What is longwall mining?
One reason longwall mining succeeds in being the scourge of southwestern Pennsylvania is that the general public doesn’t have a clear notion of what longwall mining is.
If you visualize a long wall, you probably have the wrong idea.
Longwall is a form of underground mining that differs from traditional “room and pillar” mining in that no pillars or posts are left behind after the coal has been extracted. A longwall mining machine cuts along a 1000 foot-long coal face, going back and forth along the face, taking out all the coal.
The machine has a protective shield that keeps the roof from falling on it. But as the machine advances, the roof – a layer of rock, frequently slate, sometimes sandstone or limestone – subsides more or less instantly. The machine will cut a swath 1000 feet wide and a mile or more long. As the rock immediately above the coal seam subsides, so do successive layers above that layer, until soon the subsidence reaches the surface, and the surface and contents subside. When an eight foot thick seam is mined, surface will subside from one foot to five or six feet.