| |
Longwalling
As
workings advanced, not only did the pressure increase due to
additional thickness of cover and make itself evident on the
roof, but side pressure on the ribs also increased. It then
became economically impossible to maintain roadways in room
and pillar sections. It soon became evident that some method
must be found to neutralize these difficulties and reduce the
cost of operations. Longwalling, therefore, became the only
means of continuing mining at depth.
In the longwall method, the chief principle is to extract the
whole seam in one operation and to allow the super incumbent
strata to settle in the void know as the gob or the goat. Longwalling
may be divided into two classifications - longwall advancing
and longwall retreating.
In this method of mining, the extraction of the seam starts
at the shaft pillar and works its way from the shaft bottom.
The coal is removed in a long length or face.
Longwall Advancing
In
early operations, these faces varied in lengths from 50 or 60
feet to 200 feet and finally 400 feet and more, as methods became
more secure. Access to the surface was assured by the maintenance
of two roadways, one on either side of the working face. One
of these roadways was used as a haulage level and the other
for ventilation.
As the workings advanced, these roadways were constructed by
means of continuous six by six feet softwood, stone-filled packs
on either side of the road from the floor to the roof. The stone
was usually obtained from the roof - brushed from the roadways
to make height for horses and coal tubs. Sometimes, the stone
would be taken from the gob and used to form the packwalls necessary
for road maintenance.
The coal was undercut by handpicks, hand shovelled into cars
at the face which were then drawn through cross-gates to the
main haulage level. Later, these walls were highly mechanized
with machines cutting the coal and face conveyors taking the
coal from the face to the main haulage.
In earlier years, temporary roof props were put along the face
at regular intervals from two to four feet apart. The props
were not arranged opposite each other in these rows, but alternating,
and in this way a larger surface of roof could be supported.
These props were eventually replaced by hydraulic roof supports
that were mechanically advanced with the winning of the coal.
Longwall Retreating
In the longwall retreating system, the levels, haulage roads
and airways are driven to their outer boundaries of the area
to be worked. When these extremities are reached, the longwall
faces are worked back in the direction of the shaft.
As the roads giving access to the shaft have already been constructed,
the roof is allowed to fall completely as the face retreats
and no roads are maintained through the gob or goat. The face
is supported in the same manner as with the advancing method
and the props are withdrawn as the face recedes.
This method of mining involves a high initial development cost
and a small output until the mine is completely developed.
GO
TO
1 | 2
| 3 | 4
|
|
|
