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Elk Creek Mine Subsidence Page 20 February 26, 2003 <br />would amount to 48-ft above 12-ft high "D" Seam panels. Schulte 1 <br />(1957) reported observations in a winze sunk down into a German • I <br />longwall. He reported rubble for the first three mining heights <br />up into the seam roof. He was able to trace the original bedding <br />planes above that height. The fractures and fissures he observed <br />in the roof strata decreased upward between three to nine mining I <br />heights above the original seam roof. Above nine mining heights <br />no subsidence effects were observed, except for the downward <br />displacement of the strata. ~ <br />There is no available documented mining experience <br />indicating chimney collapse breaching the ground surface through <br />more than 200-ft of overburden. Gray, Brown and Turka (1977) <br />documented the height of collapse chimney subsidence to the <br />ground surface above the Pittsburgh Seam for 127 cases. They <br />reported one case out of 127 presented where a chimney collapse <br />penetrated through 200-ft of overburden to the surface. Surface <br />weathering no doubt weakened the rock through which a portion of <br />these chimneys developed. Figure 6 presents the relative <br />cumulative frequency distribution for their data. Gray, et al <br />reported chimney subsidence height without respect to either the <br />mining height, or mining geometry, or the lithology. <br />The minimum Elk Creek Mine overburden thickness is 210-ft <br />above Panel #1. Gray, et al's Pittsburgh Seam based cumulative I <br />probability of a collapse chimney penetrating to the ground • <br />surface through the minimum 280 feet of overburden above the `D" <br />Seam is less than one percent. The probability of a collapse I <br />chimney penetrating upward from the underlying `C" Seam through <br />the minimum 188 plus feet or the mean 193 plus feet of <br />interburden and connecting with `D" Seam workings is less than <br />one percent based Gray, et. al, probability graph. The I <br />interburden thickness between the "D" Seam and the underlying "C" <br />Seam mine workings should isolate the planned Elk Creek Mine <br />workings because near surface chimneys are advancing partially I <br />through weathered rock and because of the increased horizontal <br />confinement at depth. <br />A zone of more permeable fractured and fissured tensilly ~ <br />strained rock extends upward from the rectangular chimney <br />collapse rubble, as indicated on Figures 3 and 9. Schulte's <br />observation suggests that the fractured and fissured rock above <br />12-ft high "D" Seam longwalls will probably extend 108-ft upward <br />into the overburden. Table 4 (Peng, 1992) provides predictive I <br />equations for the fracture zone height above the roof of longwall <br />panels based on rock compression strength. The compression <br />strengths of immediate "D" Seam roof and floor rock, reported in <br />Appendix C, indicates "Hard and strong rock" and "Medium hard I <br />rock". The compression strength test results suggest that the <br />fracture zone above the roof of the planned 12-ft high "D" Seam • <br />-20- ' <br />