Geology Reference
In-Depth Information
In modern day contour mining, once the coal is removed, the land is returned to the approximate original contour
with the removed overburden being returned to the mined pits in roughly the same order in which it was removed.
Topsoil and subsoils, the first materials to be removed, are then spread over the restored surface and re-vegetated.
Area Mining
A
rea mining is performed where the ground surface is relatively flat and the overburden is economical to remove.
The typical area mine requires the excavation of a series of pits or cuts. The overburden is cast into stockpiles.
Overburden from the first cut is usually reserved whereas overburden removal in subsequent cuts is cast into the
previous completed cut after the coal is removed. In each cut the overburden is removed until the underlying coal
seam is exposed. The coal is then mined in its entirety by large power shovels or front end loaders and hauled from
the open cut by trucks. All of the coal seam is recovered by this method of mining. The process of excavation, coal
recovery and cut backfilling continues until no additional cuts can be made at which time the reserved first cut
overburden is used to fill the last cut.
Auger Mining
A
uger mining requires boring large diameter, closely spaced horizontal holes into the coal seam that is exposed at
the base of a previously prepared highwall. A horizontal boring machine, equipped with large diameter auger
boring tools drills a pattern of horizontal boreholes into the exposed coal seam removing the coal as the bore
advances into the coal seam. Augers can bore 61 m (200 ft) or more into a solid coal seam that would otherwise
require recovery by developing an underground mine (Schmidt, 1979, p. 199).
The coal bed thus mined must be relatively uniform in thickness and relatively flat lying. The method is not so
commonly practiced today as the auger holes left in the coal seam open a very large surface area of coal exposed to
air and ground water infiltration resulting in significant production of acid mine drainage (AMD). This results
because auger holes are rarely backfilled in their entirety. Typically they are sealed at the exposed end with a low
permeability soil backfill that is placed a short distance into the auger hole. After each auger hole is filled in this
manner, the length of the exposed coal face is backfilled (buried) with a low permeability soil to seal the coal seam
and reclaim the mined area. This method is applicable when no additional overburden can be economically
removed.
Mountain Top Removal
T
he mountain top removal method of surface mining may be the mining method of choice in regions where
the rock strata are relatively flat lying and the landscape is punctuated by numerous hills and stream valleys
exposing multiple coal seams in the valley walls. Where multiple seams occur and where the overburden to
the highest elevation is within an economical overburden stripping ratio, the entire mountain top above the
coal seams may be removed. The mountain top is progressively lowered by drilling and blasting the
overburden. The overburden is pushed by dozer or cast by dragline into the adjacent valleys surrounding
the mountain top. Many hundreds of meters of overburden may be removed in this manner, depending on the
number and thickness of the potential coal seams to be recovered. Upon reaching the first layer of coal,
drilling and blasting stops while the coal is removed. Drilling and blasting then resumes until the next seam
of coal is exposed. The spoil is not used to restore the mountain top, but rather a relatively flat landscape is
created with buried stream valleys connecting the flattened mountain tops. While reclamation efforts such as
stabilization and revegetation are required for mountaintop removal, the remaining landscape is far different
from what it was before mining. The nature of the mountain top removal method of mining so drastically
alters the landscape that many oppose its use (McQuaid, 2009).
