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DRAFT <br />The lowering of the ground surface over and around a supercritical longwall panel is trough <br />shaped, as shown on Figure 4. Plan View of Typical Subsidence Over a Longwall Panel in <br />Affected Environment/Subsidence. Figure 4 shows a supercritical width panel with the maximum <br />subsidence (Smax) as a narrow area around and along the center of the panel and inside the <br />1.00 times Smax contour line. The maximum subsidence (Smax) over a critical or subcritical <br />longwall panel occurs along a line roughly at the center of the panel, as shown on Figure 7. <br />Critical Panel Width for Maximum Trough Subsidence in Affected Environment/Subsidence. <br />Table 8. Maximum Vertical Subsidence (Smax) for Planned Red Cliff Mine Longwall Panels <br />presents the Smax results of applying Figure 8. NCB Panel Width/Depth Maximum <br />Subsidence (Smax) Prediction in Affected Environment/Subsidence and the location of Smax with <br />respect to the individual panel centerline through application of Figure 9. NCB Subsidence <br />Profile Graph in Affected Environment/Subsidence. Figure 19. Maximum Vertical <br />Subsidence (Smax) With Respect to Panel Width and Depth is a plot of the predicted <br />maximum subsidence for the potential range of panel widths at the anticipated longwall mining <br />depths at the Red Cliff Mine Project Area. <br />In Table 8, the Panel Width in the first column and Overburden Depth in the second column are <br />given in both English and metric units because the NCB graphs are in metric units. Column 5 <br />presents both the subsidence factor and immediately below the predicted maximum number of <br />feet of vertical subsidence in a parenthesis for the planned maximum 11-foot mining height. <br />The conservative NCB predicted maximum horizontal tensile (+E) and compressive (-E) strain <br />values presented on Table 9. Maximum Tensile (+E) and Compressive (-E) Strains for <br />Planned Red Cliff Mine Longwall Panels were estimated using Figure 11. NCB Maximum <br />Strain and Slope Prediction Graph in Affected Environment/Subsidence. The locations of the <br />maximum tensile and compressive strains with respect to the individual panel centerlines were <br />estimated using Figure 12. NCB Horizontal Strain Profile Graph in Affected <br />Environment/Subsidence. The maximum tensile and compressive strains are important because <br />if they can be conservatively predicted steps can be taken to reinforce critical surface structures <br />or modify the mining plan to reduce the maximum tensile and compressive strains. For example, <br />high pressure natural gas pipelines have been undermined by longwalls by maintaining a <br />smooth pipeline through the period when the trough is forming under the pipeline, while the <br />longwall face advances across or along the pipeline. This has been accomplished by digging <br />up, temporarily supporting the section of the pipeline ahead of the advancing longwall face and <br />reburying the pipeline after the longwall face has advanced well past the elevated section of the <br />pipeline. This procedure prevents the buried pipeline from being pulled apart at an open <br />fracture. Many countries with significant longwall coal mining operations have recommended <br />and(or) established allowable strains for particular surface features. Some of these are included <br />in APPENDIX A. RECOMMENDED LIMITS FOR SUBSIDENCE INDUCED STRAIN AND TILT. <br />Page 31 of 57 <br />