Geology Reference
In-Depth Information
supporting the roof and the quantity of coal to extract for profit. As a result, underground collapses of the workings
which often took the lives of the miners, were common.
The areal extent of the early underground mines was further limited by the seepage of ground water into the
workings. As the mines dipped below the natural ground water level, seepage and subsequent flooding was soon to
follow. Large-scale, coal mining would not begin until improvement in the steam engine allowed water to be
efficiently pumped from the mine workings.
As coal became more important as a fuel, and as its production increased, so did the difficulty and complexity of
extracting it. Despite the problems, the mining industry grew very rapidly with the Industrial Revolution.
Eventually, experience and technological advancement led to the development of various mining methods and
techniques to safely maximize coal production. These methods evolved over the years in response to a variety of
geological, environmental, economic and engineering considerations. Every coalfield has its own set of physical
constraints that govern the mineability of its coal. As a result, specialized variations in mining techniques evolved
to optimize coal recovery for a given set of conditions or constraints. Optimizing coal recovery requires a mining
plan. The goal of any mining plan is to recover the maximum amount of coal in the safest possible manner at the
lowest possible cost. Before a mining plan is implemented, the mineability of a particular coal seam must be
determined. The mineability, methods of coal recovery, and potential profits can be estimated by providing
definitive answers to the following:
What is the coal thickness?
What is the grade or coal rank?
What is the potential net recovery?
What is the thickness of overburden or cover and how does it vary?
What will be the preferred method of mining?
What are the environmental impacts and consequences of mining?
What is the cost of implementing an optimized-mining scheme?; and
Where is the coal market?
Of these considerations, only the methods of mining are considered in the following paragraphs.
Coal-Mining Techniques
T
here are only two mining techniques for recovering coal from the earth; underground mining and
surface mining. Many variations of the underground and surface mining methods exist, but the ones
discussed in the following paragraphs account for nearly all the coal mined world wide (World Coal
Institute, 2005).
Underground mining is a process whereby coal is recovered from within its underground position. Surface
mining is to remove the earth and rock strata overlying the coal. The coal can then be recovered. The method
is normally dependent on the geology of the coal seam and the nature and thickness of overburden above the
coal, as well as the geomorphology of the land surface. At depths greater than 61m (200 ft) and with a
uniform overburden consisting of rock strata, underground mining methods are generally used (Hansen,
1990, p. 28). If the coal lies at shallower depth or the overburden consists of unconsolidated materials or the
geomorphology allows for easy removal of overburden, then surface mining methods are employed. Other
factors contributing to the selection of an appropriate mining method are the uniformity and attitude or pitch
of the seam. Steeply pitching coal seams are mined quite differently than seams that are relatively flat lying
and of uniform thickness.
Underground mining methods access the coal through drift openings, inclined tunnels, or slopes and vertical
shafts. Only the drift opening permits access directly to the coal face without having to dig through overlying
rock strata. Whatever type of access to the coal seam, its purpose is to facilitate entry of men and equipment to
the mine, provide egress from the mine for the mined coal, and provide ventilation to the underground workings
which are often far removed from the original opening to the coal seam (mine). Mining through slopes and
vertical shafts requires a conveyor (slope) or an elevator (shafts) to transport men, materials, and coal into and
out of the mine.
