Page 524 - Rock Mechanics For Underground Mining

MINING-INDUCED SURFACE SUBSIDENCE

Figure 16.21 Longitudinal section,
looking north, through the centre of a
3DEC model of a hangingwall wedge,
Kidd Mine, Ontario, Canada (after
Board et al., 2000).

plane stain, non-linear (tension cut-off, ubiquitous joint) FLAC analyses have been
used by Singh et al. (1993), Lupo (1999) and Henry and Dahn´er-Lindqvist (2000). At
the Kiirunavaara Mine, Sweden, deep-seated footwall fracturing and displacement
have been recorded and studied using FLAC modelling. Board et al. (2000) and
Ran et al. (2002) used FLAC and distinct element (3DEC) modelling to study a
large hangingwall wedge failure and the remobilisation of this failure with associated
displacements on footwall structures at the Kidd Mine, Ontario, Canada. The initial
failure was estimated to involve some 30 million tonnes of rock. As illustrated by
the longitudinal section through the 3DEC model shown in Figure 16.21, the wedge
extended from surface to a depth of about 610 m. Its boundaries were defined by a
fault and a shear zone having continuous lengths in excess of 1070 m, by deep tension
cracks on the eastern face, by shearing through the host rock mass at its base, and by
mined and filled stopes on its western extremity.

16.5 Continuous subsidence due to the mining of tabular orebodies

16.5.1 Concepts and definitions
A continuous subsidence trough is produced at the surface when thin, flat-dipping,
tabular orebodies are mined by longwall methods which give 100% extraction over
relatively large panels. In order for the subsidence to be continuous rather than dis-
continuous, the relation of the depth of mining and the caving-induced stresses to the
strength properties of the rock overlying the orebody must be such that fracture and
discontinuous movement of the rock are restricted to the immediate vicinity of the
orebody. This occurs in most longwall coal mining operations and in metalliferous
longwall mining at depth as, for example, in the South African goldfields. It may
also result from the mining of other minerals such as evaporites overlain by relatively
weak sedimentary rocks.
Figure 16.22 shows a vertical section through the workings in a typical case. Su-
perimposed on the diagram are the subsidence and surface slope profiles for three
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