2023-07-14_PERMIT FILE - C1981010 (16)

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January 2, 2020 Page 6-2 Agapito Associates, Inc.  The ultimate south highwall should be pre-split to limit blast-induced damage and weakening of the rock mass.  No additional spoils should be placed on the crest of the south highwall, as its SF is close to the criterion without surcharge loading.  Following creation of the south highwall, tension cracks parallel to the crest could develop in the L Seam and M Seam overburden benches due to the adverse gradient and dilation of the slope rock mass. If such cracks occur, they should be filled as soon as possible to restrict water ingress. In general, the upslope surface drainage of the south highwall should direct surface water runoff away from the highwall crest. Monitoring of the south highwall and crest in active mining zones is recommended. Either time domain reflectometry (TDR) or extensometers can provide useful data and potentially identify critical movements of the south highwall. Periodic drone or conventional surveys are also recommended. L Pit—From a water management standpoint, HWM progression updip (north to south) is preferred. The order in which the K and Q Seams are mined makes no difference, geotechnically. Top-down mining would require greater coordination with stripping operations; however, it might be simpler from an operational standpoint to open the pit to the bottom of the Q Seam, highwall mine the Q Seam, then backfill to the K Seam and mine the K Seam from the backfill bench. Should HWM pillar failure occur, miner entrapment is more of a concern with the bottom-up sequence, as subsidence of the active seam is possible. The bottom-up sequence also has the potential to sterilize HWM reserves should overlying seams subside. From a personnel safety standpoint, the top-down sequence exposes workers to possible air blasts from web failure in overlying seams and associated rock fall. Additionally, the bottom-up sequence provides confinement to HWM areas as the pit is filled. Operational preferences may take precedence, but other factors being equal, it is AAI’s opinion that the bottom-up sequence is most advantageous. N Pit Protective Buffer—The HWM openings in the south highwall of the N Pit need to be terminated a sufficient distance away from the old F Pit perimeter. The primary concern is possible inflow of water pooled in the relatively porous backfill/spoil material of the F Pit. MSHA’s rules regarding mining into old workings (30 Code of Federal Regulations [CFR] Sections 75.388 and 75.389, US Department of Labor 2019) may apply to the proposed HWM plan. The rules require maintaining a buffer zone around old workings, unless the old workings are sampled by probe drilling and found to be safe. Since probe drilling is impractical in the HWM holes, it is unlikely that the buffer zones can be mined. The regulations indicate that a 50-ft buffer zone is adequate if the previous workings have been accurately surveyed and certified. If the previous areas are not accurately surveyed, a 200-ft buffer zone is required. Additionally, Trapper should develop a hazard management plan in the event of accidentally mining into the previously mined pits. Multiple-Seam Mining—Only a small portion of the J Pit, G2 Seam HWM area underlies the I Pit, F Seam area, with 60 to 120 ft of interburden. This thickness precludes significant seam interaction. Interaction generally becomes problematic when the interburden thickness is less than about two times the thickness of the lower seam. In the N Pit, the L-M interburden thickness ranges from 30 to 50 ft, which is much greater than the 5-ft maximum thickness of the M Seam. The M-Q- Interburden thickness is 50 to 60 ft, which is also much greater than the 11-ft maximum thickness