2025-05-05_PERMIT FILE - C1981010 (24)

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6 topography and the surrounding area for approximately 25 square miles as Figure 8, Permit Area Post- Mine topography. This overview figure shows that the proposed post-mining topography blends well with other areas surrounding the Trapper Mine and would not appear unnatural or of a different general slope or aspect. Once revegetation is established, the post-mining topography would appear similar to surrounding areas. Map M14A Sheet 1 shows several cross sections through the L Pit with the pre- mining and proposed post-mining topography. The cross sections highlight the differences in backfill height and slope between pre- and post-mining topography. Post-Mining Stability Analysis AAI performed an evaluation of the geologic and geohydrologic conditions present in the L Pit and used that information to develop a stability analysis of the backfilled L Pit under both the proposed post- mining topography and the AOC topography. The analysis is contained in the AAI report, Attachment 1, also to be included as part of Appendix B of Permit C-1981-010 and briefly described below. The stability analysis considered the factors that critically affect stability of backfill at the Trapper Mine including:  Pit floor gradients – At Trapper the pit floor varies between 6° and 13° which results in both an overall reduction in backfill stability, especially with thicker volumes of spoil placed upslope under the AOC backfill scenario. The proposed backfill under the AOC variance generally places spoil at lesser thickness upslope and buttresses the upslope backfill with thicker backfill downslope.  Backfill Saturation – Pre-mining the groundwater at the Trapper Mine occurs in alternating aquifers of more permeable rock layers confined by less permeable rock. In the pits backfilled with spoil, the groundwater percolates downward through the backfill materials and trends along the sloping floor of the pit to pool at the endwall at the toe of the slope where it meets the less permeable native bedrock. The development of a phreatic surface above the pit floor and collection of groundwater at the toe of the slope has an overall negative impact on the long-term stability of the backfilled material, particularly in thicker spoil placement upslope that would occur under AOC.  Pit Floor Geology – The coal mined at Trapper is typically underlain by a seam of weak clays and shales. These materials will deteriorate over time, creating instability in the presence of groundwater. This will be exacerbated by a deeper depth of spoil placed under AOC.  Spoil Depth Variations – Research indicates that spoil shear strength decreases with depth of spoil and the overall strength of deeper spoil is controlled by clay-sized particles. Placement of deeper spoils in areas under an AOC post-mining topography is likely to result in weakened areas under the peaks and ridges with a potential for differential settlement and deformation presenting as surface cracks. These factors along with the evidence that unmined competent rock shows some instability in the surrounding area, create a potential for instability that was studied by AAI. A detailed discussion of the AAI approach to stability is contained in Attachment 1. AAI evaluated 3 sections through the L Pit under both AOC and the proposed post-mining topography. These vertical analysis sections were taken along the true dip direction. The results indicate that for the L Pit, the Safety Factors for AOC configurations are consistently lower than the proposed post-mining topography and at or below AAI’s minimum threshold for long-term stability of 1.5. Table 2 below presents the results.