|Surface vs Underground Coal Mining|
The decision to develop a coal reserve using surface or underground mining is governed by a combination of several geologic, safety, economic, physical, regulatory and engineering considerations. For example, if a coal reserve is located very deep in the geologic column, surface mining is simply not a viable recovery method. These coal seams, which are typically accessed using slope or shaft entries (see Figure A-1), can only be recovered using underground mining methods. The deeper the coal reserve, the less likely that it can be recovered using surface mining methods. Coal seams that are “thin” or less than three feet thick cannot be recovered using underground mining methods regardless of the seam’s location in the geologic column (see Figure A-5). However, the same seams, if located close enough to the surface can be recovered using surface mining methods. Conversely, coal seams that are located closer to the surface of the land and outcrop along high ridges may be thick enough for underground mining extraction but fail roof control considerations as they are too close to the surface for effective roof control of unconsolidated strata.
Figure A-5 is a cross section diagram of the coal seams typically extracted using surface mining methods in West Virginia. The heights of the various coal seams are shown on the diagram clearly indicating that most of the seams are too thin for extraction by underground coal mining.
Cross Section of Coal Seams Extracted Using Surface Mining Methods
Another consideration that generally governs the selection of the underground or surface mining methods to extract a given coal seam and reserve is the quality of the coal seam. Very few coal seams in Appalachia and West Virginia are comprised entirely of coal. Most of the recoverable coal seams in this region have small seams of non-coal materials such as shale and clay that are imbedded within, above or below the coal seam. The presence of these non-coal materials in a coal seam affects the coal’s heating values, air emission properties and other factors that generally influence the usability of the coal. For example, the presence of non-coal impurities will affect a coal’s ability to meet air emission standards imposed on electric utilities and could make the coal unusable. Figure A-6 is an illustration of a typical seam in Appalachia showing the inherent non-coal impurities that exist within the coal seam. In underground mining, all of this material would be extracted along with the coal.
Illustration of Typical Appalachian Coal Seam Showing Non-Coal Impurities Within the Coal
In surface mining extraction, these non-coal impurities can be separated “in the pit” or separated from the coal by the surface mining equipment, negating the need for a separate coal processing plant. In underground mining methods, it is impossible to separate the shale, clay and other non-coal partings from the coal seam as the mining machinery removes the entire coal seam- partings included- to maintain a workable height for miners and sometimes to achieve maximum roof stability. This “raw coal” must be cleaned on the surface at the coal processing plant. The need to clean virtually all underground mined coal at a coal processing plant adds considerably to the cost of underground mining.