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Longwall mining is a form of underground coal mining where a long wall of coal is mined in a single slice (typically 1-2 m thick). The longwall "panel" (the block of coal that is being mined) is typically 3-4 km long and 250-400 m wide. Longwall mining produced 176 million tons of coal in 2007, about 15 percent of total U.S. production. Longwall mines are located in Appalachia as well as and in Illinois, Utah, Colorado, Montana, and New Mexico.
Southwestern Pennsylvania has six of the United States’ top 25 longwall mines, underlying 138,743 acres of rural terrain, or 15 percent of the surface area. The remaining 19 U.S. mines are scattered among West Virginia, Ohio, Virginia, Kentucky, Indiana, and western states.
- 1 History
- 2 Modern Methods
- 3 Effects
- 4 How much land is disturbed by longwall mining?
- 5 Global longwall production
- 6 Longwall mining in Illinois
- 7 Longwall mining in Pennsylvania
- 8 Citizen groups working on longwall
- 9 Resources
The basic idea of longwall mining was developed in England in the late 17th century. Miners would undercut the coal along the width of the coal face, removing coal as it fell, and using wooden props to control the fall of the roof behind the face. this was known as the Shropshire method of mining. While the technology has changed considerably, the basic idea remains the same, to remove essentially all of the coal from a broad coal face and allow the roof and overlying rock to collapse into the void behind, while maintaining a safe working space along the face for the miners.
Starting around 1900, mechanization was applied to this method. By 1940, some referred to longwall mining as "the conveyor method" of mining, after the most prominent piece of machinery involved. Unlike earlier longwall mining, the use of a conveyor belt parallel to the coal face forced the face to be developed along a straight line. The only other machinery used were an electric cutter to undercut the coal face and electric drills for blasting to drop the face. Once dropped, manual labor was used to load coal onto the conveyor parallel to the face and to place wooden roof props to control the fall of the roof.
Such low-technology longwall mines continued in operation into the 1970s. The best known example of this was the New Gladstone Mine near Centerville, Iowa. This longwall mine did not even use a conveyor belt, but relied on ponies to haul coal tubs from the face to the slope mining where a hoist hauled the tubs to the surface.
Longwall mining has been extensively used as the final stage in mining old room and pillar mines. Room and pillar is a mining system in which the mined material is extracted across a horizontal plane, while "pillars" of untouched material is left to support the overburden, leaving open areas or "rooms" underground. It is usually used for relatively flat-lying deposits, such as those that follow a particular stratum. When used in this context, longwall mining can be classified as a form of retreat mining, a term used to reference the final phase of an underground mining.
The gate road along one side of the block is called the maingate or headgate; the road on the other side is called the tailgate. Where the thickness of the coal allows, these gate roads have been previously developed by continuous miner units, as the longwall itself is not capable of the initial development. In thinner seams the advancing longwall mining method may be used. In this system the gate roads are formed as the coal face advances. Only the maingate road is formed in advance of the face. The tailgate road is formed behind the coal face by removing the stone above coal height to form a roadway that is high enough to travel in. The end of the block that includes the longwall equipment is called the face. The other end of the block is usually one of the main travel roads of the mine. The cavity behind the longwall is called the goaf, goff, or gob.
Fresh air travels up the main gate, across the face, and then down the tail gat