Rock bolting is used to retain potentially unstable rock masses by drilling and installing steel or fibre tendons into deeper stable substrates. Bolts are bonded with cementitious or resin grout and are often tensioned after installation. Bolts can be used to stabilise individual blocks or in a grid pattern to improve stability of an entire rock face.

Self-drilling anchors and rock bolts
The main features of all self-drilling anchors and rock bolts are the application of asingle-use drill bit in combination with the anchor rod being used as a drill steel. Thus,the anchor or rock bolt is connected to the hydraulic rock drill by an adapter deviceand installed in the way a conventional borehole is drilled. In modern undergroundmining and tunnelling, the following types of self-drilling anchors and rock bolts are indevelopment or already available:
− Self-drilling hollow bar bolts
− Self-drilling resin grouted bolts
− Self-drilling friction bolts
− Self-drilling mechanical anchors
Self-drilling hollow bar bolts
Self-drilling hollow bar bolts are being widely used under different trade names,
although they show a consistency in their system components:
− Single-use drill bits made of cast iron, which are available either hardened or
with carbide inserts
− Hollow anchor bars made of steel, with a continuous outside thread
− Anchor couplings made of steel (or cast iron), with a continuous inside thread
and optionally middle stops
− Anchor nuts made of steel (or cast iron)
− Anchor plates in different types and geometries
Drill bits are available in different designs and diameters what makes this system
easily adjustable to changing ground or rock conditions. The thread type depends on
the manufacturer. Besides manufacturer-specific types, R-threads according to ISO
10208 are commonly being used, which can easily be adapted to standard rock
drilling equipment [6]. Standard thread sizes used in tunnelling are R32, R38 and
R51. Self-drilling hollow bar bolts are available in different load capacity classes,
depending on the inner and outer diameter of the anchor bar (with respect to the
thread). Following the introduction of self-drilling hollow bar bolts made of steel,
glass-fibre reinforced plastics (abbr.: “GRP”) as primary material for the anchor bars
are also being used particularly for applications where the bolted ground or rock
mass needs to be excavated later on (e.g. face bolts). The self-drilling installation of
the bolts is accomplished similar to the drilling of a conventional borehole; the hollow
anchor bar is connected to the shank adapter via a coupling and is being utilized as
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drill steel; the single-use drill bit is put at the top of the hollow bar. In case where
larger bolt lengths are required, the anchor bars can easily be extended by couplings.

Self-drilling hollow bars are either injected during or after the installation process.
For simultaneously drilling and injecting, which is usually performed in cases where
the ground needs to be further improved or stabilized during installation, special
rotary injection adapters, which are placed in between the shank adapter and the
hollow anchor bar, are being used. These adapters allow that the bolt is installed selfdrilling
while the injection medium is added at the same time. Normally, self-drilling
hollow bar bolts are injected after the installation process is completed. In doing so,
the anchor head is connected to an injection adapter which is furthermore linked to a
grout pump. The injection flow goes inside the hollow bolt upwards until the bolt’s toe,
from there on the bolt is grouted outside bottom up until the grout flows out at the
bolt’s head. The threaded bolt geometry ensures an optimum bond between the bolt
and the injection medium.