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• ~ Three particle size distribution curves were determined as shown on Figure 5. Curve 1 is generally unsorted spoil and consists of the following: Cobbles and Boulders 50% Gravel 28% Sand 2% ~ Fines (- No. 200 sieve) 20% Technically, according to filter design criteria the fines control, and require a fine sized filter to prevent their movement. In a saturated flow situation some movement of the fines could occur but we do not anticipate saturated conditions. The well graded matrix should ~ be stable and piping or loss of support is not expected. Based upon the results of our observations and analyses, we recommend construction of the spoil with lift thickness of at least 50 feet. Curve 3 (Figure 5) is the drain rock. Curve 2 is sorted spoil from 8 to 13 feet above the base of a high lift spoil pile (Photo 3). Curve 2 meets filter criteria for protection of the drain rock. By extrapolation, ~ as one moves progressively up the fill slope, the particle size curve will shift to the left as particle size decreases. Each layer should provide filter criteria for the layer below. With this procedure the unsorted spoil should be protected by a well developed filter section which is created by gravity sorting of the spoil. • CONCLUSIONS The results of our investigation and analyses indicate a filtering system is formed by gravity sorting of spoil placed in thick lifts, and that construction of a separate filter system should not be required. The gravity sorting which was consistently observed on ~ high lift spoil piles was evaluated and found to meet filter criteria for the underdrain. The thickness of the filter layer observed in the test section is related to the lift height of 30 feet, compared to a recommended minimum lift height of 50 feet. At higher lift heights, a thicker filter layer will form. The high flow capacity of the drain and the permeable base layer of each lift of the fill, low potential infiltration, experience with Streeter Fill, and ~ engineering judgment all indicate that construction of a filter system other than by gravity sorting is not required. To summarize, our analysis indicates that by utilizing the planned construction procedures for the fills contamination of the underdrain will not occur as fill is placed and a separate designed filter layer is not required. Furthermore, in our opinion the ~ construction techniques used at Colowyo will result in an adegate gravity sorted drain without a specially placed drain. 5 •