A form of underground coal mining making use of a long wall. The longwall consists of a number of hydraulic jacks, called chocks, which are placed in a long line in order to support the roof. An individual chock can extend to a maximum height of 4.5m in some designs. A shearer moves along in front of the chocks, disagregating the coal. The coal is then removed from the coal face by the chain or AFC (articulated face conveyor) to the main gate. Here it is loaded onto a conveyor belt and transported to the surface.
As the shearer removes the coal faces, the chocks move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the cavity behind the longwall, termed goaf, increases. This goaf eventually collapses under the weight of the overlying strata.
