Underground Mining Methods

There are several common methods of underground coal mining. Among these are drift mining, shaft mining, room & pillar mining, continuous mining, and longwall mining. Some of these are used in combination with each other--for example, shaft mining refers to the way coal is transported, while room & pillar refers to the way it is mined, so they may both be used together in the same seam of coal.

The first among these, drift mining, refers to the way the seam of coal is entered. It is used when the coal seam intersects the earth's surface. The mining area follows the coal horizontally from the surface as it extends outward. This method is advantageous in situations where the coal seam is on the side of a hill and accessible by this method, but is obviously untenable in anything other than that specific situation.



Shaft mining is used with coal deposits which rest deep underground without intersecting the surface. Vertical shafts enter the seam of coal and elevators take miners down and coal up. These mines are commonly deeper than 1000 ft below the surface. One of their primary disadvantages is that the heavy coal must travel against gravity up the elevators to reach the surface; another is that the coal must be blown up underground to put it in pieces small enough to be transported, a potentially dangerous practice. However, it's commonly practiced as it is by far the easiest way to reach seams of coal which are far underground.

Room and pillar mining is a method combined with both drift and shaft mining, as it refers not to the way the mine is entered but to the way coal is extracted. In room and pillar mining, the roof of the mine area is supported by pillars of coal left in the mine. This is a wildly inefficient method, leaving nearly half the coal behind in the mine. It's also dangerous, because soft bituminous coal can compress under the weight of the roof, putting extra stress on other pillars and leading to roof falls. Even with bolting in the roof for support, it's a dangerous method. It is, however, the most convenient way to mine a seam of coal, being quick and easy if not very financially sound.

Continuous mining refers less to a technique of mining and more to the technology used with it: continuous mining machines are run and can mine up to 5 tons per minute. These machines have been in use various places since the 1940s and are generally used with conveyor systems to transport the coal. This is an extremely fast method of mining; however, the machines are expensive and difficult to upkeep.

The final form of mining discussed here today is longwall mining. It's extremely efficient: machines mine a seam while supporting its roof with hydraulics, and after the coal is extracted the seam is allowed to collapse behind the retreating machine. Because no support pillars are necessary, much more coal is extracted than in room and pillar mining. It also resolves the problem of filling in mines after they've been used up by causing a cave-in. However, it suffers from the same machine cost problem as continuous mining.




 Sources: http://en.wikipedia.org/wiki/Drift_mining http://library.thinkquest.org/05aug/00461/shaft.htm