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Tonia Perkins <br />July 31, 2024 <br />Page 21 <br />Agapito Associates, Inc. <br />FLAC3D Analysis <br />A model was constructed in FLAC3D to perform additional checks on the safety factors of the <br />web pillars and evaluate the global stability of the slope by assessing the likelihood of a block <br />sliding failure along the mined coal seams. <br />The modeled geometry represents the fully excavated state of the N-Pit south wall, with the pit <br />floor at the base of the Q Seam and mining completed in both the M and Q Seams. The south wall <br />was chosen to evaluate slope stability because the bedding dips out of the highwall and represents <br />the more conservative condition from a kinematic perspective compared to the North wall. <br />Figure 12 shows an oblique view of the FLAC3D model colored by geologic domain. The domains <br />in the model include fill at the ground surface, coal seams L, M, Q, and R, and various overburden <br />and interburden units referred to as “shale”. <br />Model Inputs <br />The physical properties of the overburden and interburden units were calculated from weighted <br />averages of the shales, sandstones, and mudstones between each coal seam, as described in the <br />2016 site investigation program for the N-Pit mining area.1 A list of the geologic units and their in <br />situ strength parameters is provided in Table 2 for reference. <br />Table 3 provides a summary of the homogeneous overburden and interburden strength parameters <br />applied to each domain in the FLAC3D model. The materials are listed in order from top to bottom <br />of the model. <br />Model geometry <br />The FLAC3D model represents a 63-ft wide “slice” of the south N-Pit wall. The model width was <br />chosen to satisfy symmetry conditions at the model boundaries in both the M and Q Seams using <br />the same web pillar designs from the LaModel analysis. The 63-ft width allows for 3 full pillars <br />and entries in the Q Seam and 3.5 pillars and entries in the M Seam. Figure 13 shows the excavated <br />state of the M and Q Seams in the FLAC3D model. <br />Barrier pillars are not represented in the FLAC3D model. The 63-ft slice approximately represents <br />the middle third of a 10-room panel, where influence of the barrier pillars is minimal. <br />Results – Base Case <br />Using base case material strengths, summarized above, the model came to equilibrium with no <br />indication of yield in any of the coal pillars. This result verifies that the safety factors are well <br />above 1.0 but requires further analysis to identify the actual range of safety factors. Figure 14 <br />shows an oblique view of the model with yield indicators visible only in the fill near the surface <br />after excavation is complete. Figure15 shows yield indicators in a cross section through the pillars <br />at a distance of roughly 570 ft from the entries. Again, no yield indicators are present in the coal. <br />Figure 16 shows vertical displacement contours after completion of mining in both seams, with <br />magnitudes on the order of 0.025 ft, or 0.3 inches, above the M Seam. Vertical displacement was