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"D" Seam Longwall Subsidence Page 18 August 17, 1998 CHIMNEY SUBSIDENCE POTENTIAL Chimney, or roof collapse, subsidence from either the planned "D" Seam longwall panels or the abandoned underlying "C" Seam and "B" Seam room and pillar workings does not present any hazard to the ground surface because the minimum overburden exceeds both available experience and theoretical possibility and has no documented precedent or theoretical basis. The potential for chimney collapse penetrating through the interburden from either the abandoned underlying "C" Seam or "B" Seam workings into the planned "D" Seam longwall panels is extremely remote because of the thickness and type of the interburden and the elapsed time since the workings were abandoned. Gray, Bruhn and Turka (1977) documented the height of collapse chimney subsidence above the Pittsburgh Seam for 127 cases. Their compilation of the height of collapse chimneys was doubtless conservative because all their cases breached the ground surface. Surface weathering no doubt weakened the rock through which a portion of these chimneys developed. Gray, et. al. reported chimney subsidence height without respect to either the mining height or the lithology. One of the 127 cases breached the ground surface 200 feet above the Pittsburgh Seam. Figure 3 presents the relative cumulative frequency distribution for their data. Their Pittsburgh Seam based cumulative probability of an uncontrolled • chimney penetrating to the ground surface through the minimum 240 Piggott and Eynon (1977) presented a mathematical method of predicting the theoretical worst-case height of chimney development based on the type of working, the mining height and the percent swell of the collapsing rock, Figure 4. Conical chimney collapse is for the intersection between entries, wedge collapse for roadways (long and narrow workings) and rectangular collapse for longwall panels (long and wide workings). Table 9 presents percent swell for various rocks. If twelve feet of coal is extracted as planned from the underlying "C" Seam and 60 percent the interburden is sandstone (67$ free swell) and 40 percent is shale/mudstone (33~ free swell), the maximum height of potential conical, worst-case, chimney collapse is approximately 67 feet. Therefore, it is not theoretically possible for conical chimney collapse above "C" Seam to penetrate the approximately 194 feet of interburden between the underlying "C" Seam and the planned "D" Seam workings. Similarly, it should be impossible for conical chimney collapse to reach the surface above the planned 8.5-foot high "D" Seam Main Entry and feet of overburden above the "D" Seam is less than one percent. The possibility of a collapse chimney penetrating upward from the underlying "C" Seam through the minimum 188 plus feet of interburden and connecting with "D" Seam workings is less than one percent based on available experience. gateroads or rectangular collapse above longwall panels. le