Civil Engineering Reference
In-Depth Information
14.3 Case Study II—Cable anchoring of
plane failure
The two most attractive options for long-term
remediation were reinforcement using tensioned
cables or bar anchors, or reduction of the slope
face angle. Reinforcement was rejected because
of the high cost, and the uncertainty of long-
term corrosion resistance of the steel anchors. The
option finally selected was to reduce the face angle
to 35
◦
by excavating the entire block down to
the sheet joints forming the sliding surface. This
effectively removed the problem. Since good qual-
ity aggregate is always needed in Hong Kong, it
was decided to work the slope as a quarry. It took
several years to organize this activity and during
this time, the water levels in the slope were mon-
itored with piezometers. Although the road was
closed twice during this period, no major prob-
lems occurred and the slope was finally excavated
back to the sliding plane.
14.3.1 Site description
A potential rock fall hazard had developed on a
38 m high, near-vertical rock face located above
a two-lane highway (Figure 14.7). A stabilization
program was required that could be implemen-
ted with minimal interruption to traffic. The
rock falls could vary from substantial failures
with volumes of several hundred cubic meters
formed by widely spaced, persistent joints, to
falls of crushed and fractured rock with dimen-
sions of tens of centimeters. The rock falls were
a hazard to traffic because of the curved align-
ment and limited sight distance, together with the
2 m wide ditch between the toe of the rock cut
Tension crack
J
2
Joint
J
3
Sheet joint
J
1
Tension crack
38 m
Tension crack
Figure 14.7
Cross-section of the slope in
Case Study II showing movement along
sheet joints and location of tension
cracks.
Highway
