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Subsidence Evaluation For Exhibit 608 South of Divide Mining Area • Compression azches (azcuate zones of compressive stress) commonly develop, or partially develop, above the mining panels. These azches temporarily transfer overburden stresses to the panel barrier or chain pillars and to the caved gob and the mining face (Dunrud 1976). Stresses temporarily increase in the zones of these compression arches. However, the azches in a given azea commonly move upward and dissipate as longwall mining is completed in that area. Arches may not dissipate where the room-and-pillar mining method is used, because pillazs and stumps left after mining can prevent dissipation of the compression azches. The overburden rocks affected by the arches aze temporarily subjected to increased stress and strain as the arches move upwazd. In longwall mining areas, this increased stress and strain commonly aze less than in room-and-pillaz mining areas because stresses are relieved as the arches move upward and dissipate. Peng (1992, p.4) reports that the combined height of the zone of caving and fracturing ranges from 20t to 30t, and that the height of the fractured zone is greater for hazd, strong rocks than for soft, weak rocks. The height of the zone of fracturing is a function of lithology and layer thickness, according to Peng (1992, p. 6-8). For example, the zone of fracturing commonly is higher for strong, thickly- bedded, brittle sandstones than it is for thinly layered, soft, shales and claystones. Liu (1981) reports ranges of heights of the zone of fracturing for various rock types as follows: 1. Heights of 20t to 30t aze reported in strong brittle rocks, such as siliceous sandstones and limestones; a value of 28t was reported for overburden containing 70 percent sandstone. Also, because of hazdness, fractures do not close as readily in brittle rocks as they do in soft rocks during recompression. 2. Heights of 9t to l It aze reported where alt the rocks consist of soft shales and claystones. The fractures also commonly close again under stresses associated with static condifions, and become impermeable again. According to Mr. Koontz, IOt to 20t is a good projection for the height of fracturing in the Apache Rocks and Box Canyon mining azeas. However, a projected fracture height of 30 times the coal extraction thickness (30t) may locally occur (Koontz, oral communication March 2004). Within the South of Divide mining azea, the fracture zone may become less continuous in the caved zone with increasing height because of the alternating sequence of harder and brittle rocks and softer and yielding rocks. The height of the fracture zone, therefore, will likely be less-by possibly 10 to 20 percent-than the height predicted for the Apache Rocks and Box Canyon mining areas because of the presence of more shale above the E Seam mining in the South of Divide mining azea. Fractures neaz the top of the caved zone, therefore, will likely become less continuous with increasing height in the zone of fracturing. Also, with increasing height in this zone, and as lateral and vertical constraints increase, fracturing that could impact water bearing zones will tend to occur more in zones of convex upwazd curvature, along sepazated bedding planes towazd the center of the panel, and along local 831-032.620 Wright Water Engineers, Inc.