2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

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overlying and underlying coal seams. All such features have a limited tolerance for these potentially adverse effects. The magnitude of the potentially adverse impacts is directly related to the maximum subsidence, i.e. the greater the subsidence the greater the magnitude of the impact, provided the depth and panel dimensions do not change. The magnitude of the potentially adverse surface impacts is inversely related to the mining depth, i.e. the magnitude of potentially adverse impacts decrease as the mining depth increases. Great Britain has lead the world in researching these relationships because every major metropolitan area, except London, was underlain by multiple mineable coal seams. It is possible to somewhat mitigate the adverse impacts by varying panel width, by designing gateroad pillars between panels to yield when the first of two adjacent panels is mined and crush after the face of the second panel is mined past and by positioning longwall panels with respect to a particularly important surface feature. The conservative NCB maximum vertical subsidence prediction for supercritical longwall panel widths is 0.9 times the mining height (m) for overburden has been previously subsided. The NCB method specifies that the previously subsided maximum vertical subsidence prediction be multiplied by 0.9 for ground that has not been previously subsided. The adjustment for previously unmined ground is referred to as the "virgin" ground correction in Great Britain. Subsidence over the proposed Red Cliff Mine Project Area was analyzed as virgin ground because none of the proposed lease area appears to have been previously mined. The overall supercritical subsidence factor for virgin ground is 0.81 times the mining height. The lowering of the ground surface over and around a supercritical longwall panel is trough shaped, as shown on Figure 4. Plan View of Typical Subsidence Over a Longwall Panel in Affected Environment/Subsidence. Figure 4 shows a supercritical width panel with the maximum subsidence (Smax) as a narrow area around and along the center of the panel and inside the 1.00 times Smax contour line. The maximum subsidence (Smax) over a critical or subcritical longwall panel occurs along a line roughly at the center of the panel, as shown on Figure 7. Critical Panel Width for Maximum Trough Subsidence in Affected Environment/Subsidence. Table 8. Maximum Vertical Subsidence (Smax) for Planned Red Cliff Mine Longwall Panels presents the Smax results of applying Figure 8. NCB Panel Width/Depth Maximum Subsidence (Smax) Prediction in Affected Environment/Subsidence and the location of Smax with respect to the individual panel centerline through application of Figure 9. NCB Subsidence Profile Graph in Affected Environment/Subsidence. Figure 19. Maximum Vertical Subsidence (Smax) With Respect to Panel Width and Depth is a plot of the predicted maximum subsidence for the potential range of panel widths at the anticipated longwall mining depths at the Red Cliff Mine Project Area. In Table 8, the Panel Width in the first column and Overburden Depth in the second column are given in both English and metric units because the NCB graphs are in metric units. Column 5 presents both the subsidence factor and immediately below the predicted maximum number of feet of vertical subsidence in a parenthesis for the planned maximum 11-foot mining height. The conservative NCB predicted maximum horizontal tensile (+E) and compressive (-E) strain values presented on Table 9. Maximum Tensile (+E) and Compressive (-E) Strains for Planned Red Cliff Mine Longwall Panels were estimated using Figure 11. NCB Maximum Strain and Slope Prediction Graph in Affected Environment/ C-28 DBMS 320