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West Elk Mine <br />Much information has been gathered regazding subsidence at West Elk Mine due to local mining <br />of the F Seam (room-and-pillaz method) and B Seam (longwall method). Subsidence monitoring <br />of a grid network has been conducted since 1985, and has provided considerable data regazding <br />the effects of varying overburden thicknesses, mining heights, and mining methods on the <br />subsidence network. The grid has also verified MCC's predicted subsidence, and established <br />when subsidence occurs, where it occurs, and when it is complete. The grid demonstrated, in <br />regazd to longwall mining, that the majority of the subsidence was seen within the first year after <br />mining, and in most cases subsidence was completed within 12 to 18 months. This information <br />and its usefulness in predicting subsidence pazameters in the current and South of Divide <br />mining azeas is detailed in the following section. In addition, some general observations <br />obtained from West Elk Mine and neighboring mining operations are described below. <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 downwazd 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 summarizes the projected values of these <br />parameters for the Apache Rocks and Box Canyon mining areas. Table 2 of Exhibit 60B <br />• summarizes the projected values of these parameters for the .South of Divide mining areas as <br />described in the following 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) Neaz-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.St) <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 />Mr. Dunrud estimates that the caved zone in the Apache Rocks and Box Canyon mining <br />areas will range from 2 to 4 extraction thicknesses. Caved zone heights closer to 2 times the <br />mining thickness (t) are expected in dry mining conditions, whereas wetter conditions will <br />produce caved zone heights closer to 4t. An acceptable average value for the Apache Rocks <br />• and Box Canyon mining areas is 2.St. <br />2.05-/09 Revised Jane 2005 PR/0 <br />