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

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January 2, 2020 Page 2-3 Agapito Associates, Inc. The depth of cover ranges shown in Table 2-3 are primary inputs to the web and barrier pillar designs discussed later. Normally, overburden rock unit weight (usually assumed to be 162 pcf) is multiplied by depth of cover to determine cover load. Because a significant portion of the overburden above the N Pit HWM area is spoil above the I2 Seam, this approach was modified to account for the lower unit weight of spoil based on the percentage of spoil and rock overlying the N Pit target seams. For example, for 150 ft of overburden, consisting of 50 ft of spoil (unit weight of 125 pcf) and 100 ft of rock (unit weight of 162 pcf), the equivalent rock depth of cover is 50 ft × 125 pcf/162 pcf + 100 ft = 138.6 ft. The target seams dip to the north–northwest, generally in a range from about 6 to 10°, averaging about 8°, with localized dips rarely approaching 12°. The I and J Pits are generally oriented east–west (Figure 1-1), with HWM to the north; the N Pit is also oriented generally east– west, with HWM to both the north and south; and the L Pit is oriented north–south, with HWM to the east. An exception is the southern portion of the K Seam HWM area in the L Pit, where the highwall is oriented east–west, with mining to the north. Consequently, HWM for most areas will be either primarily updip or downdip, with minimal side dip, except for the L Pit, where most of the mining will have a side dip of 6° to 10°. Since the HWM machine needs to be essentially level side to side, where a side dip exists, a reduced mining height will have to be used to avoid cutting into the roof and floor at opposite corners of the opening. Assuming that the machine is oriented perpendicular to the highwall, this reduced mining height can be calculated as: ܪ௥ ൌቀ ௧ ୡ୭ୱఏ ቁെܹ௢ tanߠ (Eqn. 2-1) where Hr = reduced mining height (ft) t = true seam thickness (ft) θ = apparent side dip at highwall (°) Wo = opening width (ft) No previous underground mining of the target seams is known to AAI. Deeper coal seams below the Twentymile Sandstone have been studied in the past, but the seams have not been mined. Longwall mining of deeper seams at the Empire Mine, immediately southwest of the Trapper property, was performed within the last 40 years, but that experience is not considered applicable to the HWM effort. HWM of the H Seam in the A Pit was largely successful, although rolls in the seam prevented the miner from consistently achieving the targeted penetration of approximately 1,300 ft. Information supplied by Trapper shows that 20 HWM openings were mined (Figure 1-1), with an average penetration of 1,156 ft, or about 90% of the targeted penetration. Many of the target seams are comparable to or thinner than the H Seam in A Pit; thus, similar seam rolls, if encountered, would be expected to limit HWM penetration. Spontaneous combustion is not uncommon in Trapper’s surface mining operations, although it is not considered a serious problem. Should HWM web pillar failure occur, with related overburden fracturing to the surface, crushed coal and air circulation from natural ventilation would increase the chances for spontaneous combustion. Spoil berms are normally used to