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1. The material to be moved from K-Pit to the K-Pit Buttress Fill and the Horse Gulch <br />Fill formerly overlaid the coal seams; therefore, this material meets the definition of <br />overburden (1.04(83)). This overburden was removed during surface coal mining <br />operations and, therefore, also meets the definition of spoil (1.04 (122)). <br />2. Comparison of Trapper's premining topography map (M3) and the postmining <br />topography map (M12) demonstrates that the spoil overburden material proposed for <br />construction of the K-Pit Buttress Fill and the Horse Gulch Fill is not needed for <br />achieving approximate original contour; therefore, this material meets the definition <br />of excess spoil (1.04(43a)). <br />The engineering designs for the K-Pit Buttress Fill and the Horse Gulch Fill are <br />contained in Appendix T of the permit application. The fills will be constructed <br />entirely of excess spoil from the K-Pit area. The spoil geochemistry of the K-Pit is <br />similar to spoil elsewhere in Trapper's affected area. Neither coal mine waste <br />(1.04(22a)), nor coal processing waste (1.04(24)), will be used in the construction of <br />the fills. <br />4. The Horse Gulch Fill will be constructed in a valley where the side slopes of the <br />existing valley are greater than 20°, as measured at the steepest point; therefore, the <br />Horse Gulch Fill meets the definition of a valley fill (1.04(148)). The Horse Gulch <br />Fill will be constructed in the uppermost reaches of a hollow where the sides of the <br />gulch slope more than 20 feet; therefore, the Horse Gulch Fill also meets the <br />definition of a head-of-hollow fill (1.04(56)). <br />An underdrain will be constructed underneath the Horse Gulch Fill (Appendix T of <br />the permit application). The core of the underdrain will be constructed in advance of <br />construction of the main spoil pile. The core will be constructed of large rocks <br />retrieved from the toe of the main spoil area. All rock utilized for the underdrain will <br />have a diameter between 12 inches and 60 inches. These design features demonstrate <br />the underdrain will be constructed in compliance with 4.09.2(2). <br />6. Diversions of runoff around the Horse Gulch Fill are required by 4.09.3(1), based on <br />there being a significant difference between the elevation of the crest of the Horse <br />Gulch Fill and the low point of the adjacent ridgeline. Appendix T contains <br />engineering designs for diversion channels for the Horse Gulch Fill that will meet the <br />100-year, 24-hour storm event, as required by 4.09.2(7) for a valley fill and by <br />4.09.3(1) for a head-of-hollow fill. <br />7. Appendix T of the permit application contains a slope stability analysis for the K-Pit <br />Buttress Fill utilizing a two-dimensional numerical model. This analysis found the <br />design of the fill will provide a long-term factor of safety (the ratio of resisting forces <br />to driving forces) between 1.86 and 1.91. This analysis demonstrates the K-Pit <br />Buttress Fill is designed to attain the minimum long-term factor of safety of 1.5 <br />required for excess spoil fills (4.09.1(7)). <br />Trapper Mine 31 September 21, 2009