Page 485 - Rock Mechanics For Underground Mining

BLOCK CAVING

Figure 15.33 Conceptual model of
caving (after Duplancic and Brady,
1999).

high levels of inter-block force traverse the mass. The upper arch is stronger and
is sustained longer than the lower arch but both fail eventually by slip at the rigid
abutments.
At the other extreme, when the induced tangential stresses are high compared with
thecompressivestrengthoftherockmassandtheshearstrengthsofthediscontinuities,
failure may occur at or near the boundary of the rock mass and blocks or slabs of rock
may become free to fall under the influence of gravity. Under these circumstances,
the dominant mechanisms of failure are brittle fracture of the intact rock and slip
on discontinuities, especially those that are flat dipping (e.g. Heslop and Laubscher,
1981). This form of caving is sometimes referred to as stress caving.
Duplancic and Brady (1999) used a seismic monitoring system to study the early
stages of caving at Northparkes Mines’ E26 block cave, New South Wales, Australia.
From the data collected and analysed, they developed the conceptual model of caving
for this case shown in Figure 15.33. The model contains five regions described by
Duplancic and Brady (1999) in the following terms:

1. Caved zone. This region consists of rock blocks which have fallen from the cave
back. Material in the caved zone provides support to the walls of the cave.
2. Air gap. During continuous caving, the height of the air gap formed is a function
of the extraction rate of the material from the caved zone.
3. Zone of discontinuous deformation. This region no longer provides support to the
overlying rock mass. Large-scale displacements of rock occur in this area, which
is where disintegration of the rock mass occurs. No seismicity is recorded from
within this region. The zone was estimated to extend 15 m outside the boundary
of the cave crown.
4. Seismogenic zone. An active seismic front occurs due to slip on joints and brittle
failure of rock. This behaviour is due to changing stress conditions caused by the
advancing undercut and progress of the cave.
5. Surrounding rock mass. Elastic deformation occurs in the rock mass ahead of
the seismic front and surrounding the cave.
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