Coal extraction in West Virginia is accomplished generally by underground or surface mining. Both methods of coal extraction require the placement of fill structures, commonly referred to as “valley fills”, acknowledging the steeply-sloped terrain that exists in West Virginia, within CWA section 404 jurisdictional waters. Construction of these valley fills necessitates section 404 authorization from the Corps of Engineers (Corps).
A.1 UNDERGROUND Coal Mining
Underground mining techniques, which account for the majority of West Virginia’s coal production, utilize machinery to drive tunnels under the surface of the land. These tunnels are situated in coal seams so as the machinery advances further underground, coal is removed. Using a combined system of vehicle haulage and conveyor belts, the coal extracted from the seam is transported to the surface. Underground mining is practical from engineering and safety standpoint for coal seams that are located more than 100 feet below the surface of the land. Underground mining in shallower coal reserves will often encounter mine roof integrity problems as the geology and overburden at such shallow depths is less consolidated and more susceptible to cracking and failure (roof falls) than the strata above deeper coal seams.
Underground mines are usually characterized by the way the coal seam is accessed from the surface and the underground extraction method used to mine the coal. The method for accessing a coal seam for underground extraction is largely dependent on its vertical position relative to the surface. The three methods of underground mine access are summarized in Figure A.1.
Figure A-1.
Methods of Entry to Underground Coal Mines
An individual underground mine may have more than one of these access methods, depending on the characteristics of the individual coal seam, the coal haulage system used at the mine, the ventilation, roof control and safety plans for the mine and the mine’s supply and employee transport considerations.
Underground mining coal extraction methods can generally be divided into two categories: room and pillar mines and longwall mines. The type of extraction practiced is dictated by the characteristics of the coal seam and its related geology. Generally, longwall mining is restricted to seams with uniform geology and coal seam thickness while room and pillar mining is more adaptable to seams with varying thickness, uniformity and geologic settings.
A.1.a. ROOM and pillar underground Mining
Room and pillar mining is defined by the fact that portions of the coal seam being extracted are left in place to support the roof of the mine. Room and pillar mines are developed by using machinery to extract the coal seam in a grid like pattern through the development of a parallel series of extraction areas referred to as entries. The entries are connected together by extracting the coal perpendicular to the parallel entries in crosscuts. To support the coal extraction areas in the entries and crosscuts, roof support is installed by using machines to install long “bolts” into the roof of the mining area to consolidate and “tighten” the overlying rock strata. Referred to as “roof bolting”, this is the central point of a mine’s roof control plan that is developed by safety regulatory agencies and the operator to assure a maximum amount of safety by minimizing the possible occurrence of roof fractures and slips or roof falls. Figure A-2 provides an illustration of typical room and pillar mine in Appalachia.
Figure A-2.
Room and Pillar Mining
Coal extracted from the entry and crosscut areas or “working faces” of the room and pillar mine is loaded onto special underground rubber tired haulage vehicles called shuttle cars. The shuttle cars move the coal from the working face to a centrally-located conveyor belt loading point where the coal is unloaded and transported to the surface for processing at the coal preparation plant. Some mines use shuttle cars to move the extracted coal to a loading point where it is transferred to narrow gauge railroad cars for movement to the surface. In slope and shaft mines there may also be a third transfer point where the coal from the working face is transferred again from the conveyor belt or rail car to an elevator-like haulage system where the coal is lifted vertically to the preparation plant.
The initial development of a room and pillar mine, where entries and crosscuts are driven in the coal seam is referred to as primary extraction. Mines using primary extraction only can usually recover 40 to 60 percent of the coal in a given seam and reserve. Depending on the characteristics of the coal reserve, its overlying strata or roof and its underlying strata or floor, secondary production may be possible in room and pillar mines. In secondary production, some of the coal pillars that were established by the entries and crosscuts and were left in place for added roof control are removed. Extraction of the coal in the pillars, if possible at all, is governed by roof control and safety conditions as extraction of the pillars usually results in some level of roof collapse. Where it can be practiced secondary extraction is designed to remove the coal resource in the pillars in a calculated and designed fashion to allow for controlled roof collapse that will not endanger the safety of coal miners or compromise the overall geologic integrity of the entire mine. Where it is possible, secondary extraction can allow for recovery of 70 to 80 percent of the coal in a given coal seam.
