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Elk Creek Mine Subsidence Page 97 February 26, 2003 Water flowing in Bear Creek should rapidly wash valley fill alluvium into open subsidence fractures crossing its channel. In alluvium, at the ground surface 2,700-ft over the northwest corner of Panel #12 the minimum predicted Elk Creek Mine compressive strain is a nearly undetectable 1470µE. In the Bear Creek channel near the center of Panel #1, the maximum predicted Elk Creek Mine compressive strain is 26200µe. The predicted result is a roughly 2-ft high compression ridge across the Bear Creek drainage. This compression ridge will be roughly perpendicular to Bear Creek. A 2-ft high subsidence induced compression ridge could not temporarily impound more than 13-ft of water upstream in the Bear Creek drainage because of Bear Creek's 15~ southerly gradient across Panel #1. Water flowing in Bear Creek should cut through alluvial compression ridges as they develop during longwall face advance. •I I Chimney subsidence that may eventually develop in the roof of planned entries, longwall panels and gateroads should not breach the ground surface. The theoretical maximum predicted conical collapse I is ten times the mining height. The theoretical conical chimney height over a 9.5-ft high entry and crosscut intersections for minimum 30$ coal measure rocks is 95-ft. The high percentage of sandstone, approximately 60~, in the interburden and the high I percent free swell of sandstone when broken, 67~, further reduces the potential for adverse chimney subsidence effects. The highest available record, 200-ft, of a collapse chimney breaching the ground •) surface above coal mine workings would not breach the minimum 210 feet of overburden over planned Panel #1 where its south ribside will cross under Bear Creek. The possibility of chimney subsidence I breaching any `D" Seam workings from abandoned underlying workings is extremely remote. Reported rectangular collapse chimney heights over longwall panels range from 36-ft to 48-ft. Obviously, the I rectangular collapse above a longwall panel will not breach the ground surface through 210-ft of overburden to Bear Creek. The minimum nearly 200 feet of interburden and the 60~ sandstone in the interburden will also dissipate any stress concentrations above barrier pillars in the abandoned underlying high-extraction advance and retreat room and pillar mine workings and prevent multiple-seam stress concentrations from adversely affecting the planned "D" Seam workings. The landslide located over the extreme southwest corner of Panel #1 (Junge, 1978) should not be destabilized by the predicted surface subsidence. The west to east mining of the longwall panels should result in a minor increase the stability of the landslide mass because the subsidence will tilt the south facing ground surface slope containing the upper 1,500-ft of the landslide back into the slope. -47- L ~L