Environmental Engineering Reference
In-Depth Information
mining has ceased. Exposure to respirable coal dust has long been recognized as the cause of
pneumoconiosis, or black lung disease, which reduces lung capacity and may result in death by
suffocation (Dvorak 1977). This is a much greater risk to workers in underground mines than in
surface mines, but not a significant risk to the general public.
SURFACE MINING
The process of removing earth, rock, and other material, collectively known as overburden, to expose
an underlying mineral deposit is surface mining. Large earthmoving equipment is used to remove
or strip away overburden and expose the coal seam. Topsoil must first be removed and stored for
later reuse. To remove overburden, blasting or ripping may be required, depending on the material
involved. Overburden material is then loaded and hauled to a disposal site. Because overburden
handling is a major cost item in surface mining, it is common to use the spoil from a fresh cut to
refill nearby older workings, thereby avoiding handling the same material twice and saving on costs
of fuel and maintenance of equipment. In some instances, overburden handling is a single operation.
For instance, a dragline removes material from a fresh cut and deposits it as fill in older cuts in a
single movement. In other cases, overburden material is loaded on trucks by front-end loaders and
hauled to a dumping site where it is spread and graded by bulldozers. An alternative method uses
self-loading scrapers that haul the material and spread it (Christman et al. 1980, 77).
After overburden has been removed, the coal seam is cleaned to remove any remaining material
that would contaminate the coal. Coal is then broken up, loaded, and hauled to a preparation plant
or shipping site. If necessary, the coal is drilled and blasted before loading. After the overburden
or spoil has been used to refill an area, it is graded and recontoured, topsoil is redistributed, and
vegetation is planted to hold surface soil together and reduce erosion of spoil.
Strip mining is a specific kind of surface mining in which overburden is removed in strips,
one cut at a time. Three types of strip mining methods are used to mine coal: area, contour, and
mountaintop removal. The choice of method depends primarily on topography and coal seam
structure (OSM 1979).
Area Mining
In area mining, overburden is removed in long cuts to expose the coal. The spoil from the first cut
is deposited in an area outside the planned mining area; spoil from subsequent cuts is deposited
as fill in the previous cut after coal has been removed. This method is most suitable for areas with
flat terrain. Many separate operations are involved in this type of mining, and a wide range of
equipment options is available, such as dragline, truck and shovel, front-end loader, and bucket
wheel excavator (Christman et al. 1980, 78).
In the western states, coal seams are commonly ten to twenty feet thick, with up to 100-foot seams
in the Powder River Basin of Wyoming; in the Midwest coal seams are typically three to seven feet
thick and seventy-five to 100 feet below the surface. Because of the large size of area mines and their
relatively unrestricted sites, enormous equipment may be used to remove overburden and reconstruct
the land. The life of some area mines may be more than fifty years (OSM 1987, 4).
Contour Mining
The contour mining method consists of removing overburden from the seam in a pattern follow-
ing the contours along a ridge or around a hillside. This method is most commonly used in areas
 
 
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