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December 9. 2003 Page 2-8 2.9 Reasonably Foreseeable Development Scenario In order to effectively assess the potential surface impacts of subsidence, a RFDS for the West Flatiron LBA tract must be assumed. This RFDS is a conceptual mine plan which projects the mining configuration most likely to occur on the tract. MCC foresees mining in the West Flatiron LBA tract by extending the longwall panels currently planned in the adjacent lease to the west. MCC foresees portions of three longwall panels extending into the lease tract. The panels are oriented on anorthwest-southeast trend, and would extend up to 2,500 feet into the tract, generally in the west half of Section 13. The bleeder entries and start up room for an additional two panels would extend into the LBA tract in the extreme southwest corner of Section 13, and the extreme northwest corner of Section 24. This mine plan is for reserves in the B coal seam. No mine plans for other seams are currently proposed. Subsidence does occur in areas above longwall mining. The amount of subsidence above longwall mining depends on many factors including mine plans, coal thickness, geologic strata and overburden depth. As a general rule, with greater overburden thickness, there is less potential for subsidence to affect the surface. The overburden (the amount of rock separating the surface from the coal seam being mined) in the West Flatiron LBA tract ranges from 500 to 2,100 feet. The 500-foot overburden contour occurs beneath Raven Creek in the northeast corner of the tract, and increases dramatically to the west and south, reaching 2,100 on the western tract boundary, and 1,800 feet on the southern tract boundary. Figure 2-2, Overburden Isopach Map, shows the overburden contours of the West Flatiron LBA tract. Subsidence at any given point on the surface begins when the longwall face (i.e., active mining) is beneath that point. Subsidence is generally 90 percent complete when the longwall face has proceeded a distance (usually 1.2 to 1.4 times the depth) past the point of mining. For example, at 500-foot depth of overburden, the subsidence due to longwall mining would be 90 percent complete when the longwall face is 600 to 700 feet beyond that point on the surface (usually a month from a time standpoint). Ground movement associated with longwall mining is generally complete within a year after passage of the longwall face. The amount of subsidence in the West Flatiron LBA tract would depend on the extent of the coal extraction and on overburden depth. The subsidence should be relatively uniform across the lease tract; however, on the fringes of the subsidence area, some tension cracks may be visible but most should heal with time. Some cracks, especially in bedrock, may never heal. Assuming a mining height of twelve feet in the LBA tract, it can be expected that the land surface would lower between six and eight feet. In addition, subsidence causes changes in the rock strata after the coal is removed and the overlying rocks move to fill the mine void. Immediately above the mine, the rock rubblizes to fill the mine void, creating what is known as the caving zone. This caving zone can extend vertically above the mine a distance of eight times the mining height, in this case about 100 feet. Above the caving zone is an area known as the fracture zone, where the rock fractures. This zone can extend vertically from the mine level up to 30 times the mining height, or 360 feet in this case. Enaircmment~ I Assessmcnl V~'e51 t-!atiron LEF Tracfi Gunr-iscn County, Ca(erado