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WCOLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />Date: June 17, 2020 <br />To: Robin Reilley <br />CC: Jared Ebert and Jason Musick <br />From: Zach Trujillo <br />RE: Trapper Mine, DRMS File No. C-1981-010 <br />Trapper Mine PR -09 Review Memo <br />Robin, <br />As requested I have reviewed Trapper Mine's (Trapper) proposed AOC variance and associated <br />engineering report submitted with PR -09. Part of the Trapper's recent submission, they included Steep - <br />Slope Mining Variance from Approximate Original Contour (Appendix B) and Agapito Associates Inc. <br />(AAI) Geotechnical Evaluation of L -Pit and Ash -Pit in Support of Trapper's Steep -Slope Mining <br />Variance Application (Attachment 1). Since my request was specific to engineering principles and <br />applications, my review focused on Section 2.0 — Steep Slope Analysis found in Appendix B and the <br />entirety of Attachment 1. Additionally, Section 2.0 in Appendix B has been constructed as a summary of <br />Attachment 1 which goes into depth of the geotechnical analysis performed by AAI. Due to this, my <br />comments will be directed at Attachment 1. <br />Per Rule 4.27.3(3), [t]he highwall shall be completely covered with compacted spoil and the disturbed <br />area graded to comply with the provisions of 4.14, including but not limited to the return of the site to the <br />approximate original contour. The person who conducts the surface coal mining operation must <br />demonstrate to the Board or Division, using standard geotechnical analysis, that the minimum static factor <br />of safety for the stability of all portions of the reclaimed land is at least 1.3. In both Appendix B and <br />Attachment 1, Trapper has committed to a minimum factor of safety 1.5 to ensure long-term global <br />stability which exceeds the minimum requirements of the above mentioned rule. <br />In AAI's Attachment 1, AAI conducted pre- and post -variance stability analysis for the F -Pit, L -Pit and <br />A -Pit. While the F -Pit is reclaimed, AAI included an analysis for the F -Pit to demonstrate long-term <br />stability with previously approved AOC variances. For the L -Pit, at total of three critical cross-sections <br />was evaluated and one critical cross-section for the A -Pit. As part of the analysis, AAI took into <br />consideration pit floor gradients, backfill saturation of groundwater, weak underlying strata, and spoil <br />depth variability. When modeling their slope stability analysis, AAI used a conservative approach <br />modeling continual groundwater flow 10 feet above pit floor based on site well data from Well NP -1. <br />Additionally, AAI used a three layered "depth -based" approach with spoil backfill. Coarser particles are <br />deposited at the bottom of the backfill with finer material being deposited as depth decreases. Each of the <br />Trapper Mine PR -09 Memo June 17, 2020 <br />OF C�lo <br />0 <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http://mining.state.co.us I * * <br />Jared S. Polis, Governor I Dan Gibbs, Executive Director I Virginia Brannon, Director *+►* <br />