Rock Mechanics.pdf - Mining and Blasting

SUPPORT AND REINFORCEMENT DESIGN
of rock mass classification-based design rules and more comprehensive numerical
analyses. Quite often in practice, these approaches are used in combination.
Rock-support interaction calculations. These may be carried out using the methods
discussed in section 11.3 and the calculation procedures set out in Appendix C.
Although idealisations of the problem have to be made, and some factors and techniques
cannot be specifically allowed for in the calculations, use of this approach
permits the designer to develop a clear understanding of the relative merits of candidate
reinforcement systems in a particular application. In most cases, it will be
necessary to carry out a series of calculations for a number of trial designs before an
appropriate design can be selected for a field trial.
Empirical design rules. A wide range of empirical support and reinforcement design
rules have been developed over the last 50 years. These rules, which are based on
precedent practice, generally apply to permanent underground excavations rather than
to temporary mining excavations such as stopes. They are geometrically based and
do not account explicitly for the stress field induced around the excavation or for the
quality of the rock mass. For these reasons, they must be used with extreme caution
and only for making preliminary estimates which must be checked by making more
complete assessments.
The range of empirical design rules available has been reviewed by Stillborg (1994)
and by Rachmad et al. (2002) in the context of their application to the support and
reinforcement of production drifts in a block caving mine. One of the most useful
and long-lived set of empirical design rules is that developed by Lang (1961) for
pattern rockbolting of permanent excavations during the construction of the Snowy
Mountains Hydro-electric Scheme in Australia. Although Lang’s rules are described
here as empirical, they were established on the basis of a range of laboratory, field
and theoretical studies which have been reviewed by Brown (1999a). Lang (1961)
gives the minimum bolt length, L, as the greatest of
(a) twice the bolt spacing, s;
(b) three times the width of critical and potentially unstable rock blocks defined by
the average discontinuity spacing, b; or
(c) 0.5B for spans of B < 6m, 0.25 B for spans of B = 18–30 m.
For excavations higher than 18 m, the lengths of sidewall bolts should be at least
one fifth of the wall height. The maximum bolt spacing, s, is given by the least of
0.5L and 1.5b. When weld or chain mesh is used, a bolt spacing of more than 2 m
makes attachment of the mesh difficult if not impossible.
Figure 11.18 shows a preliminary layout of a rockbolting pattern for a horse-shoeshaped
excavation in jointed rock, prepared using Lang’s rules. This figure also illustrates
the basis on which Lang’s rules were developed, namely the establishment of a
self-supporting compressed ring or arch around the excavation. If a highly compressible
feature such as a fault or a clay seam crosses the compression ring, it is possible
that the required compression will not be developed and that the reinforcement will
be inadequate.
Rock mass classification schemes. Schemes such as those due to Barton et al.
(1974) and Bieniawski (1973, 1976) were developed as methods of estimating support
333