Page 503 - Rock Mechanics For Underground Mining

TYPES AND EFFECTS OF MINING-INDUCED SUBSIDENCE

Figure 16.2 Types of discontinuous Zambian copper belt, in previously caved material or in regularly jointed rock which
subsidence: (a) crown hole; (b) chim- progressively unravels. Chimney caves have been known to propagate upwards to
ney caving; (c) plug subsidence; (d)
surface through several hundreds of metres.
solution cavities; (e) block caving; (f)
If chimney formation is sudden rather than progressive, the phenomenon is some-
progressive hangingwall caving.
times known as plug subsidence (Figure 16.2c). Generally, plug subsidence is con-
trolled by some structural feature such as a dyke or a fault which provides a plane of
weakness whose shear strength is overcome at some critical stage of mining. Under-
ground air blasts are generally produced by this form of subsidence. As discussed in
section 15.5.1, sudden subsidence of a weak, leached rock mass above a block cave
at the Northparkes E26 mine, New South Wales, Australia, produced a massive air
blast in which four men were killed.
Chimney caving was the cause of the Mufulira Mine disaster in which 89 men were
3
killed when 450 000 m of mud (tailings) flooded part of the mine on 25 September
1970. A chimney cave propagated upwards by about 500 m to connect the sublevel
caving mining area with the overlying tailings pond. The most disastrous consequence
of this was the loss of so many lives, but a further important consequence was the
sterilisation of a major part of the mine which was subsequently isolated between
concrete bulkheads. The Final Report of the Commission of Inquiry (1971) makes
salutory reading for aspiring mining engineers.
Chimney caves are sometimes known as sinkholes. Szwedzicki (1999), for exam-
ple, defines a sinkhole as a cylindrical or conical depression caused by discontin-
uous subsidence of a rock mass over a mined out excavation, and provides nine

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