Page 383 - Rock Mechanics For Underground Mining

UNDERGROUND MINING METHODS

Figure 12.12 Longwall mining in
coal and soft rock (after Hamrin,
2001).

seam detaches from the hangingwall rock, separates into constituent blocks behind
the line of supports, and falls (or caves) into the mined void. The caving process is
accompanied by an increase in the void ratio of the displaced rock, from effectively
zero, to about 50%. The caved immediate roof occupying the mined void acts as
a natural bed of backfill against which the main roof deflects. Therefore the major
roles of the near-field rock are to cave and swell to fill the mined void, to restrain
the displacement of the main roof, and to maintain the mechanical integrity of the
main roof. As the stope face advances, the resultant deflection of the main roof causes
consolidation of the caved waste rock. It is the mobilised consolidation pressure in
the waste which can maintain pseudo-continuous behaviour of the main roof stratum
after relatively large strains have occurred.
Longwall coal mining can be undertaken in geomechanical conditions different
from those favouring room-and-pillar mining. As implied by the preceding discussion,
the preferred condition is that the immediate roof rock for the coal seam consists of
relatively weak shales, siltstones or similar lithologies, with sufficient jointing to
promote easy caving. Quite different properties are required of the main roof, which
must be sufficiently competent to bridge the span between the mine face and the
consolidating bed of caved roof rock. The seam floor rock must have sufficient bearing
capacity to support the loads applied by the roof support system at the face line.
Acharacteristicoflongwallmining,commontobothcoalandmetalliferousmining,
is that support loads are mobilised in the rock mass by large-scale displacement
of pseudo-continuous country rock. Thus although gravitational potential energy is
dissipated by restrained displacement of the country rock, the method also results
in the accumulation of strain energy in the near-field rock. The method clearly falls
between the extremes of fully supported and complete caving methods of mining.
365