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Subsidence Evaluation for the <br />Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 10 <br /> <br />831-032.923 Wright Water Engineers, Inc. <br />December 2021 <br />temporarily increase in the zones of these compression arches. However, the arches in a given <br />area commonly move upward and dissipate as longwall mining is completed in that area. Arches <br />may not dissipate where the room-and-pillar mining method is used, because pillars and stumps <br />left after mining can prevent dissipation of the compression arches. The overburden rocks affected <br />by the arches are temporarily subjected to increased stress and strain as the arches move upward. <br />In longwall mining areas, this increased stress and strain commonly are less than in room-and- <br />pillar mining areas because stresses are relieved as the arches move upward and dissipate. <br />Peng (1992, p.4) reports that the combined height of the zone of caving and fracturing ranges from <br />20t to 30t, and that the height of the fractured zone is greater for hard, strong rocks than for soft, <br />weak rocks. <br />The height of the zone of fracturing is a function of lithology and layer thickness, according to <br />Peng (1992, p. 6-8). For example, the zone of fracturing commonly is higher for strong, thickly- <br />bedded, brittle sandstones than it is for thinly layered, soft, shales and claystones. Liu (1981) <br />reports ranges of heights of the zone of fracturing for various rock types as follows: <br />1. Heights of 20t to 30t are reported in strong brittle rocks, such as siliceous sandstones and <br />limestones; a value of 28t was reported for overburden containing 70 percent sandstone. <br />Also, because of hardness, fractures do not close as readily in brittle rocks as they do in <br />soft rocks during recompression. <br />2. Heights of 9t to 11t are reported where all the rocks consist of soft shales and claystones. <br />The fractures also commonly close again under stresses associated with static conditions, <br />and become impermeable again. <br />Considering the lithology of the areas, Mr. Koontz estimated that 10t to 20t was a good projection <br />for the height of fracturing in the Apache Rocks and Box Canyon mining areas. However, a <br />projected fracture height of 30 times the coal extraction thickness (30t) may locally occur (Koontz, <br />oral communication March 2004). <br />Within the Southern Panels and Sunset Trail mining areas, the fracture zone may become less <br />continuous in the caved zone with increasing height because of the alternating sequence of harder <br />and brittle rocks and softer and yielding rocks. The height of the fracture zone, therefore, will <br />likely be less—by possibly 10 to 20 percent—than the height predicted for the Apache Rocks and <br />Box Canyon mining areas because of the presence of more shale above the E-seam mining in the <br />Southern Panels and Sunset Trail mining areas. Fractures near the top of the caved zone, therefore, <br />will likely become less continuous with increasing height in the zone of fracturing. <br />The maximum height of fracturing above longwall panels in the Southern Panels and Sunset Trail <br />mining area is estimated to range from about 10t to 20t. This is near the mid-range of 9t to 30t as <br />reported by Peng (1992, p. 7). This estimate may be conservative for the particular rock strata or <br />lithology above the E-seam. When considering a conservative 10 percent reduction for the softer <br />rocks overlying the E-seam, the effective height of fracturing in the Southern Panels and Sunset <br />Trail mining areas is estimated to range from 9t to 18t.