Civil Engineering Reference
In-Depth Information
greater than the working strength at the rock-
grout interface. For this reason, the required
anchor length is typically determined from the
stress level developed at the rock-grout interface.
overall slope; its primary function is surface pro-
tection. Another component of a shotcrete install-
ation is the provision of drain holes to prevent
build-up of water pressures behind the face.
12.4.3 Reaction wall
Reinforcement.
For permanent applications,
shotcrete should be reinforced to reduce the risk
of cracking and spalling. The two common meth-
ods of reinforcing are welded-wire mesh, or steel
or polypropylene fibers. Welded-wire mesh is fab-
ricated from light gauge (
Figure 12.4, item 3 shows an example where there
is potential for a sliding type failure in closely
fractured rock. If tensioned rock bolts are used
to support this portion of the slope, the frac-
tured rock may degrade and ravel from under
the reaction plates of the anchors, and eventu-
ally the tension in the bolts will be lost. In these
circumstances, a reinforced concrete wall can be
constructed to cover the area of fractured rock,
and then the holes for the rock anchors can be
drilled through sleeves in the wall. Finally, the
anchors are installed and tensioned against the
face of the wall. The wall acts as both a pro-
tection against raveling of the rock, and a large
reaction plate for the rock anchors. Where neces-
sary, reinforced shotcrete can be substituted for
concrete.
Since the purpose of the wall is to distribute
the anchor loads into rock, the reinforcing for
the wall should be designed such that there is no
cracking of the concrete under the concentrated
loads of the anchor heads. It is also important
that there are drain holes through the concrete
to prevent build-up of water pressure behind
the wall.
3.5 mm diameter) wire
on 100 mm centers, and is attached to the rock
face on about 1-2 m centers with steel pins, com-
plete with washers and nuts, grouted into the
rock face. The mesh must be close to the rock
surface, and fully encased in shotcrete, taking
care that there are no voids behind the mesh. On
irregular surfaces it can be difficult to attach the
mesh closely to the rock. In these circumstances,
the mesh can be installed between two layers of
shotcrete, with the first layer creating a smoother
surface to which the mesh can be more readily
attached.
An alternative to mesh reinforcement is to use
steel or polypropylene fibers that are a compon-
ent of the shotcrete mix and form a reinforce-
ment mat throughout the shotcrete layer (Morgan
et al
., 1989, 1999). The steel fibers are manu-
factured from high strength carbon steel with a
length of 30-38 mm and diameter of 0.5 mm. To
resist pullout, the fibers have deformed ends or
are crimped. The proportion of steel fibers in the
shotcrete mix is about 60 kg
/
m
3
, while compar-
able strengths are obtained for mixes containing
6 kg of polypropylene fibers per cubic meter of
shotcrete. The principal function of fibers is to
significantly increase the shear, tensile and post-
crack strengths of the shotcrete compared to
non-reinforced shotcrete; shotcrete will tend to
be loaded in shear and tension when blocks of
fractured rock loosen behind the face.
The disadvantages of steel fibers are their tend-
ency to rust at cracks in the shotcrete, and the
hazard of the “pin cushion” effect where per-
sons come in contact with the face; polypropylene
fibers overcome both these disadvantages.
∼
12.4.4 Shotcrete
Shotcrete is a pneumatically applied, fine-
aggregate mortar that is usually placed in a
50-100 mm layer, and is often reinforced for
improved tensile and shear strength (American
Concrete Institute, 1995). Zones and beds of
closely fractured or degradable rock may be pro-
tected by applying a layer of shotcrete to the
rock face (Figure 12.4, item 4). The shotcrete
will control both the fall of small blocks of rock,
and progressive raveling that could eventually
produce unstable overhangs. However, shotcrete
provides little support against sliding for the
