Strip mining is a type of surface mining that exposes coal by removing earth above each coal seam. The long strips of earth removed is referred to as “overburden.” These strips are deposited either outside the planned mining area, known as out-of-pit dumping, or in empty areas left from mining the coal and removing other overburden. This second method is known as in-pit dumping. It is often necessary to fragment the overburden with explosives before carting it away. Once the overburden is removed, the coal seam is exposed and can be mined. However, equipment to be used depends on geological conditions since some overburden may be looser than at other locations. (Coal mining, n.d.; U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement, 1987)

The surface mining method called contour mining consists of removing overburden from the seam in a pattern following the contours along a ridge or around the hillside. This method is most commonly used in areas with rolling to steep terrain. It was once common to deposit the spoil on the downslope side of the bench thus created, but this method of spoil disposal consumed much additional land and created severe landslide and erosion problems. To alleviate this issue, other techniques are now used, namely refilling mined-out areas and leaving a barrier of natural material. The limitations of contour mining are both economic and technical. When the operation reaches a certain quota, it is no longer profitable to continue. Additionally, depending on the equipment used, it may not be technically feasible to exceed a certain height of highwall. At this point, mining coal via the augering method is more economical. (Coal mining, n.d.)

Mountaintop coal mining is a surface mining practice involving removal of mountaintops to expose coal seams, and disposing of associated mining overburden in adjacent “valley fills.” Valley fills occur in steep terrain where there are limited disposal alternatives.

Mountaintop removal combines area and contour strip mining methods. In areas with rolling or steep terrain with a coal seam occurring near the top of a ridge or hill, the entire top is removed in a series of parallel cuts. Overburden is deposited in nearby valleys and hollows. This method usually leaves the ridge and hilltops as flattened plateaus. The process is highly controversial for the drastic changes in topography, the practice of creating head-of-hollow-fills, or filling in valleys with mining debris, and for covering streams and disrupting ecosystems. (Coal mining, n.d.; U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement, 1987)

Longwall mining accounts for about 50 percent of underground coal production. The longwall shearer has a face of 1,000 feet (300 m) or more. It is a sophisticated machine with a rotating drum that moves mechanically back and forth across a wide coal seam. The loosened coal falls onto an armored chain conveyor or pan line that takes the coal to the conveyor belt for removal from the work area. Longwall systems have their own hydraulic roof supports which advance with the machine as mining progresses. As the longwall mining equipment moves forward, overlying rock that is no longer supported by coal is allowed to fall behind the operation in a controlled manner. The supports make possible high levels of production and safety. Sensors detect how much coal remains in the seam while robotic controls enhance efficiency. Longwall systems allow a 60-to-100 percent coal recovery rate when surrounding geology allows their use. Once the coal is removed, usually 75 percent of the section, the roof is allowed to collapse in a safe manner. (Coal mining, n.d.; Coal Mining. World of Coal, 2009)

Image Credit: Eickhoff, 2009, Retrieved from http://www.eickhoff-bochum.de/de/

Continuous mining is an underground method that utilizes a Continuous Miner Machine with a large rotating steel drum equipped with tungsten carbide picks that scrape coal from the seam. Operating in a “room and pillar” (also known as “board and pillar”) system—where the mine is divided into a series of 20-to-30-foot (5–10 m) “rooms” or work areas cut into the coal bed—it can mine as much as 14 tons of coal a minute, more than a non-mechanized mine of the 1920s would produce in an entire day. Continuous miners account for about 45 percent of underground coal production. Conveyors transport the removed coal from the seam. Remote-controlled continuous miners are used to work in a variety of difficult seams and conditions, and robotic versions controlled by computers are becoming increasingly common. (Coal mining, n.d.)

Room and pillar mining consists of coal deposits that are mined by cutting a network of rooms into the coal seam underground. Pillars of coal are left behind in order to keep up the roof. The pillars can make up to forty percent of the total coal in the seam, however where there was space to leave head and floor coal there is evidence from recent open cast excavations that 18th-century operators used a variety of room and pillar techniques to remove 92 percent of the in situ coal. However, this can be extracted at a later stage, known as “retreat mining.” (Coal mining, n.d.; Coal Mining. World of Coal, 2009)

Shortwall mining, a method currently accounting for less than 1 percent of deep underground coal production, involves the use of a continuous mining machine with movable roof supports, similar to longwall. The continuous miner shears coal panels 150 to 200 feet (45 to 60 meters) wide and more than a half-mile (1 km) long, having regard to factors such as geological strata. (Coal mining, n.d.)