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-16- angle of internal friction of the spoil on Factor of Safety was minor. We varied the cohesion of The clay from 0 psf (which may represent a saturated condition or residual strength value) To The measured S00 psf. All of these calculations indicated factors of safety below The required 1.5;therefore, we believe the clay should be removed from the toe area of The spoil prior to placement of spoil. In The bottom of the valley, the colluvial clay may extend To depths greater than 30 feet. We believe removal of at least 20 feet of clays will be sufficient to enhance stability of the spoil Toe slope. Spoil unit weight and clay unit weight did not change the computed factor of safety appreciably over the range modeled. lNe analyzed the impact of removing clay along with variation of free water levels and These Tests indicated a factor of safety over 1.5 is maintained as long as free water levels less than 20 feet above the existing ground surface occur, along with removal of clay below the toe of the spoil. The valley near the Toe of the proposed spoil fill is relatively flat. If the recommended underdrain is constructed at the bottom of the excavation resulting from removal of the valley clays, a long trench will be necessary to "daylight" The drain. To avoid this condition and enhance stability at the Toe, we recommend the excavated clays be used to construct a comparatively impervious, buttress fill below the Toe of the spoil. The clays in the buttress fill should be placed in thin, loose lifts, moisture treated to within 2 percent of optimum moisture content, and compacted to at least 90 percent of maximum ASTh1 D 1557 dry density. The placement and compaction of this fill should be observed and tested during construction by a geotechnical engineering firm familiar with this type of construction; CTL/Thompson, Inc. can perform this service.