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5.2 Material Properties <br />Two material types were used in the stability models: (i) coal mine waste, and (ii) native <br />foundation soil. Material properties required for the stability analyses include shear strength <br />parameters and unit weight. Material parameters used in these revised stability evaluations are <br />discussed below and presented in Table 4. <br />Shear strength parameters for the coal mine waste were determined from triaxial shear testing <br />conducted on both remolded and thin-walled samples of placed coal mine waste. The triaxial <br />shear testing is discussed in Section 4.0 and by BGI (2006). Effective strength parameters were <br />used for the coal waste for these analyses. An angle of internal friction of 34.0 degrees was <br />used for the stability analyses, as this is the average friction angle determined from the CU <br />triaxial shear tests. However, we also evaluated the sensitivity of the stability model by varying <br />the friction angle for the coal waste. For these sensitivity analyses, conservative friction angles <br />of 32.0 and 30.0 degrees were used for the coal mine waste. <br />Although undrained, total stress (~ = 0) analyses were conducted and reported by BGI (2006), <br />these analyses are not considered appropriate for the coal waste pile based on the pore <br />pressure monitoring conducted over the past three years. As discussed in Section 3.1, vibration <br />wire piezometers demonstrate that pore pressures within the coal waste are dissipating and <br />therefore do not represent the undrained conditions that a total stress analysis evaluates. <br />Furthermore, the pore pressures measured by the piezometers are well below the pore <br />pressures at failure that were measured during the CU triaxial shear tests at similar confining <br />• stresses as the piezometers. For these reasons, effective stress analyses using pore pressures <br />measured by the vibrating wire piezometers were used for these updated stability evaluations. <br />A moist unit weight of 110 pcf was used for the coal waste material for the stability analyses. <br />This moist unit weight is an average unit weight determined from 212 nuclear moisture-density <br />tests conducted on the placed Bowie coal mine waste over the past three years. <br />Strength parameters for the colluvial foundation materials were conservatively estimated using <br />correlations published by Peck et al. (1974) based on standard penetration test (SPT) N-values <br />obtained during drilling (BGI, 2006). Disregarding N-values where the split spoon was <br />~~bouncing" on boulders, an average N-value of the foundation soils of approximately 38 blows <br />per foot was measured during BGI's 2005 investigation. These values are similar to those <br />reported during a previous geotechnical investigation at the site (Lambert, 2003). Using the <br />previously referenced correlation, a conservative effective friction angle of 34 degrees with zero <br />cohesion was selected for the foundation soils. The unit weight of the foundation soils was <br />estimated to be 125 pcf. <br /> <br />Bowie No 2 2008 Reevalution of Coal Mine Waste Stabiliry.doc <br />Project #OS-126-GEO <br />Page 10 of 15 <br />