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"D" Seam Longwall Subsidence Page 57 August 17, 1998 <br />• CONCLUSIONS <br />The ground surface over and near the planned "D" Seam <br />longwall panels will subside. The surface subsidence will be of <br />the type referred to as trough subsidence. The predicted <br />worst-case lowering of the overlying ground surface by the <br />subsidence accompanying longwall panel mining exceeds 15-ft as an <br />absolute maximum. The conservative longwall mining induced <br />vertical subsidence predictions are presented on Plates 1 through <br />5. It is probable that actual maximum surface subsidence will not <br />exceed 10-ft because of conservative assumptions utilized. The <br />most conservative assumption was that the full thickness of the <br />"D" Seam will be extracted wherever present. It is unlikely that <br />one set of longwall equipment will be able to fully extract both <br />the minimum 7-foot and the maximum 17-foot seam thicknesses. The <br />panel group subsidence predictions also conservatively assume that <br />all longwall panels in the group are instantaneously extracted. <br />The collapsed roof rock accompanying the extraction of one panel <br />will consolidate and provide considerable overburden support <br />before the adjacent panel is extracted. The expansion of the <br />collapsed roof rock will prevent the maximum possible predicted <br />subsidence assumed from developing when the gateroad pillars crush <br />out as planned. <br />The maximum predicted horizontal tensile strains at the <br />ground surface will result in open surface fractures roughly along <br />the maximum tensile strain lines on Plates 6 through 10. The <br />width of these open fractures will range from a barely visible <br />small fraction of an inch (predicted tensile strains in the range <br />from 1400µe to 3000µe) to approximately 2 feet in width (predicted <br />tensile strain maximum of about 40000µe). The magnitude of the <br />maximum predicted compressive strains at the ground surface range <br />from nearly undetectable (1400µe) to compression ridges approaching <br />1-ft in height (30000µe). <br />Chimney subsidence that may eventually take place in the <br />planned entries, longwall panels and gateroads should not breach <br />the ground surface. The minimum 240 feet of overburden above the <br />planned "D" Seam workings is too great for a collapse chimney from <br />developing to the ground surface. The possibility of chimney <br />subsidence breaching any of the "D" Seam workings from the <br />underlying workings is extremely remote. The nearly 200 feet of <br />interburden between the "D" Seam and the closest underlying <br />abandoned high-extraction advance and retreat room and pillar "C" <br />Seam workings is sufficient to prevent chimney collapse from <br />affecting the planned "D" Seam workings. The high percentage of <br />sandstone, approximately 608, in the interburden and the high <br />percent free swell of sandstone when broken further reduces the <br />• potential for adverse chimney subsidence effects <br />5~ <br />