Civil Engineering Reference
In-Depth Information
It was considered that the cause of the movement
was a combination of water pressures and ice
jacking on the joints, seismic ground motions over
geologic time and blast damage during construc-
tion. Also, failure could have been progressive
in which movement of one block would drag
the adjacent block(s), and as movement occurred
crushing of rock asperities along the sliding
surfaces reduced the friction angle.
The stability of the sliding blocks was stud-
ied using a plane stability model in which it was
assumed that the cross-section was uniform at
right angles to the slope face, and that sliding took
place on a single plane dipping out of the face. In
order to apply this model to the actual slope, a
simplifying assumption was made in which the
three blocks were replaced by a single equivalent
block that had the same weight as the total of the
three blocks and the same stability characteristics.
The shape and dimensions of the equivalent
single block were defined by the following para-
meters (Figure 14.10):
traffic on the highway during construction, and
the long-term reliability of the stabilized slope.
The prime advantage of the blasting operation
was that this would have been a long-term solu-
tion. In comparison, the service life of the rock
anchors would be limited to decades due to cor-
rosion of the steel and degradation of the rock
under the head. However, the disadvantage of the
blasting operation was that removal of the rock
in small blasts required for the maintenance of
traffic on the highway might have destabilized the
lower blocks resulting in a large-scale slope fail-
ure. Alternatively, removal of all the loose rock
in a single blast would have required several days
of work to clear the road of broken rock, and
to scale and bolt the new face. Bolting of the new
face would probably have been necessary because
the sheet joints would still daylight in the face and
form a new series of potentially unstable blocks.
It was decided that the preferred stabilization
option was to reinforce the slope by installing
a series of tensioned rock anchors extending
through the sheet joints into sound rock. The
advantages of this alternative were that the work
could proceed with minimal disruption to traffic,
and there would be little uncertainty as to the
condition of the reinforced slope.
The rock anchoring system was designed using
the slope model shown in Figure 14.10. For static
conditions and the tension crack half-filled with
water (
z
w
25
◦
; tension crack,
Sliding plane, dip
ψ
p
=
85
◦
; slope face, dip
ψ
f
70
◦
;
dip
ψ
t
=
=
25
◦
; height of face,
upper slope, dip
ψ
s
=
=
H
18 m; distance of tension crack behind
crest,
b
=
10 m.
Stability analysis of this block showed that the
factor of safety was approximately 1.0 when the
water in the tension crack was about 1 m deep,
and a pseudo-static seismic coefficient of 0.15
g
was applied. The static factor of safety for these
conditions was 1.53, and reduced to 1.15 when
the water level in the tension crack was 50% of
the crack depth (
z
w
=
=
7.8 m), it was calculated that an
anchoring force of 550 kN per meter length of
slope was necessary to increase the static factor of
safety to 1.5. With the application of the pseudo-
static seismic coefficients, the factor of safety was
approximately 1.0, which was considered sat-
isfactory taking into account the conservatism
of this method of analysis. The anchors were
installed at an angle of 15
◦
below the horizontal,
which was required for efficient drilling and grout-
ing of the anchors. The factor of safety of 1.5
was selected to account for some uncertainty in
the mechanism of instability, and the possibility
that there may have been additional loose blocks
behind those that could be observed at the face.
The arrangement of anchors on the face was
dictated by the requirements to reinforce each
7.8 m).
14.3.7 Stabilization method
Two alternative stabilization methods were con-
sidered for the slope. Either, to remove the
unstable rock by blasting and then, if necessary
bolt the new face, or reinforce the existing slope
by installing tensioned anchors. The factors con-
sidered in the selection were the need to maintain
