2008-03-21_GENERAL DOCUMENTS - C1980007 (3)

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Structures identified in the PR-12 (Dry Fork lease) inventory are: U.S. Forest Service Roads 711 and 711.5, the Deep Creek Ditch, the Minnesota Creek Ditch Rider's cabin, trails, stock ponds, and MCC's hydrologic monitoring stations. The renewable resource lands identified in the PR-12 inventory are the water-bearing bedrock units in and adjacent to the Dry Fork lease area. b. Possible Subsidence Consequences and Mitigation of Impacts Possible subsidence consequences are described in Section 2.05.6(6)(b)(I) of the permit application. Additional information is contained in a report in Exhibit 55 titled "Subsidence Evaluation for the West Elk Mine". Information specific to the West Flatiron area can be found in Exhibit 60C, "Subsidence Evaluation and 2004 Geologic Hazard Field Observations, West Flatiron Lease Area." Information specific to the South of Divide and Dry Fork lease areas is in Exhibit 60E, "Subsidence Evaluation for the South of Divide and Dry Fork Mining Areas". MCC's predictions of possible subsidence are based on historical observation from past mining, conceptual analytical modeling (relation between extraction height and workings depth, adjusted for lithologic variation), and numerical modeling (computed influence function). MCC predicts the angle of draw for longwall mining in the E seam in the Dry Fork lease area will be 21 degrees. (The angle of draw is the angle between a vertical line at a panel edge and a line extending from the panel edge to the point of zero subsidence at the ground surface.) MCC predicts 95 percent of subsidence will have occurred at a location in the Dry Fork lease area when the longwall face has moved from the location a distance equal to 1.0 to 1.2 times the depth of mining. The depth to mining in the Dry Fork lease area will range between 800 and 1,400 feet, with maximum vertical displacement on the land surface of 7.0 feet. Maximum surface crack depth is predicted to occur in brittle sandstone ridges, as observed elsewhere in the permit area, with maximum crack depth of 50 feet. Maximum crack depth is predicted to be 5 to 15 feet on gently sloping land (<30%). Surface cracks are predicted to not occur where mining depth is is several hundred feet and alluvium is more than a few feet thick. Possible subsidence effects on ground water has been previously discussed in this Findings document under the heading "Probable Hydrologic Consequences". MCC predicts the mining nearest State Highway 133 (600 feet horizontal distance) will probably not re-activate existing landslide deposits in the area because the mining there consists of room-and pillar development entries which have a relatively small subsidence potential. The angle of draw of longwall mining activity does not intersect landslide bodies in the area. MCC monitors monuments it has installed on a landslide mass in the mine's surface facilities area. 47