2024-06-25_PERMIT FILE - C1980007 (7)

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April 22, 2024 Page 3-2 Agapito Associates, Inc. In the Sunset Mains, full extraction of the existing mains pillars is considered the most appropriate mining method from a geotechnical and economic standpoint. Constraints imposed by the reach of the continuous miner and geometry of the existing pillars will result in a <10 ft wide remnant left after mining of each pillar. Because of the extremely low w/h ratio, the remnant pillars will almost certainly undergo significant deformation during the extraction process resulting in their inability to retain any significant amount of load. In turn, the vertical load will transfer to the stiffer surrounding unmined pillars and abutments. This geomechanical concept has been assessed using numerical methods in subsequent sections of this report. The most widely practiced pillar extraction processes in the industry include split-and-fender, pocket-and-wing, outside lifts, Christmas tree, and open-ending (Kauffman et al. 1981). Considering the full extraction approach recommended for use in the Sunset Mains and the geometry of the existing pillars, the most suitable pillar extraction process is therefore assessed to be the Christmas tree method. In this process, alternating lifts are taken from each side of the entry into the adjacent pillars and are sufficient to extract the majority of the pillar without going beyond the supported roof (see Figure 3-2). The lifts for each pillar are initially taken next to the gob, and retreat is outby, toward the unmined pillars. No additional roof bolting is required in this process. The gob edge is controlled by a set of (two) mobile roof supports (MRS) placed on the gob side of the lift, which are kept as close as possible to the continuous miner during the removal of each sequential lift. As the removal of lifts approach the outby intersection, the continuous miner is protected by a second set of two MRS installed in the outby crosscut between the pillar being mined and the previously mined pillar in the same pillar row. Supplemental lifts can also be taken from the crosscuts into the remaining pushout stump if conditions are favorable. In order to preserve the integrity of the life-of-mine roadways required for the bleeder entry and seal inspections, the neighboring pillars must remain in a long-term stable condition. The location of the travel-ways is shown in Figure 3-3. In regard to cut depths in the lifts, this assessment assumed the maximum safe reach of the continuous miner will be no more than 37 ft beyond the ribline. The mine has indicated that this is the standard cut-out distance used for roadway development. It is also assumed that the cutting head on the continuous miner will accommodate the excavation of 12.5-ft-wide lifts.