2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (39)

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CHAPTERFOUR Environmental Consequences and Mitigation fragmented soil cover and, therefore, 1-foot wide surface fractures opening when undermined by a longwall panel at the shallower depths, under approximately 500 feet. For any mining panel width and coal extraction thickness, the maximum subsidence, tilt, and strain at the ground surface should decrease with increasing overburden depth. By itself, simply vertically lowering the ground surface would not be a problem. However, the ground surface is lowered over and near a longwall panel only as the coal between the panel headgate and tailgate pillars is progressively extracted and the longwall face is advanced. The surface subsidence trough advances with the longwall face and all sides of the longwall panel deflect downward toward the center of the panel, where the vertical subsidence is maximum. The bending of the overburden develops as the longwall panel progresses and forms a stable semi-permanent trough after the panel is completely mined. The maximum vertical subsidence over a panel is of major importance because it contributes to the magnitude of extension, compression, and tilting. The magnitude of potentially adverse impacts decreases as the mining depth increases. Table 8, Appendix C presents predicted maximum vertical subsidence for a variety of panel widths and overburden depths. Predicted vertical subsidence ranges from a maximum of 8.9 feet to a minimum of 3.2 feet. Figure 19, Appendix C shows the relationship of panel width to vertical subsidence. The maximum horizontal tensile strains are the most serious potential hazard with respect to anticipated subsidence impacts from longwall mining in the proposed Red Cliff Mine lease area. Table 10, Appendix C shows predicted maximum surface fracture widths ranging from almost 20 inches to less than 1 inch. The conservative predicted single panel maximum slope angle changes resulting from longwall mining of the proposed project area, potentially ranging from approximately 0.5 to 12 percent (0.3° to 7°), would present significant hazards to overlying industrial, business, and residential uses. However, there are no such land uses over the Red Cliff Mine and none are planned. The principal tilting hazard posed by longwall mining to the undeveloped surface would appear to be tilting cliff-forming sandstone beds outcropping on the canyon walls with the potential for toppling sandstone boulders toward the canyon floors. The slopes of Big Salt Wash canyon, the major canyon in the project area, are as steep overall as 32°, with walls as high as 920 feet. Because CAM has not submitted a mine plan for the Red Cliff Mine, a reasonable plan has been projected in order to estimate potential subsidence impacts (Appendix C). This plan assumes that the minimum overburden depth will be 200 feet above the Main Cameo Seam and the maximum overburden depth will be 2,000 feet. The planned minimum overburden depth for longwall mining is 200 feet in order to minimize (1) the potential for chimney caving to the ground surface, (2) the interception and diversion of ground water through the mine workings, (3) the loss of surface water to the fracture zone overlying completed longwall panels, and (4) the potential development of up to 20-inch wide surface fractures along the sides of the panels. It also assumes that the planned coal mining height ranges from 8 to 11 feet. The 11-foot maximum height was used as a conservative maximum thickness in the subsidence analysis. Rockfall Hazards The primary geologic hazard is quantifying the risks associated with slope instability hazards within the proposed Red Cliff Mine site. During field reconnaissance, large boulders to small cobbles were observed as source material along the near vertical cliffs, benches, and steeper 4-72 DBMS 627