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Subsidence Evaluation for the <br />Exhibit 60E South of Divide and Dry Fork Mining Areas Page 9 <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 <br />affected by the arches are temporarily subjected to increased stress and strain as the arches move <br />upward. In longwall mining areas, this increased stress and strain commonly are less than in <br />room - and - pillar mining areas because stresses are relieved as the arches move upward and <br />dissipate. <br />Peng (1992, p.4) reports that the combined height of the zone of caving and fracturing ranges <br />from 20t to 30t, and that the height of the fractured zone is greater for hard, strong rocks than for <br />soft, 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 l It 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 />According to Mr. Koontz, 10t to 20t is a good projection for the height of fracturing in the <br />Apache Rocks and Box Canyon mining areas. However, a projected fracture height of 30 times <br />the coal extraction thickness (30t) may locally occur (Koontz, oral communication March 2004). <br />Within the South of Divide and Dry Fork mining areas, the fracture zone may become less <br />continuous in the caved zone with increasing height because of the alternating sequence of <br />harder and brittle rocks and softer and yielding rocks. The height of the fracture zone, therefore, <br />will likely be less —by possibly 10 to 20 percent —than the height predicted for the Apache <br />Rocks and Box Canyon mining areas because of the presence of more shale above the E -seam <br />mining in the South of Divide and Dry Fork mining areas. Fractures near the top of the caved <br />zone, therefore, will likely become less continuous with increasing height in the zone of <br />fracturing. <br />The maximum height of fracturing above longwall panels in the South of Divide and Dry Fork <br />mining areas is estimated to range from about l Ot to 20t. This is near the mid -range of 9t to 30t <br />as reported by Peng (1992, p. 7). This estimate may be conservative for the particular rock strata <br />or lithology above the E -seam. When considering a conservative 10 percent reduction for the <br />softer rocks overlying the E -seam, the effective height of fracturing in the South of Divide <br />mining area is estimated to range from 9t to 18t. <br />831 - 032.810 Wright Water Engineers, Inc. <br />