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while the clay-rich samples gave a 10°-12° range (samples F1-1 and Fl-5). <br />• Figure 4 is a plot of the Mohr failure envelope from these series of <br />consolidated drained direct shear tests for the clay-rich samples. Figure 5 <br />is the Mohr envelope for the coal-rich samples. <br />The cohesion ranges from 0, for F1-5, a clayey-silt sample, to 288 pcf <br />(2 psi) for the coal-rich refuse material. Table 2 summarizes these strength <br />values for the refuse material. The test details are in Appendix C. <br />The values in Table 2 were compiled and plotted as Mohr envelopes as shown <br />in Figures 4.and 5. Based on these drained direct shear tests, the effective <br />strength parameters for the refuse material are 30° friction angle and <br />2.0 psi cohesion (288 psf). This is for the refuse material consisting of high- <br />ash coal reject, mine roof shaly rock, clay-rich floor material, and coal <br />processing waste. In the presence of large fragments from the roof layers, <br />the friction angle is expected to increase to 40° while the presence of clay <br />from the mine floor will increase the cohesive strength up to 5 psi(720 psf). <br />It is therefore reasonable to project refuse material of 30° friction angle <br />and 288 psf cohesion, and consisting of the following rock aggregates: <br />• Component Range <br />High-ash (low Btu) coal 40%-60% <br />Shale-siltstone (mine roof) 30%-50% <br />Clay-shale (mine floor) 10%-30% <br />Coal Processing Waste 10%-50% <br />At the later stages of the mine-life, during the pillar-recovery phase, the <br />clay-rich portion may increase to 30% as more floor heave is possible. <br />Bedrock <br />The bedrock is 15 to 80 feet below the existing surface. These layers of <br />shale, mudstone and siltstone are sufficiently competent but the bedding <br />planes dip towards the toe of the proposed refuse pile. However, the dip of <br />the bed is relatively flat, 3° to 5°, and the joints appear to have cohesion <br />and friction angles higher than those of the colluvium. Therefore, the <br />strength of the weathered bedrock is more important than the intact beds or <br />• bedding plane joints. <br />-17- <br />