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Coal mine waste samples obtained by WESTEC from OXBOW were tested in a laboratory • contracted by WESTEC. A Standard Proctor compaction test (ASTM D-698) was performed on a sample of the waste. The results were used to remold another sample to 90 percent compaction, as the waste will be placed in the waste bank, and direct shear tested using ASTM D-3080. The results were a peak internal angle-of-friction of 33 degrees, and a peak cohesion of 2.0 kips per square foot (ksf-. Based on the Standard Proctor test, a wet unit weight of 91 pounds per cubic foot (pcf) was used with a saturated unit weight of 100 pcf. The coal mine waste is shown as material "1" on Figures 4.3 and 4.4. Material properties for the colluvial sandstone above the Bowie Member shale underlaying the site was obtained using SPT blowcounts obtained through the WESTEC borehole investigation. At WBH-S01, a 21 blows per foot measurement was recorded within the colluvial material. Based on the depth, the blows were adjusted to 1 ton per square foot ltsf) overburden according to procedures outlined by Peck, Hanson and Thornburn (Peck et al., 1974). The resulting corrected blowcount was related to an intemal angle of friction of 33 degrees (Lambe and Whitman, 1969). The material displayed non-cohesive characteristics in the field, so the cohesion component of strength was modeled as 0 psf. Average unit weights for the material • was assumed to be 115 pcf wet, and 125 pcf saturated (Coduto, 1994). This colluvial material is shown as material '2" in Figures 4.3 and 4.4. The Rollins Sandstone was proven a relatively strong, continuous strata through borehole investigations and rock outcrop mapping. No previous failures have occurred in the area involving the Rollins Sandstone and construction of this magnitude founded directly on it. By historical records (Anderson, ] 995), the Bowie Member/Rollins Member interface is gently dipping north, away from the toe of the coal mine waste bank. This is generally a favorable situation, since there will be a natural tendency for resistance to movement along this interface, ruling out this mode of failure. For these reasons, failure within the Rollins Sandstone Member was not selected as a failure mode for this stability analysis. Rollins Sandstone below the site was modeled artificially high so the computer program would not chose a failure surface passing through as a critical failure surface. Parameters chosen for the sandstone were an internal angle-of-friction of 50 degrees, a cohesion of 5000 psf, and wet and saturated unit weights of 150. The Rollins Sandstone Member is identified as material "3" on Figures 4.3 and 4.4. 95256/1424.RPT Coal Mine Weate Bank DeeiQn for the Sanborn Creek Mine October 1998 WESTEC, Ina. 20