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stability. This is because the relatively narrow width of the valley deposits compared to the wide <br />face of the spoil will result in underestimation of increased resistance due to three - dimensional <br />effects within the spoil. <br />6.1.2 Carbonaceous Mudstone (CMS) at Base of Spoil Model <br />This model includes a weaker CMS /CS layer 5 feet thick at the base of the spoil. The dip <br />of this layer was varied at either 3 percent or 5 percent to consider the possible bedrock dip in <br />this area. These dip estimates are based on the X -seam floor contours at the location shown on <br />Figure 2. The toe of this model is at a slightly higher elevation compared to the above model. <br />This is due to the offset of the X -Seam mining from the centerline of the valley where the lowest <br />tip of the spoil toe is located. <br />6.2 Selection of Model Parameters <br />6.2.1 Spoil <br />The spoil will consist of excavated bedrock composed of sandstone, siltstone, and <br />mudstone. The gradational characteristics of spoil vary due to sorting as the spoil is end dumped <br />over the crest of each lift. The relatively high lift thicknesses (30 to 100 feet) and end - dumping <br />over the edge of the lift produce significant sorting of the spoil. Unsorted spoil was estimated to <br />have the following gradational characteristics (CTL /Thompson, Inc., 1992a). <br />Cobbles and boulders <br />50% <br />Gravel <br />28% <br />Sand <br />2% <br />Fines (No. 200 sieve) <br />20% <br />Very coarse spoil (median grain size D 50 of 10- inches or more) is typically present at the <br />base of each lift of spoil. <br />During previous work at the site we measured and surveyed end dumped spoil slopes in <br />the West Pit. These indicated spoil material has steep overall angles of repose on the order of 38 <br />degrees. CNI performed large scale (12 inch by 12 inch shear box) direct shear testing of spoil <br />samples from the west pit (CNI, 2005a), the results are summarized on Figure 6. These tests <br />were performed on spoil samples with particle sizes less than 1 -inch and normal forces up to 80 <br />psi. We used the mean CNI spoil test results of friction angle 34.9 degrees and 750 psf as the <br />primary spoil shear strength estimate (friction angle was rounded to 35 degrees in our analysis). <br />23 -1- 01 1 0 1- 400- R1.docx/wp /lkn 23 -1- 01101 -400 <br />Exh. 23 -1 -15 Revision Date: 08/05/09 <br />Revision No.: PR -03 <br />