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

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CHAPTERFOUR Environmental Consequences and Mitigation Lease Area The McClane Canyon Mine is located within the mine lease area, and is currently being mined using room-and-pillar advance mining techniques to overburden depths of just over 1,500 feet. In some of the panels, pillars were robbed (mined) on retreat to maximize coal recovery. No observations have been reported of surface subsidence effects over the McClane Canyon Mine. Estimates of maximum subsidence, tensile and compressive strains, and maximum slope changes were made over the five selected panels and are indicated by number on Figure 7 in Appendix C. The predicted maximum surface subsidence for the five panels ranged from 1.52 feet to 2.56 feet (Table 1, Appendix C). The predicted tensile strains would result in estimated 1-inch to 2-inch wide tensile cracks at the ground surface (Table 2, Appendix C). The McClane Canyon Mine has extracted approximately 36 percent of Cameo Seam coal by advance room-and-pillar mining at approximately 225 feet of depth, apparently without any chimney collapse to the overlying ground surface. After a mine is closed progressive deterioration of the roof can result in chimney failures, which at shallow depths can and frequently do breach the ground surface. Areas where the overburden thickness is less than 200 feet above the Cameo Seam may exhibit subsidence at some point in the future. The additional impacts of mining on the lease area would be essentially the same as those described in the Mine and Facilities section. Mining the tract would result in the removal of the coal resource. In addition, any methane within the coal seam would be lost. Recoverability of any oil and gas resource present in the geologic formations below the coal seams would be reduced due to the limiting of drill pad locations. Total loss of the resource would not occur because of the possibility to directionally drill into the lower horizons. Railroad No appreciable impact to the geologic and mineral resource is anticipated. Water Pipeline No appreciable impact to the geologic and mineral resource is anticipated. Transmission Line No appreciable impact to the geologic and mineral resource is anticipated. Temporary and Permanent Impacts Geologic Hazards A landslide is a geologic hazard characterized by a perceptible downslope sliding or falling of a relatively dry mass of earth, rock, or a mixture of the two. Rockfalls are geologic hazards, as well, characterized by free falling rock masses. The degree of risk posed by landslides or rock- falls to proposed development is variable, ranging from low (very old, well drained, gentle slopes) to high (overhanging rocky cliffs with loose rock material on steep slopes and poorly consolidated surficial deposits). In most cases the risk of future movement can be reduced by appropriate design and construction practices (engineered excavation and grading) and by active mitigation techniques, such as: control of surface and subsurface drainage; rock tieback anchors, rock scaling, and buttressing. A large rockfall hazard area and a landslide have been identified within the bounds of the proposed Red Cliff Mine site and rail alignment. See Figures 4-9 and 4-10. 4-66 DBMS 621