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Conical chimney collapse height (H) Piggott and Eynon (1977) • <br />Sandstone overburden <br />3(h) (100) <br />H = where; <br />~S h = mining ]zeight(9.5 ft.) <br />H 3(9. 6)7(100) $S =percent free swell(67~) <br />H = 42 ft. <br />Shale overburden <br />H = 3(h)(100) <br />~S where; <br />H = 3(9.5)(100) h =mining height(9.5 ft.) <br />33 ~S =percent free swell(33~) <br />H = 86 ft. <br />Gray, Bruhn and Truka (1977) physically documented the height of collapse <br />chimney subsidence above the Pittsburgh Seam for 127 cases. Gray, et. al. reported <br />chimney subsidence height without respect to the actual mining height or the lithology. Six <br />of the 127 cases breached the ground surface 110 feet above the Pittsburgh Seam. Table <br />15-3 presents the relative cumulative frequency distribution for their data. Gray's (1977) <br />chimney data from the Pittsburgh Seam indicates that a chimney has a minor, <br />approximately 4.7 percent, potential to penetrate 110 feet of overburden. <br />The Bowie #2 Mine mining activities should be in overburden thickness greater than <br />110 feet. Therefore, it is unlikely that chimney collapse wilt penetrate to the surface above <br />the mine workings. <br /> <br />- 14 - 10/00 <br />~'R-o y <br />