Civil Engineering Reference
In-Depth Information
possible to reverse the process. For this reason,
reinforcement of rock slopes is most effective if it
is installed prior to excavation—a process known
as pre-reinforcement.
Shear keys on a much larger scale have been
used for the stabilization of dam foundations and
abutments (Moore and Imrie, 1982). A tunnel
was driven along a distinctly defined shear zone,
with the excavation extending into sound rock on
either side of the tunnel. The tunnel was then filled
with concrete to create a high strength inclusion
along the sliding plane.
12.4.1 Shear keys
Reinforced shear keys provide support for blocks
of rock up to about a meter thick, as well as zones
of loose and weathered rock at the crest of the
slope (Figure 12.4, Item 1). Shear keys are used
where the support required is limited by the size of
the blocks, and to prevent raveling and loosening
of closely fractured, weak rock. If rock bolts were
to be installed in this rock, the raveling would
soon expose the head of the bolt resulting in loss
of support.
Shear keys comprise lengths of reinforcing steel
about 25-32 mm diameter and about 1000 mm
long fully grouted into holes about 500-750 mm
deep drilled into stable rock. The holes are located
close to the toe of the rock to be supported, and
are spaced about 500-1000 mm apart, depending
on the support required. Lengths of reinforcing
bars, about 6-8 mm diameter, are then placed
horizontally and secured to the vertical bars.
Finally, the reinforcing steel is fully encapsu-
lated in shotcrete, or concrete poured in intimate
contact with the rock.
The support provided by the shear key is equal
to the shear strength of the vertical steel bars, and
possibly the cohesion of the rock-concrete sur-
face. The shear key acts as a resisting force in
the limit equilibrium equations (see Section 6.3),
and if the magnitude of this shear force is
R
k
, then the factor of safety for a block with
weight
W
is
12.4.2 Rock anchors
Typical applications of rock anchors, as shown
in Figure 12.4, items 2 and 3, are to prevent slid-
ing of blocks or wedges of rock on discontinuities
dipping out of the face. It is important to note that
the primary function of rock anchors is to modify
the normal and shear forces acting on the sliding
planes, rather than to rely on the shear strength of
steel where the anchor crosses this plane. In this
chapter, the term “rock anchor” refers to both
rigid bars and flexible cables that can be used in
bundles; the design principles and construction
methods are similar for both materials.
Rock anchors may be fully grouted and unten-
sioned, or anchored at the distal end and
tensioned. The different applications of unten-
sioned, pre-reinforcement bolts and tensioned
anchors are shown in Figure 12.5 (see also
Figure 4.13). Pre-reinforcement of an excavation
may be achieved by installing fully grouted but
untensioned bolts (dowels) at the crest of the cut
prior to excavation. The fully bonded dowels pre-
vent loss of interlock of the rock mass because
the grouted bolts are sufficiently stiff to pre-
vent movement on the discontinuities (Moore and
Imrie, 1982; Spang and Egger, 1990). However,
where blocks have moved and relaxed, it is gener-
ally necessary to install tensioned anchors to pre-
vent further displacement and loss of interlock.
The advantages of untensioned bolts are their
lower cost and quicker installation compared to
tensioned anchors.
Tensioned rock anchors are installed across
potential slide surfaces and bonded in sound rock
beyond the surface. The application of a tensile
force in the anchor, which is transmitted into the
+
W
cos
ψ
p
tan
φ
R
k
FS
=
(12.3)
W
sin
ψ
p
where
ψ
p
is the dip of the base of the block and
φ
is the friction angle on the base of the rock block
(assuming a dry slope). The factor of safety calcu-
lated by equation (12.3) could be for a unit length
of the slope, or a specified length, depending on
how forces
W
and
R
k
are defined.
