COAL: Ancient Gift Serving Modern Man
American Coal Foundation
How Coal Is Produced
Coal found close to the surface can be uncovered and removed by large machines in a process that is called surface mining. Surface mining techniques account for 60 percent of coal produced in the United States -- 75 percent in Western states, where some deposits are up to 100 feet thick.
Only recently has surface mining played an important role in the U.S. coal industry. The development and use of large power equipment provided the impetus that moved surface mining into prominence, and during the 1970s it became the leading method of coal mining.
Today's surface mines are large, intensively engineered, and highly efficient mechanized operations. When an area is to be mined, topsoil and subsoil are removed first and set aside to be used later in reclaiming the land. Then specially designed machines -- draglines, wheel excavators, or large shovels -- remove the rock and other material, call overburden, to expose the bed of coal. Smaller shovels load the coal into large trucks that remove the coal from the pit.
Once the coal is removed, the area is reclaimed. First the overburden and then the soils are replaced and the area is restored as nearly as possible to its original contour. Vegetation currently suitable to the area is planted to anchor the soil and return the land to a natural, productive state. Reclaimed lands are a valuable resource that can support farm crops, provide new wildlife habitats, enhance recreational opportunities, and even serve as sites for commercial development.
The complete mining operation is scheduled so that as one area is being mined, another is being reclaimed where the coal was removed. Thus, even at the largest surface mines only a relatively small area is disturbed by active mining at any one time. Since 1977 an estimated 2 million acres of coal mine lands have been reclaimed in this manner.
Underground mining methods are used where the coal seam is too deep or the land too hilly for surface mining. Most underground mining takes place east of the Mississippi, especially in the Appalachian mountain states. Coal production was once dominated by underground mining methods, but the growth of coal mining in the West changed that. Now, only 40 percent of our coal comes from underground mines.
Underground mines differ according to how the coal seam is situated with respect to the surface. If the coal deposit outcrops (appears at the surface) on a hillside, a drift mine can be driven horizontally into the seam. Where the bed of coal is relatively close to the surface, yet too deep to be recovered by surface mining, a slope mine can be constructed, with the mine shaft slanting down from the surface to the coal seam. The most common type is the shaft mine. To reach the coal, which may be as deep as 2,000 feet, vertical shafts are cut through the overburden to the coal bed, which is excavated by machines.
In deep mines, the seam is mined in carefully engineered patterns that keep as much as half of the coal in place to help support the roof of the active mining area. This "room and pillar" method requires that large columns of coal remain between mined-out areas, or rooms, which are created when the coal is mined, either by continuous mining machines or conventional methods.
The largest amount of coal taken from underground mines is produced using continuous miners. This machines has a large, rotating, drum-shaped cutting head studded with carbide-tipped teeth that break up the seam of coal. Large gathering arms on the machines scoop the coal directly onto a built-in conveyor for loading into waiting shuttle cars.
In conventional mining, a machine resembling an oversized chain saw cuts into the coal. This gives the coal an area to expand into during blasting. Holes are drilled for explosives, which blast loose large chunks of coal. Machines called loaders scoop the coal onto conveyors which dump it into shuttle cars that haul the coal out through the shaft. This traditional method of mining accounts for about 11 percent of total production.
In both continuous and conventional mining, the roof over the mined-out area is supported for safety. The most important development in roof support -- both in terms of safety and cost -- has been the "roof bolt." Roof bolts are long rods driven into the roof to bind several layers of weak strata into a layer strong enough to support its own weight. Roof bolts also can anchor a weak immediate roof to a strong, firm structure above. Machines are used to drill holes, position the bolts and tighten them.
An increasingly popular and more efficient means of underground mining -- introduced from Europe in the early 1950s -- is longwall mining technology. Longwall today accounts for about one-third of total underground coal tonnage. In a continuous, smooth motion, a rotating shear on the mining machines moves back and forth along the face -- or wall -- of a block of coal, cutting the coal, which drops onto a conveyor and is removed from the mine. The block of coal being mined is several hundred feet wide, thus the name longwall.
Where longwall mining machines are used, room-and-pillar arrangements are not created throughout the entire mine (although pillars of coal are left to support the roof in haulage ways used by people and machines moving about the mine). The longwall miner itself has a hydraulically operated steel canopy which holds up the roof and protects miners working at the face. As the miner cuts progressively deeper into the block of coal, the shield advances with it, allowing the unsupported roof in the mined-out area behind it to collapse in a controlled and safe manner.