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Sanborn Creek Project Paqe 8 August 21, 1992 • Excavation of mains, sub-mains and advance rooms in the "C" and °B" Seams Mine have the potential to produce the chimney collapse type of subsidence. The chimneys that will form if the mine workings are not supported will progress upward over time, until the natural expansion of the collapsed roof rock fills the individual mine opening and plugs the collapse chimney. Robbing of pillars during retreat from planned panels is anticipated to produce pillar failure and, therefore, trough subsidence within the overlying strata. Controlled pillar failure is preferable during retreat from a panel than that pillar failure occurring at some unknown time in the future. Prediction of chimney collapse above mine openings in the "C° and "B" Seams in the Sanborn Creek Mine is important because a chimney collapse has the potential to provide a path for the sudden release of water that may be impounded in overlying workings. This possibility was analyzed in the case of the Oliver No. 1 Mine, in the "D" Seam, and the Hawk's Nest East Portal and West Portal Mines, in the "E" Seam. Prediction of trough subsidence is important because the tensile strains in the overlying strata that would follow pillar collapse will increase the vertical permeability of the strata and may gradually and progressively release water impounded in the overlying workings. CHIMNEY COLLAPSE PREDICTION Piggott and Eynon (1977) presented a mathematical method of predicting the height of chimney development based on the mining height and the percent free swell of the collapsing roof rock, Figure 1. Table 2 presents percent swell for various rocks. If twelve feet of coal is extracted from the "B" Seam and the interburden is entirely sandstone, 67 percent swell, the maximum height of potential conical, worst-case, chimney collapse would be approximately 54 feet. with the same 12 feet of "B" Seam coal extracted but with the entire interburden shale, 33 percent swell, the maximum height of potential conical, worst-case, chimney collapse is approximately I20 feet. It would appear that uncontrolled chimney collapse has the potential of penetrating between the underlying "B" Seam and overlying "C" Seam. Chimney collapse will not develop if the roof of the "B" Seam is supported to prevent roof collapse. However, long-term deterioration of the roof of the "B" Seam could, and would be anticipated to, breach the floor of the "C" Seam floor if not maintained to prevent such uncontrolled chimney collapse. Conservative application of Piggott and Eynon's (1977) worst-case conical chimney height prediction method indicates that even a worst-case conical chimney collapse should not breach the interburden between the "C" Seam and the "D" Seam. This conservative analysis assumes that the planned 12-foot "B" Seam . mining height is added to the planned 8-foot "C" Seam mining