A.1.b. LONGWALL underground Mining
The other major form of underground coal extraction is longwall mining. In longwall mines, equipment used in Room and Pillar mining is used to develop two or three parallel entries in a coal seam. The longwall mine entries or headings are usually separated by several hundred feet of solid coal. At a certain distance advancing into the coal seam (depending on mine-specific geology), the headings are connected by driving a crosscuts, or in longwall mines, cross headings to connect the entries to form a block of solid coal called a panel. Specialized equipment, known as longwall machines, that are usually custom-built for specific mines then begin extracting the coal along the face of the panel. These machines include a shear or plow device mounted on a track that moves back and forth across the panel cutting the coal with each pass of the machine, referred to as the cutting head. As the cutting head extracts the panel of coal, the longwall machine advances further into the coal seam extracting more of the seam as it advances through successive passes across the panel, allowing for recovery of up to 85 percent of a given coal seam and reserve. The cutting head is attached to a special type of armored conveyor that gathers the coal as it is cut by the head and moves it to a loading point where it is transferred to the main haulage system for movement to the surface and the coal preparation plant. Both the cutting head and the special conveyor are protected by a special form of hydraulic roof support devices known as shields. The shields support the roof immediately above the longwall machine and are designed to advance as the cutting head moves deeper into the coal reserve keeping the roof support parallel to coal face. Figure A-3 is a photograph of a longwall shear and Figure A-4 provides an illustration of a Longwall mining system.
Figure A-3
Longwall Mining Machine with Shear Cutting Head
Figure A-4
Longwall Mining Illustration
A.2 SURFACE Coal Mining
Surface coal mining techniques, which account for the majority of national coal production and roughly 42 percent of West Virginia’s coal production, involve the mechanical removal of overlying native earth and rock or “overburden” to reach underlying coal seams. Surface mining can be further categorized into three basic operational methods: contour mining, area mining and mountaintop removal mining. All three forms of surface mining are generally referred to as “mountaintop mining”. Secondary extraction methods can also be employed with surface mining, usually through a process known as highwall mining. While surface mining includes a variety of methods, certain criteria used to determine the exact operation, layout and the extent and form of coal extraction are applicable to all forms of surface mining.
Included in the criteria that are transferable to all forms of surface mining are mining and reclamation considerations. Before any permit is issued under SMCRA or a state delegated program, the operator must submit a specific mining and reclamation plan for review and approval by the regulatory authority. The components of the mining and reclamation plan include a variety of issues such as initial mine development, final mine design, backfilling and regrading, achieving Approximate Original Contour or “AOC” (explained in greater detail in subsequent sections) and erosion and sediment control. Reclamation considerations, along with stripping ratios (explained in subsequent paragraph), are usually the dominant factors that dictate the form of surface mining that is used to extract a coal reserve.
In addition to permitting considerations, determining the extent to which a coal seam or block of coal seams is economically feasible for mining is an integral part of determining the method of surface mining used to extract the reserve. The typical method of assessing surface mining economics for a coal seam is a calculation of overburden (native rock and soil above a coal seam) moved per clean ton of coal produced, commonly referred to as “mining ratio”. The higher the mining ratio (or more overburden moved per clean coal ton), the higher the cost of producing coal. In situations where a given coal reserve includes more than one coal seam, the recoverable coal volume from the multiple seams is factored together to lower the overall mining ratio for the entire coal reserve. Because mining more than one seam lowers the mining ratio for a mining project and increases its economic potential, it allows for extraction of coal seams that would be uneconomic to recover by themselves.
The economic viability of a surface mining project, calculated through the mining ratio, is highly mine and coal reserve dependent. Site-specific factors such as overburden type and depth, excavation costs, coal value and overburden haulage distances and costs are all considered in the development of stripping ratios for a particular mine site and coal reserve.
The ability to construct valley fills is paramount to the economic and physical viability of a surface mining project. As explained in more detail in the following sections, all surface mining methods need valley fills for placement of excess overburden. Without the ability to construct fills, surface mining becomes physically impossible in the steep terrain of Appalachia and West Virginia.
A.2.a. 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.
Figure A-5
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.
Figure A-6
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.
West Virginia Coal Mining Methods - test
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