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West Elk Mine <br />Cracks and bulges caused by landslides are other types of gravity-induced features that <br />may appear to be related to subsidence, particularly in areas that have been, or are being, <br />undermined. However, landslide-induced features are related to the geometry of the <br />landslide rather than the mine geometry. For example, cracks are most common in the <br />upper area of a landslide, whereas, bulges are most common in the lower area of the slide. <br />This spatial and geometric relationship to a landslide footprint on steep, unstable slopes, <br />rather than the mine geometry can usually be used to differentiate between gravity-induced <br />and mine-induced surface features. <br />Subsidence Prediction Based on Local Mining Experience - 2.05.6 (6)(e)(i)(C) <br />Much information has been gathered regazding subsidence at West Elk Mine due to local mining <br />of the F Seam (room-and-pillar method) and B Seam (longwall method). Subsidence monitoring <br />of a grid network has been conducted since 1985, and has provided considerable data regarding <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 />regard to longwall mining, that the majority of the subsidence was seen within the first yeaz after <br />mining, and in most cases subsidence was completed within 12 to 18 months. This information <br />and its usefulness in predicting subsidence parameters in the current and South of Divide mining <br />azeas is detailed in the following section. In addition, some general observations obtained from <br />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 defimed within the subsidence azea; 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 parameters <br />for the Apache Rocks and Box Canyon mining areas. Table 2 of Exhibit 60B summarizes the <br />projected values of these parameters for the South of Divide mining azeas as described in the <br />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) 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 />2.05-109 RevisedNwember 1004 PRIO <br />