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West E!k Mine <br />Detailed Description of Predicted Subsidence Phenomena - 2.05.6 (6) (e) (i) (D) <br />Subsidence, as it relates to mining, is defined as the local downward displacement of the surface <br />and the overburden rock in response to mining under the influence of gravity. The following text <br />includes a general discussion of the vazious zones defined within the subsidence area; predicted <br />maximum vertical and horizontal displacements, tilt, curvature and horizontal strain; predicted <br />zones of tensile strain related to mine geometry; predicted rates and duration of subsidence; the <br />effects of topography on subsidence; and the predicted angle of draw. A summary of these values <br />as determined from the present mining area subsidence monitoring data is presented in Table 1, <br />Exhibit 60. Table 2 and Table 3 in Exhibit 60 summarizes the projected values of these <br />parameters for the Apache Rocks and Box Canyon mining areas in the following subsidence <br />discussion. <br />Subsidence Zone Description <br />For purposes of describing subsidence effects on overburden material and the ground surface, <br />subsidence can be divided into four zones (see Figure 1 in Exhibit 60 for details): (1) Caved zone, <br />(2) Fractured zone, (3) Continuous deformation zone, and (4) Near-surface zone. <br />Caved Zone <br />As coal is extracted and a void is produced, the roof rocks break along bedding planes, joints, and <br />_. fractures and fall to the mine floor. Rotation of the caved debris occurs during the fall so that the <br />caved fragments tend to pile up in a random fashion. This caved zone, according to Peng (1992), <br />occurs for the first 2 to 8 mining thicknesses (2 to 8t) in the roof rocks. In the current West Elk <br />Mine longwall panels, the caved zone is estimated to be 2.5 mining thicknesses (2.St) based on <br />roof track observations from directly behind the current longwall equipment. Any water present <br />in this zone will drain into the mine almost immediately after caving occurs. <br />The B and E-Seam roof rocks commonly consist of thinly bedded carbonaceous shales, sandy <br />shales, claystones, and sandstones. A soft shale that is susceptible to air slaking forms the <br />immediate roof of the B-Seam in most areas. Thick sandstones locally form the immediate roof <br />of the E-Seam, in addition to the shales and sandstones. <br />The ratios of shale to sandstone are quite similar in the first 20 feet of roof in the B and E-Seams. <br />The shale to sandstone ratio of the first 20 feet of B-Seam roof averages about 2:3, the shale to <br />sandstone ratio of the fast 5 feet averages 3:2. The shale to sandstone ratio of the first 20 feet of <br />the E-Seam roof is 3:2; the shale to sandstone ratio of the first 5 feet is also 3:2. Although the <br />percentages of shale to sandstone are similar in the B-Seam and E-Seam roof rocks, a much <br />higher degree of local variability occurs above the E-Seam. <br />The B and E-Seam roof rocks above the first 20 feet consist of shale, siltstone, lenticular <br />sandstones, and thin coal beds. A marine sandstone, locally consisting of a lower and upper <br />tongue and ranging from about 30 to 125 feet thick, underlies the D and E-Seams; the D-Seam <br />• occurs a foot, to as much as 50 feet below the E-Seam. <br />2.05-112 Revitedhm. 1995 PR06; 1/96RN03; RevisedJm,. 1998PR08 <br />