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percent compaction, as the coal waste will be placed in the waste bank, and direct shear <br />• tested using ASTM D-3080. The results were a peak internal angle-of-friction of 36 degrees, <br />and a peak cohesion of 0.8 kips per square foot (ksf). Based on the Standard Proctor test, <br />a wet unit weight of 65 pounds per cubic foot (pcf) was used with a saturated unit weight <br />of 78 pcf. The coal waste is shown as material '1' on Figures 4.3 and 4.4. <br />Material properties for the colluvial sandstone above the Mancos shale underlying the site was <br />obtained using SPT blowcounts obtained through the WESTEC borehole investigation. At <br />WBH-B2, a 30 blows per foot measurement was recorded within the colluvial material. Based <br />on the depth, the blows were adjusted to 1 ton per square foot Itsf) overburden according to <br />procedures outlined by Peck, Hanson and Thornburn (Peck et al., 1974). The resulting <br />corrected blowcount was related to an internal angle of friction of 36 degrees (Lambe and <br />Whitman, 1969). The material displayed non-cohesive characteristics in the field, so the <br />cohesion component of strength was modeled as 0 psf. Average unit weights for the material <br />was assumed to be 120 pcf wet, and 125 pcf saturated (Coduto, 1994). This colluvial <br />material is shown as material '2' in Figures 4.3 and 4.4. <br />• Fill located at the toe below the old coal loading area was field observed to be aloes-plasticity <br />clayey material. Atypical SPT blowcount within this material was 21 bpf. Using the same <br />corrections and correlations as the colluvial material, the internal angle-of-friction was <br />determined to be 25 degrees, with 150 pounds per square foot (psf) cohesion. The unit <br />weights for this fill material was determined to be 110 pcf wet and 115 pcf saturated. The <br />fill material is denoted as material "4' on Figures 4.3 and 4.4. <br />The material properties for the Mancos shale is important to the overall stability of the coal <br />waste bank. Geological hazard maps of the area (USGS, 1979b) indicate that swelling soils <br />exist at the site, the source of which is the Mancos shale. Mancos shale is reactive to water <br />such that a loss of strength usually occurs if there is not adequate drainage. Although there <br />are several systems at the site designed to transport water away from waste bank and keep <br />materials involved dry and at peak strength, the shale was modeled at a lower strength than <br />demonstrated in the field. During the WESTEC borehole investigations, the Mancos shale <br />demonstrated high strength (50+ SPT blow counts per 6 inches) and dry conditions. As a <br />contingency, the shale was modeled with a 20 degree friction angle, and 2,000 psf cohesion <br />based on previous WESTEC experience in the area. Unit weight for the shale was assumed <br />• <br />96266/1424.RPT Coal Mine Weste Benk Finel Design for the Bowie No. 2 Mine <br />June 1998 WESTEC 21 <br />wcQUACx-o1 <br />