Civil Engineering Reference
In-Depth Information
The following is a brief discussion of some
construction issues that may have a significant
influence on stabilization work.
next. A significant cost issue on active transporta-
tion routes is the use of cranes to access the slope.
If the work is done from a platform suspended
from a crane located on the road, this may block
two to three lanes of traffic. In contrast, traffic
closures can be minimized by having the construc-
tion crews work off ropes secured behind the crest
of the slope.
Waste disposal.
The least expensive method
of disposing of waste rock produced by excava-
tion and scaling operations in mountainous ter-
rain is dumping rock down the slope below the
site. However, disposing of waste rock in this
manner has a number of drawbacks. First, a
steeply sloping pile of loose rock may be a visual
scar on the hillside which can be difficult to veget-
ate. Second, the waste rock may become unstable
if not adequately drained or keyed into the exist-
ing slope; if it fails, the material may move a
considerable distance and endanger facilities loc-
ated down-slope. Third, where the site is located
in a river valley, the dumped rock may fall in the
river and have a deleterious effect on fish pop-
ulations. In order to minimize these impacts, it
is sometimes required that the excavated rock be
hauled to designated, stable waste sites.
Another problem that may need to be
addressed in the disposal of waste rock is acid-
water drainage. In areas of North Carolina and
Tennessee, for example, some argillite and schist
formations contain iron-disulfides; percolation of
water through fills constructed with this rock pro-
duces low pH, acidic runoff. One method that has
been used to control this condition is to mix the
rock with lime to neutralize the acid potential and
then to place the blended material in the center of
the fill (Byerly and Middleton, 1981). Sometimes
it is necessary to encase the rock-lime mixture in
an impervious plastic membrane.
Aesthetics.
A series of steep, high rock cuts
above a highway may have a significant visual
impact when viewed both by the road user and
the local population. In scenic areas, it may be
desirable to incorporate appropriate landscaping
measures in the design of the rock cuts in order to
minimize their visual impact (Norrish and Lowell,
1988). Examples of aesthetic treatments of rock
Blasting.
Damage to rock faces by excessively
heavy blasting is a frequent cause of instability in
the years following excavation of a slope. Meth-
ods of controlled blasting, such as pre-shearing
and trim blasting as described in Chapter 11,
can be used to excavate a slope to a specified
line with minimal damage to the rock behind
the face.
Topography.
If there is a steep slope above
the crest of a cut, then stabilization work that
involves laying back the cut will have the effect
of increasing the height of the cut. This increase
in the cut height will require a larger catch ditch,
and may result in additional stability problems,
especially if there is a substantial layer of soil or
weathered rock at the surface.
Construction access.
Determine the type of
equipment that is likely to be required to carry
out the work, and how this equipment will be
used at the site. For example, if it is planned to
excavate a substantial volume of rock in order to
lay back a slope, then it is likely that airtrac drills
and excavators will have to work on the slope.
In steep terrain, it may be found that the con-
struction of an access road for this equipment is
costly and causes additional instability. Further-
more, a cut width of at least 5 m is required to
provide sufficient working width for this equip-
ment. Also, if stabilization work is planned using
large diameter rock bolts, then it is essential that
suitable drilling equipment can access the site. For
example, on steep faces, holes with a diameter
larger than about 100 mm will have to be drilled
with heavy equipment supported from a crane.
Where it is not possible to use such heavy drilling
equipment, it would be necessary to drill smal-
ler diameter holes with hand-held equipment, and
use a larger number of smaller size bolts.
Construction costs.
Cost estimates for stabil-
ization work must take into account both the
costs of the work on the slope, and indirectcosts
such as mobilization, traffic control, waste
disposal, and environmental studies as discussed
