2025-05-05_PERMIT FILE - C1981010 ‎(24)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 5/5/2025
• Doc Name: AOC Variance Summary
• Section_Exhibit Name: Appendix B AOC
• Media Type: D
• Archive: No

7
Table 2 L Pit Safety Factors
Section AOC Safety Factor AOC Variance Safety Factor
LA-LA’ 1.5 1.58
LB-LB’ 1.3 1.55
LC-LC’ 1.49 2.14

The proposed post-mining topography under the AOC variance yields an overall more stable, long-term
backfill than the AOC.
2.2.2 ASH Pit AOC Variance
Post-Mining Topography
A postmining topography was developed for the Ash Pit that takes into consideration the proposed post-
mining topography which includes shallower reclaimed slopes. The post-mining topography is shown on
Map M12 Sheet 2. The proposed post-mining topography would have an overall slope of 4 to 14° and
would have the same general aspect with slopes facing north/northwest. As with the L Pit, the Ash Pit
would be backfilled in the same manner as described in the Trapper Mine Permit Section 3.5, except that
the Ash Pit has additional requirements related to coal ash placement. Those requirements include a
minimum cover of five feet of spoil and a foot of topsoil over the ash body. In addition, construction of
post-mining drainages must be at least 50 feet from the edge of the ash placement. The backfill would
eliminate all highwalls, and spoil/refuse piles. Once backfilled, the spoil would be graded to blend with
the surrounding topography, and drainage controls would be established as described in Section 4.0
below. It should be noted that post-mining drainages must be located to avoid placement over the top of
historical ash disposal. Figure 7 of the Mattern report (Attachment 2) shows the proposed pre- and post-
mining drainage patterns for the Ash Pit.
Figure 8 of the Mattern report shows a view of the proposed post-mining topography and the surrounding
area for approximately 25 square miles to show that the proposed post-mining topography blends well
with other areas surrounding the Trapper Mine and would not appear unnatural or of a different general
slope or aspect. Map M14A Sheet 3 shows two cross sections through the Ash Pit with the pre-mining
and proposed post-mining topography.
Post Mining Stability Analysis
AAI performed an evaluation of the geologic and hydrologic conditions present in the Ash Pit and used
that information to develop a stability analysis of the backfilled pit under both the proposed post-mining
topography and the AOC topography. The analysis is contained in the AAI report, Attachment 1, also to
be part of Appendix B of the Trapper Mine Permit C-1981-010 and briefly described below.
The stability analysis considered the same factors as described above for the L Pt including pit floor
gradients, backfill saturation, pit floor geology, and spoil depth variations. As discussed above, a detailed
discussion of the AAI approach to stability is contained in Attachment 1. AAI evaluated one section
through the Ash Pit under both AOC and the proposed post-mining topography. The vertical analysis
section was taken along the true dip direction. The results indicate that for the Ash Pit, the Safety Factor
for the AOC configuration is 1.36, below AAI’s minimum threshold for long-term stability of 1.5. The
Safety Factor for the AOC variance post-mining topography is 1.75 indicating long-term stability.
3.0 Post-Mining Land Use
For Trapper Mine there are three different types of post mining land uses that occur within the permit
area: 1) cropland, 2) rangeland dominated by mountain shrubs before mining, and 3) rangeland