Page 356 - Rock Mechanics For Underground Mining

ROCK SUPPORT AND REINFORCEMENT

Figure 11.24 Distributions of (a) Examination of the distributions of radial displacement u r and radial and tangential
stress and (b) displacement around a stresses r and t indicates the function of the radial reinforcement. It substantially
circular excavation for unreinforced reduces the radial displacement u r , and generates a higher magnitude of r in the
and reinforced near-field rock (after
fractured zone, resulting in a higher gradient in the t distribution. The effect is
Brady and Lorig, 1988).
to shift the plastic–elastic transition closer to the excavation boundary. Thus, both
closure of the excavation and the depth of the zone of yielded rock are reduced.
The density of reinforcement used in this demonstration problem is greater than
would be applied in mining practice. However, it confirms the mode of action of rein-
forcement and the prospect for application of computational methods in reinforcement
design.

11.6 Materials and techniques

11.6.1 Overview
The emphasis in this chapter has been on the principles of the support and reinforce-
ment of rock masses and on design analyses. However, if support and reinforcement
is to be fully effective, it is necessary that suitable materials be used for a particular
application and that these materials be installed or applied using satisfactory tech-
niques. The details of these techniques and materials are largely beyond the scope of
this book. Only the central principles and some illustrative examples will be given
here. For full practical details, the reader should consult texts such as those by Hoek
et al. (1995), Hustrulid and Bullock (2001), Hutchinson and Diederichs (1994), Kaiser
et al. (1996), Melbye and Garshol (1999), Proctor and White (1977) and Stillborg
(1994), and the proceedings of specialty conferences such as those edited by Kaiser
and McCreath (1992) and Villaescusa et al. (1999). In this section, brief accounts
will be given of the essential features of rockbolts and dowels, cable bolts, shotcrete,
wire mesh and steel sets. Details of the support techniques used in longwall coal and
metalliferous mining will be given in Chapter 15.

11.6.2 Rockbolts and dowels
A single tensioned rockbolt usually consists of an anchorage, a steel shank, a face
plate, a tightening nut and sometimes a deformable plate. For short term applications,
the bolt may be left ungrouted, but for permanent or long term applications and use
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