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three layers of spoil are assigned material strength parameters based on test results and numerical <br />modeling. These parameters differ based on the depth of the assigned layer and are outlined in Table 1 of <br />Attachment 1. The "depth -based" approach would not be considered the most conservative method <br />however, it's more representative and an additional layer of conservancy has already been included by <br />using a minimum safety factor criteria of 1.5. <br />Results of the slope stability analyses for each pit are shown in the following table: <br />Slope Stability <br />Analysis Results <br />Safety Factor <br />Pit <br />Cross - <br />Section <br />Pre -variance <br />Post -variance <br />F -Pit <br />FA -FA' <br />1.48 <br />1.66 <br />L -Pit <br />LA -LA' <br />1.5 <br />1.58 <br />LB -LB' <br />1.3 <br />1.55 <br />LC -LC' <br />1.49 <br />2.14 <br />A -Pit <br />AshA-AshA' <br />1.36 <br />1.75 <br />All results surpass the minimum requirement of 1.3 per Rule 4.27.3(3) and the requirement of 1.5 <br />committed to by Trapper. In all cases, the safety factor increases from the pre -variance AOC to the <br />proposed post -variance AOC. These results indicate greater potential for long-term stability using the <br />proposed AOC variances. <br />As stated earlier in this memo, a "depth -based" approach using spoil was used in each analysis. However, <br />based on a Division pre -site visit and inspection in November of 2018, it was observed that the A -pit is <br />already partially filled with compacted ash material from the power plant. There has been limited <br />discussion of the ash material in Appendix B (cover depth requirements) and no discussion in Attachment <br />1. In addition, the ash material was not part of the slope stability analysis. With that said, please add the <br />following comments to your adequacy letter associated with PR -09. <br />1) During a Division pre -site visit and inspection in November of 2018, it was observed that the A - <br />pit is partially filled with compacted ash material. However, there was no discussion or inclusion <br />of this material in the stability analysis. Please provide the Division with additional discussion <br />about the ash material found in the A -pit and include it in the stability analysis performed by <br />AAI. <br />2) Based on the "depth -based" approach, spoil of coarser/heavier particles are deposited at the <br />bottom with finer/lighter particles are deposited at the top. It is unsure to the Division how <br />Trapper plans to implement this during backfilling and grading of the L- and A -pit. Please <br />provide the Division with discussion on how Trapper plans to backfill and regrade to ensure spoil <br />participle distribution matches that which has been modeled using the "depth -based" approach. <br />This concludes my review of Trapper's proposed AOC variance and associated engineering report <br />submitted with PR -09. If you have any questions, feel free to contact me. <br />Sincerely, <br />Trapper Mine PR -09 Memo June 17, 2020 <br />