2007-11-08_PERMIT FILE - C1981041 (2)

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• Pillar design was specifically done for mining projections in the Roadside South Portals. The Roadside North Portals are in the same seam with similar conditions. Pillar width is the primary determining factor. The 70 ft. X 70 ft. centers for mining result in a 50 ft. X 50 ft, remaining pillar. Using the formulas in Appendix 20-3 and substituting a pillar height of 7.5 ft., a pillar width of 50 ft., a pillar length of 50 ft, and a 500 ft. overburden depth , a safety factor of 5.27 was calculated for the maximum load over the pillars located under Jerry and Coal Creeks. In the unlikely event that protective pillars retained below the creeks fail, subsidence of predictable magnitude would occur. Referring to Figure 20-1, an 11 ft. X 80 ft. pillar would be selected to provide the necessary support at a 200 ft. depth. Figure 20-2, Table 20-2, and Figure 20-3 are used to predict subsidence based on pillar size, depth, and mining height. Assuming failure of an 11 ft. X 80 ft. pillar at 200 feet of depth, the maximum vertical subsidence predicted is 1.74 feet. The maximum span or width (w) of a typical room-and-pillar panel at the Roadside South Mine is 1,650 feet. Referring to Figure 20-4, the maximum horizontal compressive and tensile strains that would occur coincident with this subsidence are 4,437 and 5,655 micro-strain, respectively. Calculations used to derive these predictions are provided in Appendix 20-4. Since the greatest potential for damage to the pipeline structures over the creeks would be from extension at shallow overburden depth, the worst case predicted for the protected areas is subsidence causing extension of 5,655 micro- strain. According to the classification of permissible horizontal strain in Table 20-3, the predicted 5,655 micro-strain is less than the 0.6 percent (6,000 micro-strain) considered permissible for main roads, railway tracks, main sewers, etc. • 20-5 (Rev. 01/01)