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West Elk Mine <br />The cracks may stay open or close in areas above gate roads with a combination rigid- <br />pillar/yield-pillar configuration. However, as discussed in Exhibit 60B, it is unlikely that cracks <br />will occur in colluvium and alluvium in the stream valleys of the South of Divide mining area. <br />Compression features (bulges and warps) also occur above the longwall mining panels in areas <br />where the ground surface undergoes compression in the subsidence process. The compression <br />features occur toward the center of the mining panel in zones of maximum compression, and are <br />usually more difficult to recognize. They often are masked, or absorbed, by soil and colluviurm,' <br />or are hidden in the brush and grass. They also may be indistinguishable from natural humps and <br />mounds in the soil and colluvium. <br />Pseudo Subsidence Features (Gravity-Induced Tension Cracks) <br />Cracks have been observed on high, steep ridges, near cliffs, and in landslides, in the Box <br />Canyon and Apache Rocks mining areas. These cracks look very much like subsidence cracks, <br />but cannot be, since no mining occurred in the area where they were observed. A good example <br />of a gravity-induced crack is the extensive crack that Mr. Dunrud observed on the narrow ridge <br />of West Flatiron in August 2002. This crack was as much as 3.5 in wide and 150 ft long. This <br />was not a mining-related crack because no mining had occurred in the area. The possibility of <br />gravity-induced cracking in the rugged country above planned mining activities at the West Elk <br />mining areas is a good reason to perform baseline studies of the area prior to mining so that these <br />features can be documented prior to any mining. <br />Cracks and bulges caused by landslides are other types of gravity-induced features that may <br />appear to be related to subsidence, particularly in areas that have been, or are being, undermined. <br />However, landslide-induced features are related to the geometry of the landslide rather than the <br />mine geometry. For example, cracks are most common in the upper area of a landslide, whereas, <br />bulges are most common in the lower area of the slide. This spatial and geometric relationship to <br />a landslide footprint on steep, unstable slopes, rather than the mine geometry can usually be used <br />to differentiate between gravity-induced 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 regarding 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 year after <br />minim, and in most cases subsidence was completed within 12 to 18 months. This monitoring <br />ceased in 1997. This information and its usefulness in predicting subsidence parameters in the <br />current and South of Divide mining areas is detailed in the following section. In addition, some <br />general observations obtained from lVest Elk Mine and neighboring mining operations are <br />described below. <br />0 Detailed Description of Predicted Subsidence Phenomena - 2.05.6 (6)(e)(i)(D) <br />2.05-122 Revised Jw7e 2005 PRIO, Rev. March 2006: Mai, 2006 PR70, Nov. 2006TRI07,Apri12007TR108;Sep. 2007 PR12; Feb. 2008 PR-12