Page 516 - Rock Mechanics For Underground Mining

MINING-INDUCED SURFACE SUBSIDENCE

the slope of the ground surface;

any prior surface mining;

the placement of fill in a pre-existing or newly produced crater; and

nearby underground excavations.

The extent of surface disturbance is defined by the angle of break or angle of sub-
sidence which is the angle made with the horizontal at a section by a straight line
drawn from the undercut level to the extremity of surface cracking or disturbance as
shown in Figure 16.10. The complement of the angle of break is known as the angle
of draw. The caved or subsided zone defined by the angle of break is a zone in which
large-scale or macro-deformations occur. Outside this zone there is a zone within the
overall zone of influence of the cave in which small-scale or micro-deformations oc-
cur (Figure 16.10). Although small compared with the deformations occurring within
the caved or subsided zone, these deformations may be large enough to damage exca-
vations and mine infrastructure located within the zone of influence. The rock within
this zone could become highly or over-stressed by the redistribution of stresses that
accompanies the development and upwards progression of the cave, and then become
de-stressed (at least in the lateral direction towards the cave) as the cave develops fully.
The limits of the zone of influence of a block cave are best estimated using a com-
bination of monitoring, local experience and numerical modelling. Butcher (2002), as
reported by Brown (2003), has suggested that the following steps be used in making
these predictions:

1. Project the perimeters of the orebody-country rock contact on all mining levels
to the surface to establish the area that will be clearly destroyed by caving.
2. Make a preliminary estimate of the angle of break using an empirical or experi-
ential method such as that due to Laubscher (1994).
3. Calibrate this estimate against observed angles of break in this or similar mines.
4. Check the estimated angle of break using other methods of analysis appropriate
to the likely failure mechanisms such as that developed by Hoek (1974) and
extended by Brown and Ferguson (1979) and Lupo (1997).
5. Modify the current estimate of the angle of break to take account of local geo-
logical features such as faults, topography and the amount of broken material in
the crater.
6. Use numerical modelling (using the FLAC code, for example) to check the angle
of break and to estimate the stresses and displacements induced in the rock mass
around the caved zone (e.g. Karzulovic et al., 1999, Lupo, 1999).

An approach such as this is used in making subsidence predictions at CODELCO-
Chile’s block and panel caving copper mines (Karzulovic et al., 1999, Rojas et al.,
2001). Figure 16.11 shows an aerial view of the horse-shoe shaped crater produced at
the EL Teniente Mine. The geometrical parameters used by Karzulovic et al. (1999)
in their subsidence analyses for El Teniente are defined in Figure 16.12. Note that they
differ in some respects from the general definitions given in Figure 16.10. A chart
showing the variation of the angle of break, , with height above the crater floor, H, for
the Ten4–Regimiento Sector is shown in Figure 16.13. The angle of break is higher
for the stronger Braden breccia than for the other rocks, and decreases markedly with
increasing height above the crater floor. Rojas et al. (2001) give a similar chart for
the El Teniente Sur and Esmeralda Sectors.
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