Page 505 - Rock Mechanics For Underground Mining

CHIMNEY CAVING

The first mechanism occurs in weathered or weak rock, or in previously caved rock.
It is a progressive mechanism that starts with failure of the stope roof or hanging
wall on inclined surfaces. If a stable, self-supporting arch cannot be formed, the
failure may progressively propagate towards the surface as shown in Figure 16.3. As
material falls from the roof or from the propagating cave, it will bulk and will tend
to fill the stope void. Unless the stope is initially large and open, or unless sufficient
material is progressively drawn from it, the stope will eventually become filled with
caved material which will provide support for the upper surface and so arrest the
development of the cave. It is for this reason that the development of chimney caving
is so closely associated with draw control.
This progressive failure mechanism has been well established in model studies of
the failure of shallow tunnels in sand and clay (Atkinson et al., 1975) and in model
studies of the mining of steeply dipping, tabular orebodies. It is most likely to occur
when the mechanical properties of the hangingwall material are similar to those of
a soil. Once initiated, propagation of the failure to surface can be very rapid. This
can give the impression that the cave reaches the surface instantaneously and that the
mechanism is that of sudden plug subsidence rather than a progressive one.
This mechanism was first postulated by Crane (1931) who studied chimney caving
development, particularly in the northern Michigan iron ore mining district. Much of
the chimney caving that has occurred on the Zambian copper belt is of this type. Some
of these caves appeared some time after the areas beneath them had been mined out. A
possible explanation of this is the progressive deterioration with time of the narrow rib
pillars that were left between the open stopes. Eventually the pillars collapse, leaving
an unsupported span that is wider than the critical width at which progressive caving
Figure 16.3 Progressive vertical
chimney cave development in ho- initiates. More typically, the chimney caves, or sinkholes as they are called locally,
mogeneous material (after B´etourney form almost simultaneously with the blasting of rib and crown pillars of two or three
et al., 1994). adjacent stopes. Here again, the maximum stable span may be suddenly exceeded.
The second mechanism is also progressive, but occurs as a result of the unravelling
of a discontinuous rock mass. The rock material itself may be quite strong and may
not fail except in flexure. The mechanism is controlled by the regular discontinuities
in the rock mass. As in the previous case, a sufficient void must be maintained beneath
the cave if it is to continue to propagate. Mechanisms of this type have been observed
in physical model studies using the base friction apparatus, for example. They have
also been studied using distinct element numerical models.
Rice (1934) describes an example of this type of chimney cave development over an
2
openinghavingaroofareaof4.3 × 8.6m inagraphiticslatedippingatapproximately
60 . The excavation was created to provide backfill material, and as the broken slate
◦
was progressively drawn, the cave propagated vertically through some 300 m of slates
to surface in approximately one year, maintaining the cross-sectional area of the initial
opening.
The third mechanism, that of plug subsidence, differs from the other two in that
it is controlled by one or more major structural features which provide low shear
strength surfaces on which the plug of undercut rock may slide under the influence of
gravity. In this case, the mass of rock will undergo essentially rigid-body displacement
without breaking up or dilating. Thus, a vertical displacement at the stope boundary
will result in a vertical displacement of similar magnitude at the surface. Although
an initial void must exist for this mechanism to occur, the development of this type of
chimney cave is not as closely associated with draw control as are the other two types.
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