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Coal Research and Development: to Support National Energy Policy
outside the mining area for placement and storage. In the Midwest, where the surface topography and coal seams are generally flat, it is common to employ area strip mining in which the fragmented overburden is placed directly by large draglines in the space created where coal has been mined (Figure E.1). In some situations in the eastern United States, a coal seam occurring near the top of mountains is exposed by removing the top of the mountain (Figure 4.3) and transporting the fragmented overburden to a nearby valley.
Underground mining is usually by the room-and-pillar mining or longwall mining method (Figure E.2). Even in mines where the longwall method is the principal extraction method, the development of the mine and the longwall panels is accomplished by room-and-pillar continuous mining. The thickness of the coal seam, the depth and inclination of the coal seam, the nature of roof and floor strata, and the amount of gas contained both in the coal seam and the roof and floor strata are all important for selection of the mining method. Mining difficulties are greatly increased if seams are extremely thick or thin or are steeply inclined. Longwall mining additionally requires large coal reserves to justify the capital cost of longwall equipment.
As surface mining in the Powder River and Rocky Mountain Basins proceeds, it is likely that the stripping ratios (overburden to coal) will exceed an economic limit. If this coal is to be mined at reasonably high recovery rates, it
FIGURE E.1 Schematic depiction of the unit operations in a surface coal mine. SOURCE: Royal Utilities.