Underground Mining Methods

There are variety of different underground mining methods, all having their own benefits and frustrations.

     Drift mining is an underground method that attempts to collect coal by intersecting or mining a coal seam in a nearly horizontal shaft. The coal can then be transported out from the ground by conveyor belt to be prepared and shipped out. This method is generally used when the coal is underneath a hill, but still above the adjacent ground level and is both cheaper and safer than most other underground methods, and uses less energy in transferring coal outside of the mine. Drift mining also risks occasional flooding, however, and can't be used in every circumstance.

     Shaft mining is another method used when the coal seem is well below ground level and excavates coal in a vertical manner. Shafts and elavators are used to transport both coal and the miner from the underground seam to the surface and vice versa, where the seams are commonly below 1,000 feet underground. When the ore is dynamited and broken into chunks, it is then put on a type of pulley system to be loaded into trucks. While this method of mining is much faster and conventional, frequently with better air quality, it is also more expensive than other methods, and historically the most dangerous. 

     Room and pillar mining is a system in which the mined material is extracted across a horizontal plane making an array of rooms and pillars, where the pillars are large blocks of the extracted material. The pillars can "squeeze" and compact, making roof-falls a constant danger. It is much faster than other methods, but it also yields fewer results, with much less coal being extracted. After the rooms have been excavated, the pillars are then mined partially, leaving the mine to collapse in on itself.

     In continuous mining, machines can be used to help excavate coal (frequently used in drift and room and pillar mining). One miner can operate a continuous miner to mine 5 tons of coal per minute. These machines have been use in varying frequencies since the 1940's, and account for about 45% of the product of current mining. Continuous miners are extremely inexpensive and safe when compared to other mining techniques, but do not always provide adequate roof support and eliminate some jobs previously given to miners. These machines are slowly becoming the prefered method of mining. 

Longwall mining also uses machines and is highly efficient. Huge mining machines support the roof with hydraulics as the cutter removes the coal. Again, when the coal is removed, the machines retreat and allow the cavern to collapse. This collapse can often impact the geological features above the air, such as rivers. Still, the method remains safer, provides better roof support, and extracts more coal than any other method. 





Primary sources:
http://en.wikipedia.org/
http://www.ritchiewiki.com/wiki/index.php/Underground_mining