Page 483 - Rock Mechanics For Underground Mining

BLOCK CAVING

1 m of floor heave and progressive failure of the support and reinforcement system.
Subsequently, a new sublevel caving layout was developed for depths of greater than
600 m on the basis of operational considerations and a detailed rock mechanics study
including three dimensional non-linear modelling and performance monitoring. The
layout finally adopted has a sublevel spacing of 25 m with 4.5 m by 4.5 m cross-cuts
on 17.5 m centres. Drift support and reinforcement is by fibre reinforced shotcrete,
mesh, Split Set rock bolts installed close to the face, and grouted reinforcing bar rock
bolts installed 10 m behind the face. In areas of very weak ground, the grouted rock
bolts may be replaced by 5 m long cable bolts (Struthers et al., 2000).

15.5 Block caving

The essential features of the block caving method of mining and the basic geomechan-
ics issues involved, were discussed in section 12.4.9. The key issue to be addressed
when considering the use of block caving is the cavability of the orebody and the
waste rock. This is a function of the geomechanical properties of the rock mass and
of the in situ and induced stresses. Mine design in block caving involves important
geomechanics considerations in the choice of block dimensions, the choice of an
extraction system, the determination of drawpoint size and spacing, the design of
the ore pass and haulage system, and the scheduling of development and production
operations. Finally, the results obtained from a block caving operation depend on the
fragmentation and draw control achieved. The geomechanics of each of these major
aspects of the block caving method will be discussed in the subsequent sections. The
surface subsidence associated with block caving is considered in Chapter 16. The
discussion of these issues presented here is a digest of the detailed account of block
caving geomechanics given by Brown (2003).

15.5.1 Basic caving mechanics
It must be expected that any unsupported rock mass will cave if it is undercut over
a sufficient area. Caving occurs as a result of two major influences – gravity and the
stresses induced in the crown or back of the undercut or cave. The mechanisms by
which caving occurs will depend on the relations between the induced stresses, the
strength of the rock mass and the geometry and strengths of the discontinuities in
the rock mass. Much accumulated experience supports the contention of Kendorski
(1978) that the successful initiation and propagation of caving requires the presence of
a well-developed, low-dip discontinuity set. The structure most favourable for caving
has been found to be one in which a low-dip discontinuity set is augmented by two
steeply dipping sets which provide conditions suitable for the vertical displacement
of blocks of rock (e.g. Mahtab et al., 1973).
If the compressive tangential stresses induced in the crown of the undercut or cave
are low, or tend to be tensile, blocks of rock may become free to fall under the influence
of gravity or to slide on inclined discontinuities. These conditions may occur when
the horizontal in situ stresses are low or where boundary slots or previous mining
have relieved the stresses or redistributed them away from the block or panel being
mined. Even under these circumstances, it is sometimes possible for a self-supporting
arch to develop in the crown of the cave, especially if an appropriate draw control
strategy is not in place.
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