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Wesr Elk Mine <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 azea, <br />where the sandstone is less than 50 feet thick. <br />• Western Panels E-Seam Minine -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 mitring 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 Elk mining azea, tnaximum 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 closet <br />to 8.8 feet (0.8t) beneath the draws. Maximum subsidence above the chain pillars (S~~ is <br />predicted to range from 0.6 feet (O.OSt) where the overburden is thinnest (in the draws) to <br />about 3.6 feet (0.3t) beneath the ridge areas. Maximum subsidence may be less in the <br />northeastern part of the panel area where the Marine Sandstone beneath the D-Seam is about <br />100 feet thick. <br />• Western Panels-Combined E and B-Seam Minims -For E-Seam mining and subsequent B- <br />Seam mining, the three mining panels will range from supercritical to critical in the draw <br />areas, previously mentioned in the E-Seam mining section, to subcritical in the ridge azeas <br />adjacent to these draws. Because the total extraction thickness will average 23 feet (E-Seam, <br />11 feet; B-Seam, 12 feet), the total maximum subsidence (vertical displacement) after mining <br />both seams is projected to range from about 13.8 to 18.4 feet (-0.6t to 0.8t). <br />Box Camron Minim -Overburden depth above the projected longwall panel centers in the <br />Box Canyon mining area (in the B-Seam) ranges from about 750 feet beneath Box Canyon and the <br />~mnamed gulch west of Box Canyon to about 2,300 feet beneath West Flatiron (see Map 14 for <br />details). With a projected longwall panel width of 1,100 feet, assuming acoal-extraction <br />thickness of 12 feet and chain pillar dimensions similar to the current West Elk mining azea, <br />maximum subsidence (S~,=at) is projected as follows: <br />1. The first two northern panels will range from supercritical to critical at points of minimum <br />overburden depth beneath the canyons to subcritical beneath ridges on [he flanks of West <br />Flatiron. Maximum subsidence (S„J above panel centers is projected to range from 7.2 to 9.6 <br />feet (O.bt to 0.8t) beneath the canyons to about 6.0 to 8.4 feet (O.St to 0.7t) beneath azeas of <br />greatest overbtrden depth. Maximum subsidence above chain pillars (S~ is projected to <br />range from 1.2 to 3.6 feet (O.lt to 0.3t). <br />2. The third panel southward will be subcritical beneath Box Canyon based on width-to-depth <br />ratios [R'/d] ranging from 0.5 to 1.1. Maximum subsidence is projected to range from as <br />2.05-116 RevisedJrm. 1995 PR06; 1196 RN03; RevisedJmi. 1998 PR08 <br />