Geology Reference
In-Depth Information
2.3. Surface Coal Mining
Surface and steep pitch underground mining
in the Centralia Basin, Centralia, Pennsylva-
nia. The Centralia Mine Fire underlies a por-
tion of Centralia Borough, visible at the top of
the photograph.
Photo by Robert W. Bruhn, 1983
.
Surface Mining
T
he surface-mining method is commonly referred to as strip, open-cast, or open-cut mining and is employed in flat
to moderately rolling terrain. Modern day surface mines tend to be very large, extensively engineered and highly
mechanized operations. With this method, an economical thickness of overburden is removed down to a coal seam
by following a coal contour along or around a hill or by removing the entire hill top above the coal seam. Mining
begins by removing the topsoil and subsoil which are reserved for final reclamation. Then large-scale excavating,
drilling, and hauling equipment is employed to remove the rock overlying the coal which is transported to a spoil
disposal site. Once the overburden is removed, smaller equipment, including front-end loaders and power shovels,
loads the coal into large trucks that transports the coal from the pit and delivers it to a coal preparation plant.
Approximately 90% of the coal can be recovered by surface mining. Surface-mining techniques include contour
mining, area mining, auger mining, and mountain top removal, all of which account for about 40% of the coal
mined in the world and about 67% of the coal mined in the United States (World Coal Institute, 2005). These
methods are discussed in more detail in the following sections.
Contour Mining
C
ontour mining is used when the coal may actually outcrop on the flanks of hillsides in steep, hilly, or mountainous
terrain. The technique involves the removal of overburden by following the contour of the outcropping coal and
extending the excavation into the hillside to the point where the overburden ratio makes further mining
unprofitable. This results in a wedge of overburden that is removed and stockpiled. The excavated area becomes
a level bench on top of the coal seam. The coal is then mined and the overburden is placed back on the bench to
return the hill to its approximate natural slope and contour. In past years, the overburden was cast onto the slope
below the exposed coal where it was permanently left which resulted in fill on the outside slope and a highwall on
the inside slope with the coal seam exposed at the base of the highwall. Remaining coal behind the exposed high
wall was left since it would not have been economical to remove the overburden. This coal was partially recovered
by another surface-mining procedure known as auger mining.
