2020-05-13_REVISION - C1981010 (41)

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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 181.2 acres or 9.7%. The pre- and post- mining drainage densities will remain the same and the overall drainage gradient will increase by 0.2%. Sediment yield will decrease from 1.13 tons/acre to 0.81 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 Drainage Gradient Sediment Factors Density N Yield (mi/miz) Per Acre (tons) Pre-Mining 1,877.5 4-26 1.3 to 1.5 3.8 10.6 1.13 Post-Mining 2,058.7 4-14 1.58 to 2.14 3.8 10.8 10.81 revision: 3-43 b Approved: