Page 502 - Rock Mechanics For Underground Mining

16 Mining-induced surface

subsidence

16.1 Types and effects of mining-induced subsidence

In the present context, subsidence is the lowering of the ground surface following
underground extraction of an orebody. Subsidence is produced, to a greater or a lesser
degree, by almost all types of underground mining. Surface displacement may result
from the redistribution of stresses associated with excavation or from mining-related
activities such as de-watering.
Subsidence can be regarded as being of two types – continuous and discontinuous.
Continuous or trough subsidence involves the formation of a smooth surface sub-
sidence profile that is free of step changes (Figure 16.1). The resulting displacements
of surface points may be of only elastic orders of magnitude when compared with
the dimensions of the subsiding area or the mining depth. This type of subsidence
is usually associated with the extraction of thin, horizontal or flat-dipping orebodies
overlain by weak, non-brittle sedimentary strata. It results from the longwall mining
of coal, but has also been associated with the extraction of a wide variety of other
minerals such as sulphur and the evaporites deposited in sedimentary environments.
Methods of predicting subsidence profiles in these cases are discussed in section 16.5.
Discontinuous subsidence is characterised by large surface displacements over
Figure 16.1 Trough subsidence
over a longwall extraction. limited surface areas and the formation of steps or discontinuities in the surface
profile. It may be associated with a number of mining methods, may involve a range
of mechanisms, may develop suddenly or progressively, and may occur on a range of
scales. Figure 16.2 illustrates some of the forms of discontinuous subsidence.
Crown holes (Figure 16.2a) arise from the collapse of the roofs of generally aban-
doned, shallow open workings. Much of the surface damage in the anthracite mining
region of northern Pennsylvania, USA, is due to this cause. Crown holes are also
associated with old coal, ironstone and flint workings in England (Piggott and Eynon,
1978). Crown holes may be regarded as a special case of chimney caving which is
discussed in detail in section 16.2.
Pillar collapse in old, shallow workings may lead to similar surface expressions of
discontinuous subsidence as does crown hole formation. Such collapses may occur as
a result of a deterioration in pillar strength with time or the imposition of additional
load on the pillar by surface construction. Large-scale pillar collapse in a working
mine can produce discontinuous subsidence over a larger area with more serious
effects. A most catastrophic failure of this type occurred at the Coalbrook North
Colliery, South Africa, on 21 January 1960, when a room-and-pillar mining area
2
covering approximately 3 km suddenly collapsed with the loss of 437 lives (Bryan
et al., 1964).
Chimney caving, piping or funnelling (Figure 16.2b) involves the progressive
migration of an unsupported mining cavity through the overlying material to the sur-
face. The surface subsidence area may be of a similar plan shape and area to the
original excavation. Chimney caves may form in weak overburden materials as on the

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