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-16- <br />• angle of internal friction of the spoil on Factor of Safety was minor. We <br />varied the cohesion of the clay from 0 psf (which may represent a saturated <br />condition or residual strength value) to the measured 500 psf. All of these <br />calculations indicated factors of safety below the required I.S; therefore, we <br />believe the clay should be removed from the toe area of the spoil prior to <br />placement of spoil. In the bottom of The valley, The colluvial clay may extend <br />to depths greater than 30 feet. We believe removal of at least 20 feet of <br />clays wi(I be sufficient to enhance stability of the spoil toe slope. <br />Spoil unit weight and clay unit weight did not change the computed <br />factor of safety appreciably over the range modeled. ~.Ne analyzed the impact <br />of removing clay along with variation of free water levels and these tests <br />indicated a factor of safety over 1.5 is maintained as long as free water <br />levels less than 20 feet above the existing ground surface occur, along with <br />removal of clay below the toe of the spoil. <br />The valley near the toe of the proposed spoil fill is relatively flat. If <br />the recommended underdrain is constructed at the bottom of the excavation <br />resulting from removal of the valley clays, a long trench will be necessary to <br />"daylight" the drain. To avoid this condition and enhance stability at the toe, <br />we recommend the excavated clays be used to construct a comparatively <br />impervious, buttress fill below the toe of the spoil. The clays in the buttress <br />fill should be placed in thin, loose lifts, moisture treated to within 2 percent <br />of optimum moisture content, and compacted to at least 90 percent of <br />maximum ASTM D 1557 dry density. The placement and compaction of this <br />fill should be observed and Tested during construction by a geotechnical <br />engineering firm familiar with this type of construction; CTL/Thompsori; Inc. <br />. can perform this service. <br />