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Coal - Coal Mining Methods

mines underground surface productivity

The method used to mine coal depends on the terrain and the depth of the coal. Prior to the early 1970s, most coal was taken from underground mines. Since that time, however, coal production has shifted from underground mines to surface mines. (See Table 4.1 and Figure 4.2.)

Underground mining is required when the coal lies deeper than 200 feet below ground level. The depth of most underground mines is less than 1,000 feet, but a few go down as far as 2,000 feet. In underground mines some coal must be left untouched in order to form pillars that prevent the mine from caving in. In both underground mines and surface mines, natural features such as folded, faulted, and interlaid rock strata reduce the amount of coal that can be recovered.

Surface mines are usually less than 200 feet deep and can be developed in flat or hilly terrain. Area surface mining is practiced on large plots of relatively flat ground, while contour surface mining follows coal beds along hillsides. (See Figure 4.3.) Open pit mining is used to mine thick, steeply inclined coal beds and uses a combination of contour and area mining methods.

FIGURE 4.2

The growing prevalence of surface coal mining and the closing of nonproductive mines led to increases in coal mining productivity through the 1980s and 1990s. (See Figure 4.4.) In 2000 average productivity reached an all-time high of seven short tons per miner hour. Productivity dipped a bit in 2001 and 2002, but by 2003 it had nearly returned to the 2000 level. Because surface mines are easier to work, they average up to three times the productivity of underground mines. In 2003 the productivity for surface mines was 10.7 short tons of coal per miner hour, while underground mines produced 4.1 short tons per miner hour, as reported in the Annual Energy Review 2003, published in 2004 by the Energy Information Administration (EIA) of the U.S. Department of Energy (DOE).

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