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Elk Creek Mine Subsidence Page 16 February 26, 2003 <br />The British National Coal Board (1975) longwall subsidence <br />prediction method was developed from surface subsidence <br />measurements over 187 longwall panels, mostly in areas where <br />mining had already taken place in overlying and/or underlying <br />coal seams. The NCB subsidence predictions are for ground <br />previously subsided by mining in other coal seams. The NCB <br />reported that surface subsidence over longwall panels in virgin <br />ground is normally only 90~ of subsidence measured over longwall <br />panels extracted in previously subsided ground. Most of the <br />planned Elk Creek Mine will be in virgin ground, not previously <br />subsided. The southern end of the Elk Creek Mine will overly <br />areas of abandoned room and pillar mine workings in one or more <br />of the underlying `C2" Seam, "B" Seam and `B2" Seam and were <br />probably previously subsided. <br />In Panel Group 1, all of planned Panel #1 and Panel #2 <br />overly abandoned mine workings, between the purple lines labeled <br />"Mining Limits in Other Seams" on Plate 2. Most of Panel #3 has <br />been similarly undermined. Roughly 1,100-ft of the north side of <br />Panel #3 lies in virgin ground, i.e. not previously subsided. <br />Roughly S50-ft of the south side, 800-ft at the east end and <br />3,000-ft at the west end of the north side of Panel #3 will be in <br />virgin ground. All but about 1,500-ft of the south side of Panel <br />#4 will be mined in virgin ground. All panels in Panel Group 2 <br />are in virgin ground. In Panel Group 3, Panels #12, #13 and #14 <br />are in virgin ground, between the purple lines labeled "Mining <br />Limits in Other Seams" on Plate 7. Roughly 2,000-ft near the <br />center of Panel #15 and 3,500-ft near the west end on the south <br />side of Panel #15 overly abandoned room and pillar mine workings, <br />as indicated by the outline of the mine workings shown on Plate <br />12. Roughly 3,400-ft near the west end of the north side of <br />Panel #16 overlies abandoned room and pillar mine workings. On <br />the south side of Panel #16, only about 2,000-ft near the center, <br />500-ft on the east end and 400-ft on the west end are in virgin <br />ground. Panels #17, #18 and #19 of Panel Group 3 have previously <br />been undermined. <br />Table 2 presents the interburden thickness between the "D" <br />Seam and the underlying "C" Seam, 199-ft mean thickness, and "B" <br />Seam, 238-ft, measured in eight boreholes. The reported <br />lithologic distribution was measured by logging six of the <br />boreholes. The interburden thickness and should be sufficient to <br />prevent adverse mining induced stress concentrations during <br />planned "D" Seam longwall mining. The mean 194-ft to 238-foot <br />thicknesses of the interburden and the approximate weighted <br />average sandstone in the interburden should also prevent adverse <br />mining induced stress concentrations during planned "D" Seam <br />longwall mining in the Elk Creek Mine (Chanda, 1989). <br />-16- <br />