Page 357 - Rock Mechanics For Underground Mining

MATERIALS AND TECHNIQUES

in corrosive environments, rockbolts are usually fully grouted with cement or resin
grout for improving both pull-out strength and corrosion resistance.
Rockbolts are often classified according to the nature of their anchorages. Early
rockbolt anchors were of the mechanical slot-and-wedge and expansion shell types. It
is often difficult to form and maintain mechanical anchors in very hard or in soft rocks.
Mechanical anchors are also susceptible to blast-induced damage. Anchors formed
from Portland cement or resin are generally more reliable and permanent. A third
category of rockbolt anchorage is that utilised by friction (Split Set and Swellex)
bolts which rely on the generation of friction at the rock-bolt contact along their
lengths for their anchorage and strength. As with mechanical anchors, friction bolts
depend for their efficacy on the sizes and accuracy of the drilling of the holes in
which they are installed. They are also susceptible to corrosion. Although they may
be given a pre-tension to ensure that an anchorage is formed, friction bolts are usually
not installed with the levels of pre-tension (5–20 tonnes) used for other rockbolts. In
this case, they act a dowels rather than rockbolts. Other types of dowel are usually
grouted along their lengths on installation and develop their tension with deformation
of the rock mass in which they are installed. Grouting of Split Set bolts and dowels
may increase their load carrying capacity for longer term applications (Thompson
and Finn, 1999).
Figure 11.25 shows a number of types of rockbolt and dowel classified according
to the anchorage method used but with several types of shank illustrated. Figure 11.26
shows further details of the installation and grouting of a resin anchored and grouted
bolt made from threaded bar. Resin encapsulated rockbolts are widely used for
the reinforcement of longer term openings in metalliferous mines.(e.g. Slade et al.,
2002).

11.6.3 Cable bolts
Cable bolts are long, grouted, high tensile strength steel elements used to reinforce
rock masses. They may be used as pre-or post-reinforcement and may be left un-
tensioned or be pre- or post-tensioned. Windsor (2001) defines the following terms
associated with cable bolting:

Wire – a single, solid section element.

Strand – a set of helically spun wires.

Figure 11.25 Types of rockbolt
and dowel (after Hadjigeorgiou and
Charette, 2001).

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