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TR-9 Geotechnical Review Memo April 25, 2025 1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http://mining.state.co.us Jared S. Polis, Governor | Dan Gibbs, Executive Director | Michael Cunningham, Director Date: April 25, 2025 To: Amy Yeldell CC: Travis Marshall From: Zach Trujillo RE: Mid Continent Limestone Quarry, DRMS File No. M-1982-121 Technical Revision No. 9 – Highwall Reclamation Stabilization Review Amy, As requested I have reviewed the provided geotechnical report, “Highwall Reclamation Stabilization” (Report), associated with Technical Revision No. 9 (TR-9). The Report is conducted by Golden Geotechnics, Inc. (GGI) on behalf of RMR Aggregates, Inc. (RMR) for the Mid-Continent Limestone Quarry (Mine). The purpose of the proposed Report is to outline a plan for highwall stabilization for current site conditions in the event of revocation and forfeiture of the Mine. The Report is based on the site reconnaissance and engineering analysis provided in Kildruff Underground Engineering’s (KUE) report, “Failure Analyses and Stabilization Report” that was approved under Technical Revision No. 6 (TR-6). For more information on the referenced KUE’s engineering report, please refer to TR-6 and the Division’s associated review letters dated September 29, 2023, February 6, 2024, and March 25, 2024. As noted earlier in this memo, the proposed Report is based on the engineering analysis and site reconnaissance conducted by KUE. Most of the site parameters and conditions are carried over to the Report from the original analysis provided under TR-6. The primary differences between KUE’s analysis and the Report come from two main factors; 1) the change in designation and associated minimum factor of safety requitements of the Type of Structure/Consequence of Failure from “critical” to “non-critical” as outlined in Section 30, Table 1 of the Policies of the Mined Land Reclamation Board and 2) the use of an increased value of cohesion for the interbed material located between limestone beddings. Board Policies – Section 30, Table 1 Table 1. - Recommended Minimum Factors of Safety for Slope Stability Analyses for Operations and Reclamation outlines the minimum requirements for factors of safety for the Division. The table is broken down into four quadrants based on potential magnitude of damages (critical vs. non-critical) and reliability of geologic information (assumed parameters vs. site specific parameters). As noted in GGI’s Report, in the scenario of revocation, the site would no longer be operating and the potential for human safety risk no longer remains. With the Mine inactive, GGI uses the designation of “non-critical” for generalized, assumed, or single test strength measurements. The corresponding minimum factors of safety found with Section, Table 1 are 1.3 for static conditions and 1.15 for seismic conditions.