What is longwall mining

Longwall mining is a method whereby large rectangular blocks of coal (called panels) surrounded by a series of parallel tunnels (called entries) are mined in a continuous full extraction process. The panels are typically up to approximately 10,000 feet long and 1,100 feet wide. Nearly 100% of the coal in the panel is mined. The coal in the panel is mined using a “shearer” to cut the coal; the coal falls on a conveyor and is removed from the mine. A series of “shields” supports the roof immediately behind the shearer protecting the miners and the equipment. As the coal in the panel is removed, the strata behind the shields collapse filling the void. The collapsed roof is referred to as gob or gob area. As the strata collapses, its downward movement works its way to the surface causing the surface to fall. This movement caused by the mining that is manifested on the surface is referred to as subsidence. Under customary subsidence theory, subsidence manifests on the surface as a large trough whose areal extent and depth are determined by the mining height, the type and thickness of rock strata over the panel, the distance from the surface to the coal seam (referred to as the cover), and the geometry of the panel (length and width). In addition, the subsidence extends on the surface beyond the edges
of the panel by an angular amount referred to as the “angle of draw.”