Page 317 - Rock Mechanics For Underground Mining

THIN TABULAR EXCAVATIONS

the presence of natural discontinuities. This is demonstrated in Figure 10.20, where
the stiffness ratios for various jointed specimens are seen to be quite different from that
for an unjointed specimen. As demonstrated in Figure 10.21, various joint patterns
lead to a positive value of a pillar stability index, for a pillar width/height ratio of
0.5. It can be readily inferred, from the shape of the accompanying plots in Figure
10.21, that stable post-peak behaviour is assured at higher pillar width/height ratios.
The results as presented imply that the natural discontinuities in a rock mass have
a dominant effect on the post-peak deformation properties of the medium, and may
control the potential for mine global instability. In general, joint sets and other features
oriented to favour slip during the process of development of new fractures in a pillar
can be expected to lead to stable yield of the pillar.
Analysis of mine stability for geometrically irregular mine structures is not
amenable to simplification in the way described for the structures developed in strati-
form orebodies. It is possible that a general computational method for global stability
analysis may be formulated by incorporation of the localisation theories of Rudnicki
and Rice (1975) and Vardoulakis (1979) in some linked computational scheme.

10.8 Thin tabular excavations

Interest in thin, tabular excavations arises since they are common and industrially
important sources of ore. They are generated when coal seams or reef ore deposits are
mined by longwall methods. Energy release has been studied extensively in relation
to the mining of South African gold reefs, where, at the mining depths worked,
static stresses are sufficient to cause extensive rock mass fracture around production
excavations. Many of the original ideas associated with energy release evolved from
studies of problems in deep mining in South Africa. For example, Hodgson and
Joughin (1967) produced data on the relation between ground control problems in
and adjacent to working areas in stopes and the rate of energy release. Some of these
notions of the mining significance of energy release appear to have developed from
macroscopic application of the principles of Griffith crack theory.
The conventional treatment of a thin tabular excavation such as that by Cook
(1967a), considers it as a parallel-sided slit, as shown in Figure 10.22. Sneddon
(1946) showed that the mining-induced displacements of points on the upper and

Figure 10.22 Representation of a
narrow mine opening as a narrow slot.

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