Coal - Energy Policy Analysis: A Conceptual Framework - page 17

Coal Mining Methods
There are two basic methods of mining a coal reserve: underground mining and surface min-
ing. The choice of mining method depends primarily on depth of burial and thickness of the
coal seam. Seams relatively close to the surface, at depths less than approximately 180 feet, are
usually surface mined. Coals that occur at depths of 180 to 300 feet are usually deep mined,
although in some cases surface mining techniques can be used. For example, some western U.S.
coals that occur at depths in excess of 200 feet are mined by open pit methods, due to thick-
ness of the seam (sixty to ninety feet). Coals occurring below 300 feet are usually deep mined
(Christman et al. 1980, 74).
Coal is mined only where technically feasible and economically justifiable. Evaluation of
technical and economic feasibility of a potential mine requires consideration of many fac-
tors: regional geologic conditions; overburden (e.g., rock and soil) characteristics; coal seam
continuity, thickness, structure, quality, and depth; strength of materials above and below the
seam for roof and floor conditions; topography (especially altitude and slope); climate; land
ownership as it affects the availability of land for mining and access; surface drainage patterns;
groundwater conditions; availability of labor and materials; coal purchaser requirements in
terms of tonnage, quality, and destination; and capital investment requirements (Christman
et al. 1980, 74).
From about 1945 to 1960, use of coal for rail and water transportation and for space heating
declined, while overall demand increased for electricity generation. Today, demand for coal is
driven by the electric power generating sector, which accounts for 90 percent of coal consump-
tion. As demand increased, capital-intensive surface coal mining grew at the expense of labor-
intensive underground coal mining; accelerated use of surface mining technology in large-scale
area mines shifted production from eastern coalfields to the western United States; underground
mining shifted from conventional room and pillar mining to longwall mining, further displacing
labor; and improvements in mining equipment durability and capability increased productivity
and helped keep coal mining costs low (Bonskowski, Watson, and Freme 2006, 1-6). In the
1990s, growth in total coal demand slowed to a little over 1 billion tons per year and coal prices
decreased by 45 percent in real dollar terms, as average mine size continued to increase; produc-
tion was increasingly concentrated among fewer, larger companies; “high-grading” of reserve
properties forced the closure of less competitive properties; and productivity continued to increase
(Bonskowski 2000, 1).
UNDERGROUND MINING
When coal occurs at depths too great for economic surface mining methods, a tunnel entry to the
seam is driven horizontally into the hillside (drift mine), at an angle to intercept the seam (slope
mine), or vertically to reach the coal at depth (shaft mine). In deep underground mines, machinery,
miners, and coal are transported by rail cars, conveyors, or elevator hoists. Deep mining involves
removing the coal seam without removing the overburden. In many instances, the mine roof is
permitted to collapse after coal is removed, which may result in some subsidence, or differential
lowering of the land surface due to downward movement of overburden to occupy mined-out spaces
(Christman et al. 1980, 74-75). Underground mining methods are separated into two categories:
room and pillar, and longwall. Each accounted for about half of underground coal production in
the United States in 2007 (Weir International Inc. 2008).