2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT 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. 7.4 Angle of Draw The angle of draw defines the extent that subsidence can be detected beyond the limits of mining. The angle of draw is the angle formed by the vertical line above the outer limit of mining and the lateral limit of detectable subsidence. It has special importance to land-use planning because it indicates where the surtace will be unaffected by mining-induced subsidence. Reported angles of draw are highly variable, as indicated by Table 6. Angles of Draw for Coal Mining in the United States and Europe presented angles of draw from 19° to 45° collected from various countries and sources. The study by Abel and Lee (1984) demonstrated that the potential for error in applying the angle of draw measured in one country to another, or even within one country and(or) district, is considerable. Table 12. Angles of Draw for Mines in Flat-Bedded Sedimentary Rocks with Respect to Lithology of Overburden, from their paper, shows a wide range of angles of draw, from 0° to 40°, indicates that lithology statistically appears to plays a roll in determining the angle of draw. The various sources of data demonstrate that the NCB's 35° angle of draw is a conservative Page 37 of 57