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Sanborn Creek Subsidence Page 11 September 2, 1997 CHIMNEY SUBSIDENCE POTENTIAL ~- Chimney, or roof collapse, subsidence does not present any hazard to either the ground surface, or the planned B Seam longwall panels, or the abandoned mine workings overlying the planned _ longwall panels because of the depth of the panels and thickness of ~ the interburden. Gra Bruhn and Turka (1977 y, ) physically ~-- 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 i_ 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 L through the average 238 feet of interburden between the B Seam and Oliver No. 1 Mine workings in the D Seam is less than 1~. The cumulative probability of an uncontrolled chimney penetrating through the average 364 feet of interburden between the B Seam and r overlying Hawksnest Mine workings in the E Seam is less than 1/28. Piggott and Eynon (1977) presented a mathematical method of r predicting the theoretical worst-case height of chimney development i_~ based on the mining height and the percent swell of the collapsing rock, Figure 9. Table 3 presents percent swell for various rocks. L If twelve feet of coal is extracted as planned from the "B" Seam and the interburden is entirely sandstone, 67 percent free swell, the maximum height of potential conical, worst-case, chimney C collapse is 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 110 feet. Theoretically, a ~ collapse chimney should not penetrate through the interburden rock - between the "B" Seam and the "D" Seam 238-ft overhead. Roof collapse following an advancing longwall face has a rectangular C collapse geometry, as shown on Figure 4. The height of rectangular collapse is one-third the potential collapse height for worst-case conical collapse, i.e. 18-ft for sandstone interburden and 36-ft C. for shale interburden. Therefore, there is no possibility of a mining induced chimney L collapse connection between the B Seam workings and either the overlying Oliver No. 1 Mine or the Hawksnest Mine. Any direct connection between the Sanborn Creek Mine would have to be through i a fault zone or structure. No faults or fault zones have been '_ encountered during mining in the Sanborn Creek Mine. Longwall gateroad development should disclose any faults or fault zones crossing a panel before longwall mining of the panel begins. ` 11