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<br /> <br /> <br /> <br /> 1536 Cole Blvd., Suite 310, Bldg. 4 715 Horizon Drive, Suite 340 <br /> Lakewood, CO 80401 Grand Junction, CO 81506 USA <br /> +1 303.271.3750 +1 970.242.4220 <br />www.agapito.com <br />July 31, 2024 534-61 <br />Tonia Perkins <br />Senior Mining Engineer <br />Trapper Mining Inc. <br />25910 S. Highway 13 <br />Craig, CO 81625 <br />Re: Inwall HWM Sequence Analysis Proposed for N-Pit <br />Dear Tonia: <br />As requested by Trapper Mining, Inc. (Trapper), Agapito Associates, Inc. (Agapito) has completed <br />an analysis of highwall mine (HWM) stability in the N-Pit with consideration of a top-down <br />mining sequence. Agapito has developed the mine plan as shown in Figure 1 based on panel <br />orientations and safety factor constraints. Agapito understands that the overlying M Seam must <br />remain stable while the lower seams within the N-Pit are mined out. Because of the top-down <br />mining sequence, it is critical that the highwall mining does not significantly impact highwall <br />stability. On this basis, Agapito has utilized the numerical modeling program FLAC3D to assess <br />highwall stability during the proposed highwall mining sequences. <br />EMPIRICAL PILLAR DESIGN <br />Agapito’s approach to web and barrier pillar designs involved two iterative steps: (1) application <br />of empirical pillar design formulas, and (2) numerical modeling analysis to confirm design <br />performance and test its robustness. This section describes the empirical methods used to size the <br />web and barrier pillars for the various cover depths, anticipated mining heights, and stress <br />conditions. <br />Input Parameters and Methods <br />The in situ strength of coal serves as a critical input to the pillar design process. In situ coal <br />strengths for the M and Q seams are taken as 766 and 850 pounds-per-square inch (psi), <br />respectively, based on previous work conducted by Agapito for the N-Pit mining area.1 Other key <br />inputs include the anticipated roadway width and depth of cover. The roadway width was <br />established at 11.5 feet (ft) based on input from Trapper, and the depth of cover is accounted for <br />using a range of possible values in the design tables. <br />The web and barrier pillar widths for the M and Q seams were calculated under the guidelines set <br />forth in the Analysis of Retreat Mining Pillar Stability-Highwall Mining (ARMPS-HWM), a <br />procedure developed by the National Institute of Occupational Safety and Health (NIOSH). This <br /> <br />1 Agapito Associates, “Geotechnical Design and Operational Considerations For Highwall Mining, N Dip Pit, Trapper <br />Mine.” Project no. 534-36 (2016)