Page 26 - Rock Mechanics For Underground Mining

ROCK MECHANICS AND MINING ENGINEERING

or by fractured ore temporarily retained in contact with mined stope walls. The second
type of mine configuration recognised by Thomas is a caving structure, generated by
mining methods such as block caving and sublevel caving. In these cases, no support is
provided in the mined space, which fills spontaneously with fragmented and displaced
orebody and cover rock.
From a rock mechanics point of view, discrimination between the two generic
mining techniques, and the structures they generate, may be made on the basis of
the displacements induced in the country rock and the energy redistribution which
accompanies mining. In the technique of mining with support, the objective is to
restrict displacements of the country rock to elastic orders of magnitude, and to main-
tain, as far as possible, the integrity of both the country rock and the unmined remnants
within the orebody. This typically results in the accumulation of strain energy in the
structure, and the mining problem is to ensure that unstable release of energy cannot
occur. The caving technique is intended to induce large-scale, pseudo-rigid body
displacements of rock above the crown of the orebody, with the displacement field
propagating through the cover rock as mining progresses. The principle is illustrated
schematically in Figure 1.4. The process results in energy dissipation in the caving
rock mass, by slip, crushing and grinding. The mining requirement is to ensure that
steady displacement of the caving mass occurs, so that the mined void is continuously
self-filling, and unstable voids are not generated in the interior of the caving material.
This distinction between different mining techniques does not preclude a transition
from one technique to the other in the life of an orebody. In fact, the distinction is

Figure 1.4 Principal features of
a caving operation (after Borquez,
1981).

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