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Colowyo Coal Company, L.P. SHANNON &WILSON. INC. <br />Attn.: Mr. Juan Garcia <br />March 10, 2015 <br />Page 5 of 7 <br />REVISED DRAIN RECOMMENDATIONS <br />Little Collom Gulch in the revised location meets the requirements for a valley fill based on side <br />slopes greater than 20 degrees (Section 1.04, CMLRB, 2008). A drain is recommended in the <br />base of this valley at the location shown on Figure 1. <br />At the revised spoil fill location there is a smaller drainage along the western boundary of the fill. <br />This drainage does not meet the criteria for a valley fill with respect to side slopes or slope of <br />drainage floor. However, we recommend a smaller drain in the base of this drainage at the <br />location shown on Figure 1. <br />Based on the anticipated rock types in the excess spoil, we recommend a revised drain size for <br />the valley drain in Little Collom Gulch. CDRMS Regulations 4.0.9.2 requires a minimum valley <br />fill drain width of 16 feet and height of 8 feet when the predominant fill material is sandstone. <br />The area of a triangular drain with a base width of 16 feet and a height of 8 feet is area of 64 <br />square feet [sfJ). <br />A larger drain is recommended for predominantly shale material. In our opinion, the smaller <br />drain size (64 sf) associated with predominantly sandstone fill is applicable based on the <br />following evaluation. <br />The predominant rock types from which spoil will be derived, the percentage of rock type <br />expected based on core logs and the average Unconfined Compressive Strength (UCS) were <br />estimated based on information from the geotechnical drilling in the Collom pit (CNI, 2005). <br />This indicates the following: <br />■ Mudstone, 23 percent of spoil material anticipated, average UCS 4,413 psi. <br />• Siltstone, 36 percent of spoil material anticipated, average UCS 6,416 psi. <br />■ Sandstone, 18 percent of spoil material anticipated, average UCS 4,989 psi. <br />These rock types represent approximately 77 percent of the anticipated spoil material (the <br />reminder was classified primarily as coal with minor percentages of carbonaceous mudstone, ash <br />or burned zones). These three rock types were typically are massive bedded and are not shale <br />like. Based on the UCS values, the siltstone is typically harder than the sandstone, and the <br />mudstone UCS is only slightly lower than the sandstone. <br />23-1-01403-002 <br />