Coal - Energy Policy Analysis: A Conceptual Framework - page 18

Room and Pillar
In the room and pillar method, pillars of coal, typically fifty-foot squares, are left in place to support
the roof while rooms of coal are mined. In order that the work can proceed without interruption,
at least five rooms are mined at one time, to accommodate the following five different operations
of conventional room and pillar mining:
1. Undercutting: a slot is cut at the bottom of the coal face to facilitate breaking of the coal
when it is blasted.
2. Drilling: a series of holes is drilled in the face for the placement of explosives.
3. Blasting (shooting): explosives are placed in the drill holes and detonated to fracture the
coal and break it away from the face.
4. Loading and hauling: the broken coal is loaded on shuttle cars to be moved from the face
to the main haulage way for transport to the surface.
5. Roof bolting: the roof in front of the face is drilled and bolts are driven upward into the
overlying beds to prevent spalling, or flaking off of roof material. (OSM 1979)
Pillars left behind account for the low recovery rate (50 to 60 percent) of this mining method. As
operations retreat, coal in pillars can be extracted, allowing the roof to collapse into abandoned
rooms, but this was not common practice until recent years (Christman et al. 1980, 76).
Continuous mining differs from conventional room and pillar mining in that cutting, drilling, shoot-
ing, and loading operations are all performed by one machine, a “continuous miner.” This machine
uses a rotating drum with hardened teeth to rip coal from the face. It loads coal on a conveyor that
transports it to shuttle cars or carries it out of the mine. A roof bolter may be mounted on a continuous
miner, but more often mining temporarily ceases as the continuous miner advances while a separate
roof bolter operates to keep the roof from collapsing (Christman et al. 1980, 76).
Longwall
Longwall mining is similar to continuous mining in that coal is ripped from the face and loaded
on a conveyor. A longwall miner travels along the coal face (850 to 1,000 feet), and a conveyor
transports coal to an entry where it is loaded on shuttle cars or larger conveyors for removal from
the mine. The primary difference between room and pillar mining and longwall mining is that in
longwalling, the roof is temporarily supported by hydraulic lifts that advance automatically as
mining progresses, allowing the unsupported roof to collapse behind the active area. This provides
a controlled method of roof collapse and is particularly useful in mines with poor roof condi-
tions. Longwall mining on a fairly short face (200 feet or less) is sometimes called shortwalling
(Christman et al. 1980, 76).
Environmental Costs of Underground Mining
The environmental costs of deep mining are mostly limited to the area around tunnel openings,
ventilation shafts, and spoil piles, unless the hydrologic balance is disturbed and groundwater
is contaminated or diverted from existing wells. Mine tunnels, bore holes, subsidence fractures,
and other openings may provide cross connections between underground aquifers, allowing flows
between previously separate water supplies (Christman et al. 1980, 98). Subsidence occurs as
a result of long-wall mining or collapse of cavity or tunnel roofs, sometimes many years after