2024-05-21_PERMIT FILE - C1981010A (8)

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3-43b 3.5.3.1 L-Pit and Ash Pit Variance from Approximate Original Contour The final cut of the L Pit and the Ash Pit will be backfilled and graded using the methodologies outlined in Section 3.5 but with a final graded configuration that will vary from approximate original contour or AOC. The projected post-mining topography of the L Pit is shown on Map M12, Postmining Topography, Sheet 3. The projected post-mining topography of the Ash pit is shown on Map M12, Postmining Topography Sheet 2. Cross sections of the pre-mining and post-mining topography are shown on Map M14A Postmining Cross Sections, Sheet 1 for the L- Pit and Sheet 3 for the Ash Pit. Appendix B provides detailed information on the benefits of backfilling and grading of the L Pit and Ash Pit to gentler slopes for both long-term stability and watershed improvement. Permit C-1981-010 Technical Revision 69 cautioned that steep slopes at the Trapper Mine could potentially be anything over 14°, depending on hydrologic or geotechnical conditions. The L Pit overall pre-mining topography exhibited extensive slopes of 16° or steeper, with some slopes as steep as 26° in the southern portion of the L Pit area. Post-mining the slopes will range from 4 to 14° and will blend with the surrounding topography. The post-mining backfill configuration for the L Pit will decrease the depth of spoil placed at the head of the slope and increase the depth of spoil placed at the toe of the slope to act as a buttress to any movement. Overall surface topography that is less steep will also increase the stability of the backfilled spoil. The stability analysis (App. B, Attachment 1,) showed that the post-mining stability for the AOC variance improved Safety Factors when compared to restoring the L Pit to AOC. Further, post-mining drainage density and slope have been designed to approximate the pre- mining drainage distribution. The AOC variance drainage design will result in overall watershed improvement. For the L Pit, the drainage area for Flume Gulch, which is the major drainage associated with the L Pit, will increase by approximately 205.7 acres or 9.9%. The post-mining drainage densities will increase by 0.1%, and the overall drainage gradient will remain the same. Sediment yield will decrease from 1.13 tons/acre to 0.80 tons/acre under the proposed post-mining configuration. The table 3.5-1 summarizes the pre- and post-mining characteristics for the L Pit. Table 3.5-1 Pre- and Post-Mining Characteristics for the L Pit Condition Acreage Slopes (°) Safety Factors Drainage Density (mi/mi2) Gradient (%) Sediment Yield Per Acre (tons) Pre-Mining 1,877.5 4-26 1.3 to 1.5 3.8 10.6 1.13 Post-Mining 2,083.2 4-14 1.58 to 2.14 3.9 10.6 0.80