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• Material Properties <br />We estimated material properties based upon our review of previous studies <br />and the results of the current investigation. These were discussed in detail in the <br />preceding section D(ISTING RDA CONDITIONS. The best estimate was designated <br />as the "normal" value. For each model the normal values were evaluated and then <br />properties were varied (density, strength, water condition or earthquake <br />acceleration) while the remaining properties remained at the normal value. <br />Coal Refuse. The strength values used for the coal refuse were based <br />primarily upon the direct shear testing performed during this investigation (5 tests) <br />and test results from previous investigations (4 tests). The tests results were <br />compared to published values (Reference 15). Figure 5 shows the results of the <br />testing and the normal value selected for use in the stability analysis. The normal <br />• values were friction angle of 32 degrees and cohesion of 200 psf. As shown on Fig. <br />5, this value is considered conservative in that k is below the results of most of the <br />testing performed. We also evaluated conditions for no cohesion as a worst case <br />condition. For each of these cases (with and without cohesion) the friction angle <br />was varied from 30 degrees to 36 degrees. The density of the coal refuse was <br />estimated based upon the results of the moisture-density tests performed for this <br />investigation. The average dry density was 93 pcf at a moisture content of 11 <br />percent. This is equivalent to a moist density of 103 pcf. The parameters selected <br />were a normal moist density value of 105 pcf, with a low value of 85 pcf and a high <br />value of 125 pcf. <br />Soil Laver. The soil layer was included in the model as a continuous layer <br />to model a disturbed or mixed zone of soil/bedrock/refuse at the base of the RDA. <br />• <br />18 <br />