2022-09-29_PERMIT FILE - C1980007 (5)

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Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 36 831-032.923 Wright Water Engineers, Inc. December 2021 The probable reason for the lack of cracking in alluvium is that the fine sand-to clay-sized material and overlying soil yields without cracking or bulging as it deforms as a discrete unit, or as discrete units, in the subsidence process. This same reasoning also applies to the colluvium in the area. Although mined longwall panel subsidence cracks were locally observed in colluvium less than one foot to a few feet thick, no cracks were observed in colluvium more than about ten feet thick. No cracks were observed in alluvium above mined longwall panels in the Apache Rocks and Box Canyon mining areas. 11.3.1 Potential for Hydraulic Connection between Mine Workings and Surface Near the southwest corner of E-seam longwall panel E2, the Dry Fork channel encounters a short reach where the E-seam overburden above the longwall panel block is less than 400 feet, with a minimum of approximately 375 feet. A prudent concern is whether mining induced longwall panel subsidence could establish a hydraulic connection between the Dry Fork stream channel and the mine workings. To address this scenario, the maximum projected height of longwall panel fracturing and the maximum depth of surface cracks were considered. As discussed in Section 5.2, the effective height of longwall panel fracturing in the Southern Panels and Sunset Trail mining areas is estimated to range from 9t to 18t, or a maximum fracture height of 252 feet for a mining height of up to 14 feet. However, Peng (1992) states that the upper one-third of the longwall panel fractured zone has only minor fractures with little potential for water conductivity. Therefore, the height of the fractured zone capable of transmitting water would be two-thirds of the 18t, or 168 feet. The maximum height of the mined longwall panel caved zone is projected to be 5t, or 70 feet, for the Southern Panels and Sunset Trail mining areas with a projected mining thickness of up to 14 feet. The maximum mined longwall panel fracture zone height is projected to be 18t, or 252 feet, of which the lower two-thirds, or 168 feet, are capable of transmitting water. Therefore, the combined height of the mined longwall panel caved and fracture zones capable of transmitting water is projected to be a maximum of 238 feet. As discussed in Section 5.3.2, the maximum crack depth in the Southern Panels and Sunset Trail mining areas is estimated to be 15 feet in terrains with slopes less than 30 percent, with depths up to 35 feet occurring locally in steep topography. For the South Prong channel near the western edge of longwall panel SS4, the maximum projected crack depth is 15 feet. Consequently, the combined maximum height of the caved and fractured zones and the maximum crack depth is 253 feet. Based on previous mine plans and overburden thickness projections, the previous modeling evaluation indicated that no longwall panel subsidence impacts were projected with a minimum overburden thickness of 375 feet, leaving an estimated 122-foot “buffer” of unfractured bedrock remaining intact. It was not the intent of the previous analysis to indicate or specify that the 122- foot buffer was necessary or that longwall mining could not occur in overburden less than 375 feet. WWE does believe that it is prudent to have a “buffer” to reduce the possibility of a hydraulic connection between the ground surface and the mined longwall panels. We recommend that a factor of safety of 20 percent be added to the combined fracture height and crack depth total to yield the minimum overburden necessary to avoid a hydraulic connection. For example, if mining