2025-05-05_PERMIT FILE - C1981010 ‎(32)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 5/5/2025
• Doc Name: Inwall HWM Sequence Analysis
• Section_Exhibit Name: Appendix X-II 534-61
• Media Type: D
• Archive: No

Tonia Perkins
July 31, 2024
Page 22

Agapito Associates, Inc.
lowest near the entries and exhibited a relatively smooth transition toward the maximum
displacements in the areas of deepest cover. The absence of sharp transitions in the displacement
contours verifies that the slope remains stable in response to mining, as no shearing planes were
formed in the rock mass.
Figure 17 shows major principal stress contours through the pillars. Note that in FLAC3D,
compressive stresses are negative, and thus, the major principal stress is referred to as the minimum
principal stress because it has a large negative value. The yellow contours through the pillar ribs,
indicating a current support capacity of 60,000 to 70,000 psf, or 416 to 486psi.
Overall, the results of the base case model show that the pillars perform as expected, and the global
stability of the N-Pit slope is not impacted by highwall mining of the M and Q seams.
Results – Strength Reduction
To further evaluate the stability of the pillars and the slope, the model was run with strength
reduction factors of 2.0 and 2.5 in the coal seams. These represent 50% and 60% reductions in
strength, respectively.
Figure 18 shows yield indicators in the pillars with a strength reduction factor of 2.0. In this case,
the onset of yield through the coal is evident, but the pillars remain stable. The stability is verified
by the convergence of the model to a state of equilibrium. Figure 19 shows major principal stress
contours through the pillars, with reduced stress magnitudes at the ribs verifying the yielded state
of that material. Figure 20 shows displacements throughout the model at equilibrium, which are
slightly higher than the base case. The color scale is kept the same as the base case figure to
accommodate a comparison. Overall, the results of the model with a stress reduction factor of 2.0
suggest that the coal pillars are approaching a state of limit equilibrium, but the safety factors are
still above 1.0.
With a strength reduction factor of 2.5, the pillars in the M Seam fail before excavation commences
on the Q Seam. The model did not converge at this stage, but was terminated when displacements
surpassed 1.5 ft. Figure 21 shows the yield indicators through the pillars in the M Seam, where the
pink color indicates that shearing was occurring when the model was terminated (the “shear-n”
designation refers to “shearing now” in FLAC3D). Figure 22 shows displacements exceeding
1.0 ft throughout the overburden when the model was terminated.
Overall, the results of the strength reduction technique suggest the pillars have a safety factor
closer to 2.0. The relative quickness in which the pillars failed with a strength reduction factor of
2.5 indicates that limit equilibrium was surpassed by a significant margin.
Block Sliding Analysis
To evaluate the likelihood of a block sliding mechanism due to failure of the M Seam web pillars,
an additional model was constructed to represent half of one panel, with support from the barrier
pillar explicitly represented. The revised model accounts for a 96-ft wide “slice” of the south slope
of the N-Pit, with 5 entries, 4.5 web pillars, and half of a barrier pillar. With symmetry conditions
on each of the lateral boundaries, the model behaves as if it were situated within an infinitely long