95 percent of standard Proctor maximum dry <br />density (ASTM D 698). <br />Limitations <br />Our recommendations are based upon visual observations, and review of <br />previous investigations. The embankment design recommendations relate to slope <br />stability and foundation conditions. Hydraulic factors such as capacity, freeboard <br />and outlet or spillway structures were not within the scope of the investigation. <br />We recommend the keyway excavation, embankment area stripping and <br />preparation, and the compaction and placement of fill be observed and tested. <br />n <br />COLOWYO COAL CO.LP. <br />cTLrt za,oes 4 <br />