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Subsidence Evaluatron For <br />6chibd 606 South of Divide Mining Area Page 5 <br /> <br />5.0 SUBSIDENCE PREDICTION BASED ON LONGWALL MINING AT <br />WEST ELK MINE <br />Subsidence, as it relates to mining, is defined herein as the local downward displacement of the <br />surface and overburden rock in response to mining under the influence of gravity. For purposes <br />of describing subsidence effects on overburden material and the ground surface, subsidence can <br />be divided into four zones (Figure 2): 1) caved zone, 2) fractured zone, 3) continuous <br />deformation zone, and 4) neaz surface zone. <br />5.1 Caved Zone <br />As coal is extracted and a void is produced, the roof rocks break along bedding planes, joints, <br />and fractures and fall to the mine floor (Figure 2). Rotation of the caved debris occurs during the <br />fall so that the caved fragments tend to pile up in a random fashion. This caved zone, according <br />to Peng (1992, p. I-2) occurs for the first 2 to 8 mining (or coal extraction) thicknesses (2t to 8t) <br />in the roof rocks (for example, if t=12 feet, the caved zone would range from 24 [0 96 feet [2t to <br />8t]). According Wendell Koontz, senior geologist at West Elk Mine, this caved zone averages <br />about 2.St for longwall mining of the B Seam and all the mining areas at West Elk Mine. This <br />includes the Apache Rocks and Box Canyon mining arras (Koontz, oral communication March <br />2004). <br />. Based on the stratigraphic and lithologic information obtained from drill holes in the South of <br />Divide mining area, the rocks consist of a greater proportion of shales, siltstones, and claysWnes <br />than are present in the Apache Rocks and Box Canyon mining areas. The height of the caved <br />zone is therefore projected to range from 2t to St, depending on water conditions encountered <br />and on specific roof lithology. In a dry environment, where lenticulaz sandstones comprise the E <br />Seam roof, the caved zone will be closer to 2t In a wet environment where soft shales and <br />chtystones occur in the rooi; however, the caved zone will likely be closer to St. The average <br />height of the caved zone is projected to average 3t <br />5.2 Fractured Zone <br />A zone of fracturing and local separation along rock bedding planes and joints occurs above the <br />zone of caving (Figure 2, Enlargement 1). 1n this zone, which is transitional to the underlying <br />caved zone, lateral and vertical constraints in the adjacent overburden strata and the caved rocks <br />below prevent further large displacement or rotation of the fractured rock. Displacements in the <br />fracture zone and severity of fracturing tend to decrease upward as lateral and vertical confining <br />stresses increase. <br />Based on width and conductivity of fractures Peng (1992, p. 143) states that the upper one-third <br />of the fractured zone (in terms of height) has only minor fractures with little potential for water <br />conductivity. In the lower two-thirds of the fractured zone, water conductivity commonly <br />increases progressively downward. <br />Compression arches (arcuate zones of compressive stress) commonly develop, or partially <br />develop, above the mining panels. These arches temporarily transfer overburden stresses to the <br />831-032.690 Wdght Water Engineers; Inc. <br />