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Elk Creek Mine Subsidence Page 3 February 26, 2003 feet from the east end of the panel for the north and south • gateroad ribsides and from the north gateroad pillar ribside for the east or west room ribsides. Actual trough subsidence may differ from the worst-case predictions by as much as 108 (NCB, 1975) because of local variations in the distribution of the rock types present in the overburden and the stratigraphic position, thickness and proportion of the rock types indicated on drillhole logs. • '~ The absolute maximum predicted vertical surface subsidence and maximum surface strains will develop where the minimum overburden is present above the panels. Minimal overburden thicknesses of less than 1,000-ft over planned Elk Creek Mine workings will occur where the Bear Creek drainage crosses over Panels #1 dnd #2 in Panel Group 1 and over Panels #12 through #17 in Panel Group 3. Similar shallow overburden thicknesses will be present at the western ends of Panels #15, #16, #18 and #19 where these panels approach the east side of Hubbard Creek. The minor predicted increase in trough subsidence that will develop where the Elk Creek drainage crosses over Panel #11 in Panel Group 2 will probably be too small to be detectable because of the shallow depth of the drainage and the overburden thickness, between approximately 2,900-ft and 2,500-ft, Table 1 presents the locations of these crossings and the maximum associated predicted tensile {+E) and compressive strains (-E). The maximum predicted vertical subsidence of Bear Creek occurs where it crosses Panel #1 and is predicted to be 10.8-ft, see Plate 3 and Table Al in Appendix A. In flat terrain such a depression would impound water producing a small lake across the 1,500-ft long section of a stream course. However, Bear Creek drops 110-ft from north to south over that reach. Therefore, the overall gradient will simply be decreased by trough subsidence but no water will be impounded because gradient will still be toward the south. Bear Creek will enter the high north side of the trough depression and flow unimpeded out the south side. Predicted maximum Elk Creek Mine horizontal tensile strains (+E) occur where Bear Creek crosses over the south and north ribsides of Panel #1, see Plate 4. At these respective locations the predicted tensile strains are 334O0µ at 210-ft depth to 26200 µ at 350-ft depth, see Table A1. Tensile strain on the order of 33000µe typically results in an approximately 2-ft wide open fracture at the ground surface. Initially open fractures will probably develop along the maximum tensile strain line (+E) directly above the north and south ribsides of Panel #1, where Bear Creek crosses overhead, Plate 4. The predicted open fractures over the south and north ribsides will be roughly perpendicular to Bear Creek. At these locations Bear Creek is approximately 600-ft below the top of the west side slope and more than I, 000-ft below the top -3-