Page 340 - Rock Mechanics For Underground Mining

ROCK SUPPORT AND REINFORCEMENT

The analysis presented so far is a very simple one. It uses a simplified constitutive
model for the rock mass and applies to an axisymmetric problem modified only
by an empirical correction for the influence of gravity. A range of analytical and
semi-analytical solutions have been developed for other boundary conditions and
constitutive models, including the Hoek-Brown empirical rock mass strength criterion
and non-associated flow rules (e.g., Anagnostou and Kovari, 1993, Brown et al., 1983,
Carranza-Torres et al., 2002, Carranza-Torres and Fairhurst, 1997, 1999, Detourney
and Fairhurst, 1987, Panet, 1995, Wang, 1996). A useful means of reducing the
mathematical complexity of the solutions is to adopt the transformations and scaling
methods used by Detourney and Fairhurst (1987), Anagnostou and Kovari (1993)
and Carranza-Torrens and Fairhurst (1999). The results of these analyses are usually
presented in dimensionless form as in the example shown in Figure 11.8. In this
example, the ground reaction curves and the scaled plastic zone radius, = r e /r, are
shown for a section five diameters removed from the face of an advancing tunnel in
a rock mass that satisfies a Hoek-Brown strength criterion and is subject to an initial
hydrostatic stress field of magnitude 0 . Solutions are given for a set of selected
parameters and for three possible values of the Geological Strength Index, GSI.
Although analytical solutions such as those outlined above may be of value in pre-
liminary studies of a range of problems, most practical underground mining problems
require the use of numerical methods of the types discussed in Chapter 6 for their com-
plete solution. Finite element, finite difference and distinct element methods have all
been used for this purpose. The results of calculations carried out using the finite dif-
ference code FLAC 3D are shown superimposed on Figure 11.8. Figure 11.9 shows the
ground reaction curves calculated by Leach et al. (2000) using FLAC 3D for the more
geometrically complex case of extraction or production level drifts in the Premier
block caving mine, South Africa. The ground reaction curves shown in Figure 11.9
are for several locations along a production drift with respect to the undercut face (see
Chapter 15 for an explanation of these terms). These curves were used to estimate
the levels of support pressure required to limit drift closures to acceptable levels.

11.4 Pre-reinforcement

In some circumstances, it is difficult to provide adequate support or reinforcement
to the rock mass sufficiently quickly after the excavation has been made. If suitable
access is available, it is often practicable to pre-reinforce the rock mass in advance of
excavation. In other cases, extra reinforcement may be provided as part of the normal
cycle, in anticipation of higher stresses being imposed on the rock at a later stage in
the life of the mine.
In mining applications, pre-reinforcement is often provided by grouted rods or
cables that are not pre-tensioned and so may be described as being passive rather than
active. Such pre-reinforcement is effective because it allows the rock mass to deform
in a controlled manner and mobilise its strength, but limits the amount of dilation and
subsequent loosening that can occur. The effectiveness of this form of reinforcement
is critically dependent on the bonding obtained between the reinforcing element and
the grout, and between the grout and the rock.
The initial major use of pre-reinforcement in underground mining was in cut-and-
fill mining (Fuller, 1981). The use of cables to pre-reinforce the crowns of cut-and-fill
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