Page 520 - Rock Mechanics For Underground Mining

MINING-INDUCED SURFACE SUBSIDENCE

Figure 16.16 Progressive hanging-
wall caving at Gr¨angesberg (after
Hoek, 1974).

at the Gr¨angesberg iron ore mine in Sweden. Figure 16.16 shows the progressive
development of the subsidence zone at Gr¨angesberg with increased mining depth
over a 40-year period.
This form of subsidence is often associated with mines in which the near-surface
portions of the orebody have been extracted previously using open-pit methods.
Alternatively, underground mining operations may commence in previously unmined
ground at shallow depths below the surface. Figure 16.17 shows the sequence of pro-
gressive hangingwall failure postulated by Hoek (1974) for the cases of mining from
an outcrop (a–e) and from an open pit (f–i). It is assumed that at each new stage of
mining, a tension crack and a shear failure surface form in the hangingwall rock mass
at a critical location determined by the strength of the rock mass and the imposed
stresses. In some cases, mechanisms other than this may occur. As noted above for
the case of block caving, major discontinuities such as faults may provide preferential
shear planes. Alternatively, if a major set of persistent discontinuities dips steeply in
a similar direction to the orebody, toppling of the hangingwall rock mass may occur
(Figure 16.18).
Hoek (1974) developed a limiting equilibrium analysis for predicting the progress,
from a known initial position, of hangingwall failure with increasing mining depth.
This analysis assumed a flat ground surface and full drainage throughout the caving
mass. Brown and Ferguson (1979) extended Hoek’s analysis to take account of a
sloping ground surface and groundwater pressures in the tension crack and on the
shear plane. The idealised model used for the extended analysis is shown in Figure
16.19. The variables involved in the analysis are:

A = base area of wedge of sliding rock mass
c = effective cohesion of rock mass

H 1 = mining depth at which initial failure occurs
H 2 = mining depth at which subsequent failure occurs
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