Page 504 - Rock Mechanics For Underground Mining

MINING-INDUCED SURFACE SUBSIDENCE

case histories of such occurrences in Western Australia and the Northern Terri-
tory, Australia. However, this term is also used to describe the subsidence features
associated with pre-existing solution cavities in dolomites and limestones (Figure
16.2d). The characteristics of these features and the mechanisms of their formation
are discussed in section 16.3. Perhaps the best known examples of dolomite sinkholes
associated with mining operations are those that have occurred in the Far West Rand
gold mining district of South Africa. Jennings et al. (1965) describe the sudden col-
lapse into a sinkhole of a three-storey crusher chamber at the West Driefontein Mine
in December 1962; 29 lives were lost.
Discontinuous subsidence also occurs as a result of caving methods of mining.
Figure 16.2e illustrates the large-scale mass subsidence associated with block caving
operations, and Figure 16.2f shows the surface configuration that can be produced
by progressive hangingwall failure in a sublevel caving operation. An analysis of the
latter case is presented in section 16.4.2.
Some of the instances of mining-induced subsidence outlined above have had dis-
astrous consequences. They have led to loss of life on a large scale, loss of parts
of producing mines and loss of major surface installations. Such catastrophic con-
sequences are usually associated with sudden discontinuous subsidence. An obvious
objective of mine planning must be to limit discontinuous subsidence to areas over
which the mine operator holds the surface rights. Having done this, it is also necessary
to ensure that subsidence does not affect surface installations, transportation routes
or underground access. As the Mufulira example shows, it is especially important to
avoid discontinuous subsidence beneath surface accumulations of water or tailings.
The effects of continuous subsidence are generally not as dramatic as those of dis-
continuous subsidence. Because of the large surface areas affected, longwall coal min-
ing often influences built-up areas and services. Differential vertical movements, hor-
izontal compressive and tensile strains, and curvature of the ground surface, can cause
distress to engineered structures, domestic buildings, roads, railways and pipelines.
A wide variety of examples of the structural damage that may be caused by mining
subsidence are given by the National Coal Board (1975), Whittaker and Reddish
(1989), Peng (1992) and Holla and Barclay (2000). These and other publications also
describe the design features that have been used to limit mining subsidence damage
to structures, pipelines, roads, railways and electrical transmission lines.
It is clear from this introductory discussion of mining-induced subsidence and its
effects, that the prediction of the subsidence profile in the case of continuous subsi-
dence, and of the likely occurrence and areal extent of discontinuous subsidence, are
vitally important to the planning of underground mining operations. Before rational
predictions of these types can be made, it is necessary that the mechanisms involved
be understood. Accordingly, the remainder of this chapter is devoted to considerations
of the mechanisms involved in the major types of subsidence and to the methods of
analysis that are available for use as predictive tools.

16.2 Chimney caving

16.2.1 Chimney caving mechanisms
Three distinct chimney caving mechanisms may be identified, each associated with
different geological environments.
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