Sub-surface mining

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Sub-surface mining or underground mining refers to a group of techniques used for the extraction of valuable minerals or other geological materials from the earth. In contrast to the other main type of excavation, surface mining, sub-surface mining requires equipment and/or manpower to operate under the surface of the earth.

In underground coal mines, another major environmental risk is fires. Hundreds of coal mines smolder in the United States, China, Russia, India, South Africa, and Europe. The inaccessibility and size of these fires many impossible to extinguish or control.

Methods

• Longwall mining : Longwall mining machines consist of multiple coal shearers mounted on a series of self-advancing hydraulic ceiling supports. The entire process is mechanized. Longwall mining machines are about 800 feet (240 meters) in width and 5 to 10 feet (1.5 to 3 meters) tall. Longwall miners extract "panels" - rectangular blocks of coal as wide as the mining machinery and as long as 12,000 feet (3,650 meters). Massive shearers cut coal from a wall face, which falls onto a conveyor belt for removal. As a longwall miner advances along a panel, the roof behind the miner's path is allowed to collapse.

• Room-and-pillar mining : Room and pillar mining is commonly done in flat or gently dipping bedded ores. Pillars are left in place in a regular pattern while the rooms are mined out. In many room and pillar mines, the pillars are taken out, starting at the farthest point from the mine haulage exit, retreating, and letting the roof come down upon the floor. Room and pillar methods are well adapted to mechanization, and are used in deposits such as coal, potash, phosphate, salt, oil, shale, and bedded uranium ores.