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2024-06-25_PERMIT FILE - C1980007 (7)
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2024-06-25_PERMIT FILE - C1980007 (7)
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Last modified
7/30/2024 10:25:17 AM
Creation date
7/30/2024 10:24:24 AM
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DRMS Permit Index
Permit No
C1980007
IBM Index Class Name
Permit File
Doc Date
6/25/2024
Doc Name
Exhibit 84 Geotechnical Assessment for Pillar Extraction in Sunset South Mains
Section_Exhibit Name
Exhibit 84 Geotechnical Assessment for Pillar Extraction in Sunset South Mains
Media Type
D
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December 27, 2023 Page 3-2 <br />Agapito Associates, Inc. <br />In the Sunset Mains South, full extraction of the existing mains pillars is considered the most <br />appropriate mining method from a geotechnical and economic standpoint. Constraints imposed by <br />the reach of the continuous miner and geometry of the existing pillars will result in a <10 ft wide <br />remnant left after mining of each pillar. Because of the extremely low w/h ratio, the remnant pillars <br />will almost certainly undergo significant deformation during the extraction process resulting in <br />their inability to retain any significant amount of load. In turn, the vertical load will transfer to the <br />stiffer surrounding unmined pillars and abutments. This geomechanical concept has been assessed <br />using numerical methods in subsequent sections of this report. <br /> <br />The most widely practiced pillar extraction processes in the industry include split-and-fender, <br />pocket-and-wing, outside lifts, Christmas tree, and open-ending (Kauffman et al. 1981). <br />Considering the full extraction approach recommended for use in the Sunset Mains South and the <br />geometry of the existing pillars, the most suitable pillar extraction process is therefore assessed to <br />be the Christmas tree method. In this process, alternating lifts are taken from each side of the entry <br />into the adjacent pillars and are sufficient to extract the majority of the pillar without going beyond <br />the supported roof (see Figure 3-2). The lifts for each pillar are initially taken next to the gob, and <br />retreat is outby, toward the unmined pillars. No additional roof bolting is required in this process. <br />The gob edge is controlled by a set of (two) mobile roof supports (MRS) placed on the gob side of <br />the lift, which are kept as close as possible to the continuous miner during the removal of each <br />sequential lift. As the removal of lifts approach the outby intersection, the continuous miner is <br />protected by a second set of two MRS installed in the outby crosscut between the pillar being <br />mined and the previously mined pillar in the same pillar row. Supplemental lifts can also be taken <br />from the crosscuts into the remaining pushout stump if conditions are favorable. <br /> <br />In addition to the mains pillars, a combination of Christmas treeing and a single outside-lift is <br />possible in selected areas of the gateroad and chute road pillars located outby of the seals. Coal <br />recovery can also be extracted in the solid coal barrier pillar positioned along the outside entries. <br /> <br />In order to preserve the integrity of the life-of-mine roadways required for the bleeder entry and <br />seal inspections, the neighboring pillars must remain in a long-term stable condition. The location <br />of this travel-way is shown in Figure 3-3. In regard to the gateroad and chute road pillars, a suitable <br />stand-off distance between the roadway and the inby extent of the pillar extraction should be <br />implemented. <br /> <br />In regard to cut depths in the lifts, this assessment assumed the maximum safe reach of the <br />continuous miner will be no more than 37 ft beyond the last row of roof bolts. The mine has <br />indicated that this is the standard cut-out distance used for roadway development. It is also assumed <br />that the cutting head on the continuous miner will accommodate the excavation of 11.5-ft-wide <br />lifts. <br /> <br />
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