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

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April 22, 2024 Page 2-1 Agapito Associates, Inc. 2 GEOTECHNICAL ENVIRONMENT Boreholes indicate that the immediate roof lithology (first 10 feet [ft]) of the Sunset Mains is comprised of alternating units of soft to medium hard mudstone and shale, hard siltstone, and fine- grained sandstone. Of these units, the mudstone and shale account for between 50% and 80% of the lithology. The overburden is comprised of units of coal, mudstone, shale, siltstone, and sandstone, with the majority of the lithology being dominated by the siltstone and sandstone units. The geological structure information provided by the West Elk Mine indicates a major normal fault located to the south and trending parallel with the Sunset Mains (see Figure 2-1). The offsets for the fault are indicated to range between 36 and 70 ft. The only geological structure located within Sunset Mains is a small normal fault mapped between 20 and 23 crosscuts in entries 1E and 2E (see Figure 2-1). The throw of the fault is indicated to range between 2 and 3 ft. 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 44 and 52, which indicates weak to moderate roof competency. The depth of cover ranges between 400 ft at the inby end of the panel to 830 ft around crosscut 19, and, from there, ranges between 600 and 700 ft outby to crosscut 1 (see Figure 2-1). Despite this wide range, most of the overburden is confined to a thickness of between 550 and 700 ft. 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 that 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 in view of the amount 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 are surrounded by previously extracted longwall gob to the north and south of the panel. The gob from LW E14 extraction will be located more than 524 ft from the nearest pillars following the proposed development, while the gob from LW E7 extraction is located 300 ft away from its nearest point. Measured relationships between depth of cover and the lateral extent of abutment loading indicate that the pillars in Sunset Mains are located a sufficient distance away from the gobs to be considered outside the reach of the abutment loads. The 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