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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 Horse Gulch Fill is not needed for achieving approximate original <br />contour; therefore, this material meets the definition of excess spoil (1.04(43a)). <br />3. The engineering designs for the Horse Gulch Fill are contained in Appendix T of the <br />permit application. The fills will be constructed entirely of excess spoil from the K- <br />Pit area. The spoil geochemistry of the K -Pit is similar to spoil elsewhere in <br />Trapper's affected area. Neither coal mine waste (1.04(22a)), nor coal processing <br />waste (1.04(24)), will be used in the construction of 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 Horse <br />Gulch Fill utilizing a three- dimensional finite difference model. This analysis found <br />that horizontal lifts constructed by dumping material from a height of 400 feet will <br />result in a factor of safety for the Horse Gulch Fill of 1.45. The analysis also found <br />that contouring the dumped material to the final configuration of the design will <br />increase the long -term factor of safety to a value between 1.51 and 1.75. This <br />analysis demonstrates the Horse Gulch Fill is designed to attain the minimum long- <br />term factor of safety of 1.5 required for excess spoil fills (4.09.1(7)) and valley fills <br />(4.09.2(1)). <br />VIII. Coal Mine Waste Banks <br />No specific approvals are granted to the applicant under this section. <br />Trapper Mine 30 July 9, 2013 <br />