Civil Engineering Reference
In-Depth Information
the outermost loose rock, provided that the frac-
turing was caused by blasting and only extended
to a shallow depth. However, if the rock mass
is deeply fractured, continued scaling will soon
develop a cavity that will undermine the upper
part of the slope.
Removal of loose rock on the face of a slope is
not effective where the rock is highly degradable,
such as shale. In these circumstances, exposure of
a new face will just start a new cycle of weathering
and instability. For this condition, more appro-
priate stabilization methods would be protection
of the face with shotcrete and rock bolts, or a
tied-back wall.
excavation of “sliver” cuts in which the toe of
the new cut coincides with that of the old cut.
12.5.2 Trimming
Failure or weathering of a rock slope may form
an overhang on the face (Figure 12.17, item 2),
which could be a hazard if it were to fail. In
these circumstances, removal of the overhang by
trim blasting may be the most appropriate sta-
bilization measure. Section 11.4 discusses meth-
ods of controlled blasting that are applicable to
situations where it is required to trim blast small
volumes of rock with minimal damage to the rock
behind the trim line.
Where the burden on a trim blast is limited,
flyrock may be thrown a considerable distance
because there is little rock to contain the explosive
energy. In these circumstances appropriate pre-
cautions such as the use of blasting mats would
be required to protect any nearby structures and
power lines. Blasting mats are fabricated from
rubber tires or conveyor belts chained or wired
together.
12.5.1 Resloping and unloading
Where overburden or weathered rock occurs in
the upper portion of a cut, it is often necessary to
cut this material at an angle flatter than the more
competent rock below (Figure 12.17, item 1).
The design procedure for resloping and unload-
ing starts with back analysis of the unstable slope.
By setting the factor of safety of the unstable
slope to 1.0, it is possible to calculate the rock
mass strength parameters (see Section 4.4). This
information can then be used to calculate the
required reduced slope angle and/or height that
will produce the required factor of safety.
Another condition that should be taken
account of during design is weathering of the
rock some years after construction, at which time
resloping may be difficult to carry out. A bench
can be left at the toe of the soil or weathered rock
to provide a catchment area for minor slope fail-
ures and provide equipment access. Where a slide
has developed, it may be necessary to unload the
crest of the cut to reduce its height and diminish
the driving force.
Resloping and unloading is usually carried out
by excavating equipment such as excavators and
bulldozers. Consequently, the cut width must
be designed to accommodate suitable excavating
equipment on the slope with no danger of col-
lapse of the weak material while equipment is
working; this width would usually be at least
5 m. Safety for equipment access precludes the
12.5.3 Scaling
Scaling describes the removal of loose rock, soil
and vegetation on the face of a slope using hand
tools such as scaling bars, shovels and chain
saws. On steep slopes workers are usually sup-
ported by ropes, anchored at the crest of the slope
(Figure 12.18). A suitable type of rope for these
conditions is a steel-core, hemp rope that is highly
resistant to cuts and abrasion. The scalers work
their way down the face to ensure there is no loose
rock above them.
A staging suspended from a crane is an altern-
ative to using ropes for the scalers to access the
face. The crane is located at the toe of the slope
if there is no access to the crest of the slope. The
disadvantages of using a crane, rather than ropes,
are the expense of the crane, and on highway pro-
jects, the extended outriggers can occupy several
lanes of the highway with consequent disruption
to traffic. Also, scaling from a staging suspen-
ded from crane can be less safe than using ropes
