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

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and similarly increase the width of the predicted open fracture. The total additive tensile strain at other locations along the overlapping strain profiles could be conservatively predicted by superimposing the subsidence profiles from the two adjacent longwall panels. The rapidly changing overburden depths at the Red Cliff Mine Project Area could make estimating the total tensile strains across barrier pillars using the NCB method a time consuming process. 7.3 Maximum Tilt (G) The maximum slope or tilt change as the result of mining a longwall panel occurs at the inflection point between bending progressively more downward toward the center of the panel to bending progressively less downward closer to the center of the panel. On Figure 4. Plan View of Surface Subsidence Over a Longwall Panel in Affected Environment/Subsidence, this is the 0.50 Smax contour line. With the exception of subcritical panels, where the panel width is less than approximately 0.41 times the panel depth, the inflection line is within the sides of the panel projected to the ground surface. Table 11. Maximum Slope Angle (Tilt) Change for Planned Red Cliff Mine Longwall Panels lists potential panel widths, depths, panel width/depth ratios and the slope (tilt) change multiplier from Figure 10. NCB Maximum Strain and Slope Prediction Graph in Affected Environment/Subsidence. The calculated maximum slope angle change is presented in terms of percent grade change and degrees. The conservative NCB predicted single panel maximum slope angle changes resulting from longwall mining of the proposed Red Cliff Mine Project Area, potentially ranging from approximately 0.5% to 12% (0.3° to 7°) would present significant hazards to overlying industrial, business and residential uses. However, no such land uses are planned over the Red Cliff Mine. The principal tilting hazard posed to the undeveloped surface overlying the proposed lease area by longwall mining would appear to be tilting cliff forming sandstone beds outcropping on the canyon walls and potentially toppling sandstone boulders toward the canyon floors. Figure 18. Ribside Tension Cracks in Road Fill and Cliff Face, York Canyon Mine show a sandstone cliff failure in the combined downslope tilted and tension zone approximately 50 feet outside the underlying longwall panel ribside. Table 3. Slope Geometries Within Project Area lists some of the higher overall canyon slopes in the lease area. The slopes of Big Salt Wash canyon, the major canyon in the proposed lease area, walls are as high as 920 feet and as steep overall as 32°, which is the most impressive combination in the Project Area. It is possible to at least partially mitigate this and similar potential major toppling hazards in Garvey Canyon and along Munger Creek by retreating toward these drainages from the north and from the south. Retreating toward these drainages, would slightly flatten the slope of the canyon walls as opposed to advancing away from Big Salt Wash which would slightly steepen the canyon walls. See Figure 9. Localized Mining Induced Slope Angle Changes. The slope angle or tilt change over a barrier pillar is not additive like horizontal tensile strains over barrier pillars. The slope angle or tilt change coming from longwall panels on opposite sides of a barrier pillar are in opposite directions Therefore, where the tilting overlaps the longwall mining induced slope changes at least partially cancel each other. The maximum interaction is potentially possible complete cancellation is unlikely. C-34 DBMS 326