Civil Engineering Reference
In-Depth Information
strand is unbonded and is free to strain as tension
is applied. Figure 12.4 shows that the bond zone
is located in stable rock below the potential slid-
ing plane so that when the anchor is tensioned,
stresses are applied to this plane to increase the
factor of safety (see Section 6.4).
Methods of securing the distal end of an
anchor in the drill hole include resin, mechan-
ical, and cement grout anchors. The selection
of the appropriate anchor will depend on such
factors as the required capacity of the anchor,
speed of installation, strength of the rock in
the anchor zone, access to the site for drilling
and tensioning equipment, and the level of cor-
rosion protection required. The following is
a brief discussion of each of these anchorage
methods.
The installation method consists of inserting a
sufficient number of cartridges into the drill hole
to fill the annular space around the bar. It is
important that the hole diameter, in relation to
the bar size, be within specified tolerances so that
complete mixing of the resin is achieved when
the bar is spun. This usually precludes the use
of coupled anchors because the hole diameter to
accommodate the coupling will be too large for
complete resin mixing. The bar is spun as it is
driven through the cartridges to mix the resin and
form a rigid solid anchorage. The required speed
of rotation is about 60 revolutions per minute,
and spinning is continued for about 30 s after the
bar has reached the end of the hole. It is prefer-
able that threaded bar be rotated in the direction
that augers the resin into the hole, particularly in
up-holes.
The maximum bolt length is limited to about
12 m because most drills cannot rotate longer bars
at sufficient speed to mix the resin. It is pos-
sible to install a tensioned, resin-grouted bolt by
using a fast-setting (about 2 minutes) resin for the
anchor, and a slower setting (30 minutes) resin
for the remainder of the bar. The bolt is ten-
sioned between the times that the fast and slow
resins set.
The primary advantage of resin anchorage is
the simplicity and speed of installation, with sup-
port of the slope being provided within minutes of
spinning the bolt. The disadvantages are the lim-
ited length and tension capacity (
Resin anchors.
These comprise a plastic cart-
ridge about 25 mm in diameter and 200 mm long
that contains a liquid resin and a hardener that
set when mixed together (Figure 12.10). Setting
times vary from about 1 minute to as much as
90 minutes, depending on the reagents used. The
setting time is also dependent on the temperat-
ure, with fast-setting resin hardening in about
4 minutes at a temperature of
5
◦
C, and in about
−
25sat35
◦
C.
∼
400 kN) of
the bolt, and the fact that only rigid bars can be
used. Furthermore, the resin is not as effective as
cement grout for corrosion protection of the steel.
Unlike cement grout, resin does not provide the
high pH protective layer against corrosion, and it
cannot be verified that the cartridges completely
encapsulate the steel.
Mechanical anchors.
These comprise a pair of
steel platens that are pressed against the walls of
the drill hole. The anchor is expanded by driving
or torquing a steel wedge between the platens.
The advantage of mechanical anchors is that
installation is rapid, although not as rapid as resin
anchors, and tensioning can be carried out as
soon as the anchor has been set. Grouting can
Figure 12.10
Resin cartridges for anchoring rock
bolts (TRB, 1996).
