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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 4 4.2 Fractured Zone A zone of fracturing and local separation along rock bedding planes and joints occurs above the zone of caving (Figure 1, Enlargement 1). In this zone, which is transitional to the underlying caved zone, lateral and vertical consvaints in the adjacent overburden strata and the caved rocks below prevent further large displacement or rotation of the fractured rock. Displacements in the fracture zone and severity of fracturing tend to decrease upward as lateral and vertical confining stresses increase. Based on width and conductivity of fractures Peng (1992, p. 143) states that the upper one-third of the fractured zone (in terms of height) has only minor fractures with little potential for water conductivity. In the lower two-thirds of the fractured zone, water conductivity increases progressively downward. Compression arches (arcuate zones of compressive stress) commonly develop, or partially develop, above the mining panels. These arches temporarily transfer overburden stresses to the panel barrier or chain pillazs and also to the caved gob and the mining face (Dunrud 1976). Stresses temporarily increase in the zones of these compression arches. However, the arches in a given area 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 pillars and • stumps left after mining can prevent dissipation of the arches. The overburden rocks affected by the arches are temporarily subjected to increased stress and strain as the arches move upward. In longwall mining azeas this increased stress and strain commonly are less than in room-and-pillar mining azeas 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 20 to 30 extraction thicknesses (20t to 30t), and that the height of the fractured zone is greater for hard, strong rocks than it is 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, plastic shales and claystones. Liu (1981) reports ranges of heights of the zone of fracturing for various rock types as follows: 1. Heights of 20 to 30 times coal-extraction thickness (20t to 30t) are 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 hardness, fractures do not close as readily in brittle rocks as they do in soft rocks during recompression. 2. Heights of 9 to 11 times the coal-extraction thickness (9t to l It) are reported where all the rocks consist of soft, plastic shales and claystones. The fractures also commonly close again 831-032.181 Wright Water Engineers, Inc.