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

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CHAPTERFOUR Environmental Consequences and Mitigation There is a potential that mining subsidence could aggravate existing landslides and other geologic hazards. Mining-induced seismic events as a result of mining would likely occur. Based on existing information, these events are not expected to cause damage to surface resources or overlying structures. However, this is difficult to quantify given the existing data and research available. Impacts described subsequently are for all action alternatives. The assessment of impacts from subsidence is summarized from a comprehensive assessment included as Appendix C of this EIS. Subsidence The effects of subsidence on the surface of the landscape can take several forms. Chimney caving can cause sinkholes and troughs to open up. Both cracks and ridges can form due to tension and compression strains. Subsidence trough-like depressions occur directly above and somewhat outside the panel where the coal is being extracted. On steep slopes and cliffs, subsidence may result in landslides and rockfalls. Slope change or tilt can occur on steep slopes. The time that it takes for surface manifestations to occur can be almost immediate up to over 50 years. It is extremely difficult to quantify the impact of geology on the extraction of coal and the resulting subsidence of the ground surface. There are some obvious generalities that can be stated with complete confidence, but predicting what will happen and where is fraught with risk. The overall geology of the coal bearing Mesaverde Group is generally known, but the site- specific geologic conditions are not fully understood because it is possible to see only outcrops and the immediate roof and floor. The coal seam and the overburden lithology are changing conditions. Differing lithologies (rock types) have differing strengths; e.g., stronger sandstones and weaker shales and mudstones. Because of the rugged terrain in the project area, subsidence- related surface impacts may change several times as the overburden depth changes along the roughly 7,300-foot to 13,500-foot lengths of the longwall panels. Subsidence, strain, and tilt predictions will be less certain than would be the case in more gentle and flatter terrain. For instance, the potentially additive subsidence on ridges will increase the tensile strain and the width of open surface cracking. However, higher compression ridges, but negligible tensile fractures, are likely to occur in narrow valley bottoms, because the overburden on both sides will try to move toward the bottom of the valley as the subsidence trough approaches and then passes the valley bottom. Consequently subsidence impacts are likely to be greater on narrow ridges and lesser in narrow valley bottoms than they would be in more subdued terrain. Strains and displacements on steep slopes with thin alluvial cover, particularly cliffs, may cause surface fractures on the order of several inches to more than 2 feet wide and possibly 25 feet deep, compared to a fraction of an inch to a few inches wide and a few feet deep in valley bottoms at the same overburden depth. When the relief is subdued and terrain gentle, the surface fractures will be consistent in width and depth and generally follow a smoothed ovaloid around the panel perimeter. Cracks will tend to be widest (approaching 20 inches) and deepest (possibly 50 feet) along prominent joints and fractures on the steepest slopes and cliffs, which in turn, may become less stable and more susceptible to landslides and rockfalls. Landslides and rockfalls will be most likely to occur where mining approaches the outcrop, and the overburden depth is decreasing. It should be anticipated that longwall mining under the canyon walls will present a similar hazard for rock to roll out from undermined sandstone outcrops. The slopes of the canyon walls are certainly steep enough within the Red Cliff Mine project area to result in thin 4-71 DBMS 626