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MINING-INDUCED SURFACE SUBSIDENCE
Figure 16.4 North–south section,
Athens Mine, showing plug subsi-
dence controlled by dykes. Coordi-
nates and scale are shown in metres
(after Boyum, 1961).
Allen (1934) describes an example of chimney caving that was controlled by geo-
logical planes of weakness, but which developed progressively as in the first mecha-
nism. After three years of mining by a caving method at the Athens Mine in northern
Michigan, a cave developed and rapidly progressed to surface through 600 m of
cover. As shown in Figure 16.4, the subsidence zone was bounded on either side by
subvertical dykes.
16.2.2 Limiting equilibrium analysis of chimney caving
Atkinson et al. (1975) found that limiting equilibrium analyses of the final vertically
sided collapse configurations gave good approximations to the ultimate collapse con-
ditions observed in their tests on model tunnels in sand. By analogy, limiting equi-
librium calculations may be expected to be helpful in estimating ultimate collapse
conditions when chimney caving develops by the first of the mechanisms described.
They should also be useful in the case of plug subsidence, although in this case an
assumption of zero dilation on the slip surface should be made. The approach cannot
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