Page 319 - Rock Mechanics For Underground Mining

INSTABILITY DUE TO FAULT SLIP

Figure 10.23 Schematic represen-
tation of the advance of the active
zone in a longwall stope, after wall
convergence.
advancing single slot, mining would occur under steady-state conditions. There would
be no increase in stored strain energy and a constant rate of energy release. The process
is equivalent to translation of a locally active domain (the stope face and its immediate
environs) through the rock mass, as shown in Figure 10.23. There is no increase in
stored strain energy since previously destressed rock is recompressed, by the advance
of mining, to a state which would eventually approach, theoretically, its pre-mining
state.
Unfortunately, the provision of sufficient work spaces to sustain the typical pro-
duction rates required from a highly capitalised mine, requires that a tabular orebody
cannot be mined as a single, advancing slot. When longwall stopes advance towards
one another, high rates of energy release are generated by interaction between the
respective zones of influence of the excavations. Energy release rates for these types
of mining layouts, which also usually involve slightly more complex dispositions of
stope panels in the plane of the orebody, are best determined computationally. The
face element method described by Salamon (1964), which is a version of the boundary
element method, has been used extensively to estimate energy changes for various
mining geometries.
Cook (1978) published a comprehensive correlation between calculated rates of
energy release and the observed response of rock to mining activity, for a num-
ber of deep, South African mining operations. The information is summarised in
Figure 10.24. The data indicate a marked deterioration of ground conditions around
work places in longwall stopes as the volume rate of energy release, dW r /dV , in-
creases. The inference is that the energy release rate may be used as a basis for
evaluation of different mining layouts and extraction sequences, and as a guide to the
type of local support required for ground control in working places.
In studies similar to those of gold reef extraction, Crouch and Fairhurst (1973)
investigated the origin of coal mine bumps. They concluded that bumps could be
related to energy release during pillar yielding. It was suggested also that a boundary
element method of analysis, similar in principle to the face element method, could be
used to assess the relative merits of different extraction sequences.

10.9 Instability due to fault slip

The mechanism of mine instability considered previously results from the constitu-
tive behaviour of the rock material, and may involve shearing, splitting or crushing

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