Contour mining occurs on hilly or mountainous terrain, where workers use excavation equipment to cut into the hillside along its contour to remove the overlying rock and then mine the coal. The depth to which workers must cut into the hillside depends on factors such as hill slope and coal bed thickness.

E. Auger Mining

Auger mining is frequently employed in open-pit mines where the thickness of the overburden is too great for open-pit mining to be cost-effective [26]. Open-pit mining would require the lengthy and costly removal of the overburden, whereas auger mining is more efficient because it cuts through the overburden and removes the coal as it drills. In this technique, the miners drill a series of horizontal holes into the coal bed with a large auger (drill) powered by a diesel or gasoline engine [27]. These augers are typically about 60 m (200 ft) long and 0.6 to 2.1 m (2 to 7 ft) in diameter. As these enormous drills bore into the coal seam, they discharge coal like a wood drill producing wood shavings. Additional auger lengths are added as the cutting head of the auger penetrates farther into the coal. Penetration continues until the cutting head drifts into the top or bottom of the coal seam, into a previous hole, or until the maximum torque [28] (energy required to twist an object) of the auger is reached.

F. Satellite Aids [29] to Surface Mining

In the late 1990s some coal mining enterprises used technologies such as the global positioning system (GPS) [30] to help guide the positioning of mining equipment. Satellites operated by the United States Air Force Space Command and leased to companies for commercial use track the position of mining equipment against a map of a mine��s topography [31]. This map uses colors to distinguish soil that should be excavated, soil that should remain in place, and areas that should be filled in. The equipment driver observes this visual information [32] on a monitor [33] while operating the equipment. Some coal mining enterprises have used GPS to increase mining efficiency up to 30 percent.

5��2 Underground Mining

Underground, or deep, mining occurs when coal is extracted from a seam without removal of the overlying strata. Miners build a shaft mine that enters the earth through a vertical opening and descends from the surface to the coal seam. In the mine, the coal is extracted from the seam by various methods, including conventional mining [34], continuous mining [35], longwall mining [36], and room-and-pillar mining [37].

A. Conventional Mining

Conventional mining, also called cyclic mining, involves a sequence of operations that proceed in the following order: a. supporting the roof [38], b. ventilation [39], c. cutting [40], d. drilling [41], e. blasting [42], f. coal removal [43], and g. loading [44]. First, miners make the roof above the seam safe and stable by timbering [45] or b

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