Page 493 - Rock Mechanics For Underground Mining

BLOCK CAVING

The in situ stresses and their redistribution around the undercut influence cave
initiation and development. Assume that the undercut drifts and the direction of
cave advance are aligned with the principal horizontal in situ stress directions. If the
direction of advance is perpendicular to the direction of the major principal horizontal
stress, the levels of stress in the abutment ahead of the undercut will be high and will
increase as the undercut advances. This will increase the likelihood of damage to the
undercut drifts and the extraction level excavations. However, this effect may be an
advantage in inducing the caving of stronger rock masses as at the Andina and El
Teniente mines, Chile.
Any spatial variation in the strength of the orebody can be expected to have an effect
on the influence of the induced stresses on cave initiation and propagation. Because
caving should be easier to initiate in weaker than in stronger ore, and because the
stresses induced ahead of the undercut should increase as the undercut advances, it
is often argued that mining should take place from a weaker to a stronger section
of the orebody (Ferguson, 1979). Major structural features such as faults and shear
zones can have an influence on cave initiation and propagation and on the stability
of undercut and extraction level excavations. A circumstance to be avoided is the
isolation of large wedges of rock that may fall or “sit down” under the influence of
gravity, inhibiting cave propagation and imposing additional dead weight loads on
undercut drifts and extraction level excavations. As a general rule, it is preferable to
orient the advancing undercut face as close as possible to normal to the strike of any
persistent structural feature or set of features.
In many large caving mines, the orebody may be extracted in a series of blocks.
Where possible in these cases, new blocks should be retreated from existing blocks
rather than being advanced towards them. As illustrated in Figure 15.38, this prevents
the creation of potentially highly stressed pillars between the two caves which can
lead to the stress-induced failure of excavations in their vicinity.

Figure 15.38 The initiation of cav- Shape of the undercut face. Both mining experience and a consideration of the
ing of a block adjacent to an ex- induced stresses suggest that sharp changes to, or large irregularities in, the shape of
isting caved block for (a) the pre- the advancing undercut face should be avoided and that the lead between adjacent
ferred mining direction, and (b) an
unfavourable mining direction (after sections of the overall face should be minimised. A circular or square undercut will
Ferguson, 1979). produce a larger hydraulic radius than a rectangular undercut of the same plan area

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