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_17_ <br /> <br />After the buttress fill is placed, the surface of the fill shauld be shaped <br />To provide positive gravity flow (minimum 2°o slope) from the anticipated <br />underdrain southeast of The buttress fill area. The underdrain should then be <br />constructed over the buttress fill to the planned Toe "daylight" point. Figure <br />4 (sheet 4) shows a detail of the underdrain construction. <br />Our final analysis considered the effects of earthquakes on slope <br />stability. We estimated a Factor of Safety of 1.3 for earthquake loading, <br />assuming clay replacement and water levels aT the existing ground surface. <br />CONCLUSIONS <br />The results of our site investigation, stability calculations, and knowledge of <br />the performance of The Streeter Canyon fill at the Colowyo mine lead us to the <br />. following recommendations for construction of The West Pit Excess Disposal <br />Site. Sheet 2 (Fig. 2) and Sheet 4 (Fig. 4) illustrate these recommendations. <br />I. We recommend excavation of the clay materials in the valley bottom to <br />bedrock or a depth of at least 20 feet (whichever is less) about 700 feet <br />upstream of the toe of the proposed spoil. A buttress fill should be <br />constructed of these clays at The planned toe of the spoil. <br />2. We recommend a controlled underdrain be placed in the area where clays <br />are removed. The harder available durable sandstones should be used to <br />construct this drain. Rock ranging from I foot to 6 feet should be used <br />in the drain. <br />We do not anticipate a controlled, graded filter will be required around <br />the controlled drain. ~Ne believe the natural sorting which occurs with <br />spoil placement in thick lifts will be sufficient to protect the drain from <br />clogging. <br />3. We believe the spoil fill can be placed using comparatively thick lifts so <br />that a permeable fill is created with reduced risk of development of free <br />water levels within The fill. Fill lift thickness of 50 feet to 100 feet is <br />acceptable. The maximum permanent slope should be 3 horizontal to <br />I vertical. <br />• <br />