2021-02-05_PERMIT FILE - C1980007A (5)

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Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 37 at a thickness of up to 14 feet, then the minimum overburden cover should be 253 feet plus 20 percent, or about 304 feet. This minimum overburden is directly a function of the mining height as shown above, and can be reduced if the longwall mining height is lowered. For example, if the mining height were to be reduced to 12 feet near the western end of longwall panel SS4, then the combined height of the caved and fracture zones capable of transmitting water is projected to be 204 feet(5t=60';2/3(18t) = 144'). Adding in the maximum projected crack depth of 15 feet yields a combined height of 219 feet, or 263 feet with the 20 percent"buffer." Therefore,the mine can make operational decisions based on the actual overburden encountered in specific locations. We do not recommend that longwall mining occur where overburden thickness is not at least 250 feet, even with reduced mining height. The current projection of minimum overburden over the Sunset Trail longwall panel is approximately 280 feet at the western end of longwall panel SS4. Should this projected overburden prove to be accurate,MCC can either shorten the longwall panel at a location where the overburden drops below approximately 300 feet or reduce the mining height in accordance with the actual overburden thickness. Another factor that will help maintain a lack of hydraulic connection between the surface and the mined longwall panels is the presence of soft shales and claystones in the E-seam overburden that will increase the probability that the strata will warp rather than fracture during the subsidence process. The projected lack of hydraulic connection was confirmed as mining occurred in E-seam panel E2 without any adverse impacts to Dry Fork or any anomalous water inflows observed in the mine. With the proposed B-seam mining occurring below the E-seam, we do not foresee the potential for hydraulic connection between Dry Fork and the mine workings. 11.4 Springs and Water-Bearing Zones MCC has produced the Spring and Stock Pond Location Map that covers the Southern Panels, Apache Rocks West, and Sunset Trail mining areas. Only a few springs in the West Elk Mine area indicate a source from a local bedrock water-bearing zone. Most springs likely have sources from local water-bearing zones in surficial material (debris flows, colluvium, and possibly alluvium). In contrast to surface water containment structures, such as reservoirs,ponds, streams and ditches, springs and water-bearing zones may have water sources that are either in bedrock beneath the blanket of clay-rich surficial material (debris flows, alluvium, and colluvium), or have a source from within the surficial material. Subsidence may affect a spring or water-bearing zone source located in bedrock, whereas effects may or may not be expected where the spring source is within the surficial material. Tension cracks produced in sandstone bedrock during the subsidence process, for example, may divert water to a lower rock layer and therefore change the flow location. However, local water-bearing zones in permeable zones, which are interlayered with clay-rich zones (Wasatch clays) in the surficial deposit, may yield to tensile stresses without cracking. Therefore, spring flows are monitored for a few years (to account for seasonal variations) prior to any mining in the area. Monitoring data and field observations are provided each year in MCC's Annual Hydrology Report. 831-032.912 Wright Water Engineers, Inc. December 2020