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PN: D99009GE <br />March 23, 1999 <br />Page 2 <br />We performed random licld density tests during the material placement at the project site when <br />requested. All areas where the test results indicated that the material placed did not meet the <br />minimum project compaction requirements were re-rolled and retested until the test results <br />indicated that the required relative compaction in the areas tested were achieved. Final shaping <br />and contouring was done after our field density trsting. <br />The field for this slope stability study was performed on February 3, 1999. The field study <br />consisted of logging and sampling the soils encountered in four (4) test borings. The log of the <br />soils encountered in the test borings are presented on Figures A2 through A5. <br />We performed three (3) Direct Shear tests on the material placed to determine the angle of <br />internal friction (phi) and cohesion of the material being placed. Direct shear strength properties <br />of sleeve samples were evaluated in general accordance with tes(ing procedures defined by ASTM <br />Test Method D3080. An internal angle of friction of twenty-five (25) degrees and a cohesion of <br />250 pounds per square foot were used in our slope stability analysis. "I~he nominal average wet <br />density of the direct shear sample tested was about 120 pounds per cubic Ibot. <br />7~he material at the Oak Ridge Energy site was placed on top of and adjacent to an existing soil <br />deposition area. We previously performed a slope stability analysis for this material pile prior to <br />the current shaping and final contouring. Ow' previous analysis was presented in our December <br />21, 1995 letters. We understand that a drain system was placed below the existing pile near the <br />toe of [he slope. Long term stability of the deposition area is dependent, in a very large part, on <br />the proper location and long term performance of [he drain system. <br />The stability of any slope is dependent on many lactors. Typically the stability of a slope is <br />analyzed by calculating the anticipated gravitational forces that tend to drive the mass of soil <br />dowt>jiill and compare these forces to the internal strength characteristics of the soil along the <br />expected plane of failure which tend to resist the potential movement. If the driving forces are <br />;realer than the resisting forces then failure of the slope is likely. Slope failure can occur as slow <br />deformation, creep, or as a somewhat spontaneous and rapid movement. <br />As requested we performed Simplified Bishops Method of Slice.; to assess the theoretical slope <br />stability of [he material as placed. We analyzed three (3) cross sectional areas that currently exist <br />on the site. We analyzed areas of steepest gradient. TLc steepest gradient of the stock pile <br />material that currently exists on the site is about hvo and one-half (2'/~) to one (I) (horizontal to <br />vertical.) We used existing soil parameters, slope gradient, and material height in our analysis. <br />We did not assess the in-place support materials on the site. We understand that the stability of <br />the support materials was assessed as part of the previous moniroring at the site during the <br />operation of the coal mine. We assumed that the support materials have similar strength <br />characteristics as the material currently being placed and that the most likely mode of failure was <br />' a toe failure of the embankment. We performed our analysis assuming pseudo-static seismic <br />conditions. <br />i!I~I1lZ17L'1'1 X11111 ~`_`,'_;11Ci~1~1'S <br />CONSULTIIiG GEOTECHN ICAL ENGINEERS <br />At1D IMTERIALS TESTING <br />