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