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West Elk Mine <br />l? 3.6 feet (0.3t) beneath the ridge areas. Maximum subsidence may be less in the northeastern <br />part of the panel area where the Marine Sandstone beneath the D-Seam is about 100 feet thick. <br />• Western Panels-Combined E and B-Seam Minin - For E Seam mining and B Seam. <br />mining, the three mining panels will range from supercritical to critical in the draw areas, <br />previously mentioned in the E Seam mining section, to subcritical in the ridge areas adjacent <br />to these draws. Because the total extraction thickness will average 23 feet (E Seam, 11 feet; <br />B Seam, 12 feet), the total maximum subsidence (vertical displacement) after mining both <br />seams is projected to range from about 13.8 to 18.4 feet (-0.6t to 0.8t). <br />Box Canyon Mining Area - 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 />unnamed 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 a coal-extraction thickness <br />of 12 feet and chain pillar dimensions similar to the current West Elk mining area, maximum <br />subsidence (5,,, 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 the flanks of West <br />Flatiron. Maximum subsidence (Sm) above panel centers is projected to range from 7.2 to 9.6 <br />feet (0.6t to 0.8t) beneath the canyons to about 6.0 to 8.4 feet (0.5t to 0.7t) beneath areas of <br />greatest overburden depth. Maximum subsidence above chain pillars (Scp) is projected to range <br />r from 1.2 to 3.6 feet (0.lt to 0.3t). Panel 25 in Sylvester Gulch is expected to be in this range. <br />2. The third panel southward will be subcritical beneath Box Canyon based on width-to-depth <br />ratios [W/d] ranging from 0.5 to 1.1. Maximum subsidence is projected to range from as much <br />as 8.4 feet (0.7t) in areas of shallower overburden to about 6.0 feet (0.5t) under deepest cover; <br />SCp is projected to range from 2.4 feet (0.2t) to as much as 4.8 feet (0.4t) in areas of deepest <br />cover. <br />3. Panels 4 through 6 range from about critical to subcritical (W/d = 13 to 0.5), assuming that the <br />chain pillars support the overburden load. However, maximum subsidence is expected to range <br />from 3.6 to 7.2 feet (03t to 0.6t) because the subsidence factor above the chain pillars (Sp) is <br />projected to range from 3.6 to 7.2 feet (0.3t to as much as 0.6t). Subsidence of as much as 7.2 <br />feet (0.6t) is therefore projected because the chain pillars and caved material in the caved zone <br />(gob) are projected to compress about equally. With the increased overburden loads under West <br />Flatiron and its flanking cliffs and steep slopes, these three southern panels are expected to <br />locally subside as a super panel (i.e., multiple panels and adjacent chain pillars that behave as a <br />single panel) of supercritical width beneath areas of greatest overburden depth. <br />4. The subsidence factor is projected to be less where the B-Seam roof rocks (sandstones, shales, <br />and siltstones) cave into dry mining conditions than it will be where the roof rocks cave into wet <br />alining conditions. T f-uis . ariance is expected because the final bulking factor of caved rocks <br />under dry conditions is greater than it is for caved rocks under wet conditions. <br />lie <br />2.05-128 Revised Jmae 200.1 PR10. Rev. Adarch 2006; A1a), 2006 PRIO. A'ov. 2006TRIOTApril 2007TR108; Sep. 2007 PR12; Feb. 1008 PR-12