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West Elk Mine <br />• Subsidence, as it relates to mining, is defined as the local downward displacement of the surface and <br />the overburden rock in response to mining under the influence of gravity. The following text <br />includes a general discussion of the various 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 Exhibit 60B <br />(Table 1). Table 2 and Table 3 in Exhibit 60 summarize the projected values of these parameters for <br />the Apache Rocks and Box Canyon mining areas. Table 2 of Exhibit 60B summarizes the projected <br />values of these parameters for the South of Divide mining areas as described in the following <br />subsidence 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 2 in Exhibit 60B for details): (1) Caved <br />zone, (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, <br />and fractures and fall to the mine floor. Rotation of the caved debris occurs during the fall so <br />that the caved fragments tend to pile up in a random fashion. This caved zone, according to Peng <br />(1992), occurs for the first 2 to 8 mining thicknesses (2 to 8t) in the roof rocks. In the current <br />West Elk Mine longwall panels, the caved zone is estimated to be 2.5 Mining thicknesses (2.5t) <br />based on roof rock observations from directly behind the current longwall equipment. Any water <br />present 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 of <br />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 first 5 feet averages 3:2. The shale to sandstone ratio of the first 20 feet of the <br />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 higher <br />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 sandstones, <br />and thin coal beds. A marine sandstone, locally consisting of a lower and upper tongue and ranging <br />from about 30 to 125 feet thick, underlies the D and E-Seams; the D-Seanz occurs a foot, to as much <br />as 50 feet below the E-Seam. <br />Mr. Dunrud estimates that the caved zone in the Apache Rocks and Box Canyon mining areas <br />. will range from 2 to 4 extraction thicknesses. Caved zone heights closer to 2 times the mining <br />thickness (t) are expected in dry mining conditions, whereas wetter conditions will produce <br />2.05-123 Revised June 2005 PRIO, Rev. A9arch 2006; A1ay 2006 PR10, Alov. 2006TR707.4pri12007TR108;Sep. 2007 PRI2; Feb. 2008 PR-12