2024-01-22_PERMIT FILE - C1980007A (6)

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December 27, 2023 Page 2-1 Agapito Associates, Inc. 2 GEOTECHNICAL ENVIRONMENT Boreholes indicate that the immediate roof lithology (first 10 feet [ft]) of the Sunset Mains South is comprised of alternating units of soft-to-medium hard mudstone and shale, hard siltstone, and fine-grained sandstone. The overburden is comprised of units of coal, mudstone, shale, siltstone, and sandstone, with approximately half of the lithology being dominated by the hard siltstone and sandstone units. The geological structure information provided by the West Elk Mine indicates a large normal fault (BF Fault) is located across the north end of Sunset Mains South (see Figure 2-1). The reported offsets for the fault range between 20 and 60 ft. No other significant geological structures are indicated in the Sunset Mains South. The competency of the immediate roof has been estimated by using the Coal Mine Roof Rating (CMRR). The CMRR classification system was developed by the National Institute for Occupational Safety and Health (NIOSH) for use in bedded Coal Measure rock (Molinda and Mark 1994). The parameters of the classification system are the Unconfined Compressive Strength (UCS) of the intact rock, intensity of bedding and other discontinuities, shear strength of the bedding and other discontinuities, moisture sensitivity of the rock, and the presence of a strong bed in the bolted horizon. These parameters have been estimated from the geological descriptions recorded on the borehole logs noted above. The logs indicate that the CMRR is likely to range between 45 and 55, which indicates moderate to strong roof competency. The depth of cover ranges between 50 ft at the inby end of the mains to 650 ft around the mid- point of the mains, and, from there, ranges between 250 and 600 ft outby to the fault crossing (see Figure 2-1). In the context of coal mining in the Western United States, this depth equates to a reasonably low vertical stress regime. Previous in-situ stress measurements at the mine indicate the major horizontal stress is aligned in an east–northeast direction with a major horizontal-to-vertical stress ratio (k-ratio) around 1.5:1. At the depths considered in this assessment and taking into account the close proximity of surrounding longwall gob, it is anticipated that the horizontal stress will have a negligible effect on ground conditions during pillar extraction. As shown in Figure 2-1, the pillars in Sunset Mains South are flanked by previously extracted longwall gob to the east of the panel. The gob from LW SS2 extraction, which is the closest of the longwall panels to the mains pillars, is located more than 500 ft from the nearest pillars. Measured relationships between depth of cover and the lateral extent of abutment loading indicate that the pillars in Sunset Mains South are located a sufficient distance away from the gob to be considered outside the influence of the abutment loads. The horizontal distance to which the resulting abutment load will be redistributed away from the failed overburden onto neighboring pillars and/or barriers (D) can be estimated using the following equation presented by Peng and Chiang (1984): DD = 9.3√H (Eqn. 2-1) This relationship suggests that any measurable abutment loads from LW SS2 will be limited to a lateral extent of approximately 250 ft.