Underground Coal Mining:
Underground coal mining is the extraction of coal from below the surface of the earth. The coal is worked through tunnels passages and opening that are connected to the surface for the perpose of the removal of the coal.
Mechanical equipment breaks the coal to a size suitable for haulage.
Underground Coal mining Methods:
In underground coal mining, the working environment is completely enclosed by the geologic
medium, which consists of the coal seam and the overlying and underlying strata.
Modern underground coal-mining methods can be classified into four distinct categories:
room-and-pillar,
longwall,
shortwall, and
thick-seam.
Room-and-pillar mining:
In this method, a number of parallel entries are driven into the coal seam. These entries are then
connected at intervals by wider entries, called rooms, that are cut through the seam at right angles to
the entries. The resulting grid formation creates thick pillars of coal that support the overhead strata
of earth and rock.
Under favourable conditions, between 30 and 50 percent of the coal in an area can be recovered
during development of the pillars. For recovering coal from the pillars themselves, many methods are
practiced, depending on the roof and floor conditions. The increased pressure created by pillar
removal must be transferred in an orderly manner to the remaining pillars, so that there is no
excessive accumulation of stress on them.
Otherwise, the unrecovered pillars may start to fail, endangering the miners and mining equipment.
The general procedure is to extract one row of pillars at a time, leaving the mined-out portion, or gob,
free to subside. While extraction of all the coal in a pillar is a desirable objective, partial pillar
extraction schemes are more common. At depths greater than 400 to 500 metres, room-and-pillar
methods become very difficult to practice, owing to excessive roof pressure and the larger pillar sizes
that are required.
Longwall mining:
In the longwall mining method, mine development is carried out in such a manner that large blocks of
coal, usually 100 to 300 metres wide and 1,000 to 3,000 metres long, are available for complete
extraction.
A block of coal is extracted in slices, the dimensions of which are fixed by the height of coal
extracted, the width of the longwall face, and the thickness of the slice (ranging from 0.6 to 1.2
metres).
In manual or semi-mechanized operations, the coal is undercut along the width of the panel to the
depth of the intended slice. It is then drilled and blasted, and the broken coal is loaded onto a
conveyor at the face.
The sequence of operations continues with support of the roof at the face and shifting of the conveyor
forward. The cycle of cutting, drilling, blasting, loading, roof supporting, and conveyor shifting is
repeated until the entire block is mined out.
In modern mechanized longwall operations, the coal is cut and loaded onto a face conveyor by
continuous longwall miners called shearers or plows.
Two main longwall systems are widely practiced. In the advancing longwall method, development
of the block takes place only 30 to 40 metres ahead of the mining of the block, and the two operations
proceed together to the boundary.
In longwall mining, as in the room-and-pillar system, the safe transfer of roof pressures to the solid
coal ahead of the face and to the caved roof behind the face is necessary.
Caving of the overlying strata generally extends to the surface, causing surface subsidence.
The subsidence over a longwall face is generally more uniform than it is over room-and-pillar
workings.
If conditions are such that the roof will not cave or subsidence to the surface is not allowable, it will
be necessary to backfill the void with materials such as sand, waste from coal-preparation plants, or
fly ash.
Owing to technical and environmental reasons, backfilling is practiced in many mining countries
(e.g., Poland, India), but the cost of production is much higher with backfilling than it is without.
Shortwall mining:
In the shortwall mining method, the layout is similar to the longwall method except that the block of
coal is not more than 100 metres wide.
Furthermore, the slices are as much as three metres thick and are taken by a continuous miner. The
mined coal is dumped onto a face conveyor or other face haulage equipment.
The roof is supported by specially designed shields, which operate in the same manner as longwall
shields.
Although a great future was envisioned for shortwall mining, it has not lived up to expectations.
Thick-seam mining:
seams up to seven metres thick have been extracted by conventional mining systems in one pass.
However, when a seam exceeds these thicknesses, its extraction usually involves dividing the seam
into a number of slices and mining each slice with longwall, continuous, or conventional mining methods.
The thickness of each slice may vary from three to four metres. Many variations exist in the manner in
which the complete seam is extracted. The slices may be taken in ascending or descending order. If
the roof conditions or spontaneous-combustion liability of the seam requires that there be no caving,
the void created by mining will be backfilled. The backfill material then acts as an artificial floor or
roof for the next slice. Caving is the preferred practice.
Thick coal seams containing soft coal or
friable bands and overlain by a medium-to-strong roof that parts easily from the coal can be
fragmented by a high-pressure water jet.
For successful operation, the floor must not deteriorate through contact with water, and the seam
gradient must be steep enough to allow the water to flush the broken coal from the mined areas.
Under favourable conditions, hydraulic mining of coal is productive, safe, and economical.