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

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5 2.0 Steep Slope Analysis For the purposes of the AOC variance, Trapper contracted with Agapito Associates, Inc. (AAI) to evaluate the existing conditions and stability of the L Pit and Ash Pit based on pre-mining topography and post-mining stability under both AOC conditions and lesser slopes proposed by the variance request. AAI’s report “Geotechnical Evaluation of L-Pit and Ash-Pit in Support of Trapper’s Steep-Slope Mining Variance Application” is attached as Attachment 1 and will be included with this document inserted into Permit C-1981-010 as part of Appendix B. AAI identified that, based on the unique combination of steepness of the pit floor, spoil thickness, spoil saturation and the presence of a weak shale/clay in the floor of the mined pits, steep slopes for the final cut of the L Pit and Ash Pit at the Trapper Mine should be identified as any slope over 16°. These conditions make any reclaimed slopes at a steepness of 16° or steeper unstable as described in the following sections. 2.1 Pre-Mining Topography and Evidence of Historic Slides The pre-mining topography for the Trapper Mine and surrounding areas is shown on Map M3 of the Trapper Mine Permit M- 1981-010. Overlaying the pre-mining topography with the limits of the final cut of L Pit shows there are small areas throughout the pit and in particular in the southern portion of the pit with slopes that exceeded 20° and almost all of the pre-mining topography of the pit had slopes that exceeded 16° (see Figure 1a of the AAI report). The pre-mining topography of the Ash Pit showed slopes shallower than 16° for most of the pit area with steeper areas within the northern portion of the pit (See Figure 1.b of the AAI report). AAI studied Google Earth Imagery to evaluate historical slumping within the surrounding areas on ground not impacted by mining operations and, based on similar conditions, likely to be encountered at the Trapper Mine (oversteepened slopes, similar geology likely in combination with a weak base layer such as a clay or shale and saturated conditions). There is evidence of historical instability resulting in slumps in an area south of the G Pit; three areas to the southeast and southwest of the L Pit, and an area southeast of the Trapper Mine Office (see AAI report Figures 2a, 2b, and 2c). Based on the similarities between pre-mining slopes in these historical slumps, the similar conditions likely to be encountered at the Trapper Mine in the L Pit and Ash Pit (oversteepened slopes, similar geology likely in combination with a weak base layer such as a clay or shale and saturated conditions), and the fact that the historical slumps occurred in competent rock significantly stronger than the mined spoil material, AAI concluded that an AOC post-mining topography could result in unstable slopes in both pits. 2.2 Proposed Post-Mining Topography and Stability 2.2.1 L Pit Post-Mining Topography A proposed postmining topography was developed for the L Pit that takes into consideration the need for shallower reclaimed slopes. The post-mining topography is shown on Map M12 Sheet 3. The proposed post-mining topography would have slopes of 4 to 14°, shallower than the overall pre-mining topography of 4-26° and would have the same general aspect with slopes facing north/northeast. The L Pit would be backfilled in the same manner as described in the Permit C-1981-010 Section 3.5. The backfill would eliminate all highwalls, and spoil/refuse piles. Once backfilled, the spoil would be graded to blend with the surrounding topography, and drainage controls would be established as described in Section 4.0 below. Attachment 2 to this document is titled “An Evaluation of Pre-Mine and Post-Mine Hydrology Characteristics Associated with the Trapper Mine Approximate Original Contour Variance Application” completed by JMattern Mining, LLC. (Mattern) and includes an overview of the proposed post-mining