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West Elk Mine <br />maximum about midway between the point of inflection and the point of maximum vertical <br />displacement and decrease to zero again at the point of maximum vertical displacement. <br />Maximum Vertical Displacement (Subsidence) <br />Longwall mining panels are defined by their panel width to overburden depth ratio as subcritical, <br />critical, or supercritical. The subsidence literature indicates that a panel width to overburden <br />depth ratio between 1.0 and 1.4 (average of 1.2) generally defines the critical longwall panels. <br />Those panels with panel width to overburden depth ratios exceeding this value are defined as <br />supercritical, and those which are less are defined as subcritical. <br />Apache Rocks Mining Area - Overburden depth above projected eastern longwall panel centers (B <br />Seam mining only) of the Apache Rocks mining area ranges from about 950 feet near the head of <br />Sylvester Gulch to about 2,250 feet near the south end of West Flatiron (see Map 14 for details). <br />Overburden depth above the western panel centers ranges from about 500 feet to about 1,050 feet <br />for the E Seam and 700 feet to 1,250 feet for the B Seam. <br />With a projected longwall panel width of 950 feet, and assuming that the chain pillar dimensions <br />will be similar to the current West Elk mining area, maximum subsidence (Sm at) is predicted as <br />follows: <br />• Eastern Panels - The eastern panels, which trend about N80°W, will range from about <br />critical to subcritical in width in the west ends of panels E1, E2, E3, and E4 SE (numbered <br />• from north to south) to subcritical in the eastern part of the panels. Maximum subsidence <br />(Sm at) above panel centers is projected to range from 4.8 to 6 feet (0.4t to 0.5t) beneath <br />areas of greatest overburden depth to 7.2 to 9.6 feet (0.6t to 0.8t) near the head of Sylvester <br />Gulch. The maximum subsidence above the chain pillars (S,p apt) is projected to range <br />from 2.4 feet (0.2t) in areas of thinnest overburden to 6.0 feet (0.5t) feet in areas of thickest <br />overburden. <br />The Marine Sandstone located beneath the D Seam is about 100 feet thick in the eastern panel <br />area. This sandstone may reduce the subsidence factor, compared to the western panel area, <br />where the sandstone is less than 50 feet thick. <br />• Western Panels E Seam Mining - For E Seam mining, all three western panels (numbered <br />from south to north) will be of supercritical width (i.e., mining width greater than critical; <br />critical width is the mining width needed to cause maximum subsidence) in areas near the <br />head of Pond Gulch, in the Horse Gulch area, and the two unnamed draws east of Horse <br />Gulch. Only in areas of the higher ridges adjacent to these draws will the panels be of <br />subcritical width (mining width less than critical). With a projected longwall panel width of <br />950 feet, assuming an 11-foot coal-extraction thickness and chain pillar dimensions and <br />geometry similar to the current West Lllc mining area, maximum subsidence is projected to <br />range from 6.6 to 8.8 feet (0.6t to 0.8t). <br />Maximum subsidence is expected to be closer to 6.6 feet (0.6t) beneath the ridges and closer to <br />8.8 feet (0.8t) beneath the draws. Maxiimwn subsidence above the chain pillars (S,.p) is <br />. predicted to range from 0.6 feet (0.05t) where the overburden is thinnest (in the draws) to about <br />2.05-127 Revised June 2005 PRIG, Rev. March 2006; Map 2006 PRIO. Alov. 2006TRI07.April 2007TR108;Sep. 2007 PRI2; Feb. 2008 PR-12