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e.. RPPU ..,ILC <br />e.:Z 2Mlw <br />C.PI RMV eepoul k.P Ne.1 <br />wP 51., <br />4 . 1 ..11.1 Lw.� ♦M1 P.lun \ �� <br />0 <br />Figure 1. Geometry of Coal Waste Disposal Area No. 4 used for stability evaluation • <br />5.2 Material Properties <br />Two material types were used in the stability models: (i) coal refuse, and (ii) native foundation <br />soil. Material properties required for the stability analyses include shear strength parameters <br />and unit weight. Material parameters used in these revised stability evaluations are discussed <br />below and presented in Table 3. <br />Shear strength parameters for the coal mine refuse were determined from triaxial shear testing <br />conducted on both remolded and thin - walled samples of placed coal refuse. The triaxial shear <br />testing is discussed in Section 4.0 and by BGI (2006). Effective strength parameters were used <br />for the coal refuse. An angle of internal friction of 34.0 degrees with zero cohesion was used <br />for the stability analyses, as this is the average determined from the CU triaxial shear tests. <br />However, we also evaluated the sensitivity of the stability model by varying the friction angle <br />for the coal refuse. For these sensitivity analyses, conservative friction angles of 32.0 and 30.0 <br />degrees were used for the coal mine refuse. <br />A moist unit weight of 110 pcf was used for the coal refuse for the stability analyses. This <br />moist unit weight is an average unit weight determined from 250 nuclear moisture- density tests <br />conducted on compacted Bowie coal refuse over the past four years. <br />Shear strength parameters for the foundation soils were conservatively estimated based on <br />materials encountered during site investigation and our professional judgment. The cobble and • <br />boulder content of the foundation soils made both in -situ strength testing and soil sampling <br />Bowie No 2 Refuse Me No 4 2009 Revised stability.dx <br />PmjeR #06481 -003 <br />Page 8 of 12 <br />