Civil Engineering Reference
In-Depth Information
Aesthetics.
A requirement on some civil pro-
jects is that shotcreted faces should have a natural
appearance. That is, the shotcrete should be
colored to match the natural rock color, and
the face sculpted to show a pattern of “discon-
tinuities.” This work is obviously costly, but the
final appearance can be a very realistic replica of
a rock face.
the water (Figure 12.16). The selection of the
most appropriate method for the site will depend
on such factors as the intensity of the rainfall or
snow melt, the permeability of the rock and the
dimensions of the slope.
Surface infiltration.
In climates that experi-
ence intense rainfall that can rapidly saturate the
slope and cause surface erosion, it is beneficial
for stability to construct drains both behind the
crest and on benches on the face to intercept
the water (Government of Hong Kong, 2000).
These drains are lined with masonry or concrete
to prevent the collected water from infiltrating the
slope, and are dimensioned to carry the expected
peak design flows (see Figure 1.1(a)). The drains
are also interconnected so that the water is dis-
charged to the storm drain system or nearby water
courses. Where the drains are on steep gradients,
it is sometimes necessary to incorporate energy
dissipation protrusions in the base of the drain to
limit flow velocities. In climates with high rain-
fall there is usually rapid vegetation growth, and
periodic maintenance will be required to keep the
drains clear.
Horizontal drain holes.
An effective means of
reducing the water pressure in many rock slopes is
to drill a series of drain holes (inclined upwards at
about 5
◦
) into the face. Since most of the ground
water is contained in discontinuities, the holes
should be aligned so that they intersect the dis-
continuities that are carrying the water. For the
conditions shown in Figure 12.4, the drain holes
are drilled at a shallow angle to intersect the more
persistent discontinuities that dip out of the face.
If the holes were drilled at a steeper angle, parallel
to these discontinuities, then the drainage would
be less effective.
There are no widely used formulae from which
to calculate the required spacing of drill holes,
but as a guideline, holes are usually drilled on a
spacing of about 3-10 m, to a depth of about one-
half to one-third of the slope height. The holes
are often lined with perforated casing, with the
perforations sized to minimize infiltration of fines
that are washed from fracture infillings. Another
aspect of the design of drain holes is the disposal
12.4.5 Buttresses
Where a rock fall or weathering has formed a
cavity in the slope face, it may be necessary to con-
struct a concrete buttress in the cavity to prevent
further falls (Figure 12.4, item 6). The buttress
fulfills two functions: first, to retain and protect
areas of weak rock, and second, to support the
overhang. Buttresses should be designed so that
the direction of thrust from the rock supports
the buttress in compression. In this way, bending
moments and overturning forces are eliminated
and there is no need for heavy reinforcement of
the concrete, or tiebacks anchored in the rock.
If the buttress is to prevent relaxation of the
rock, it should be founded on a clean, sound rock
surface. If this surface is not at right angles to
the direction of thrust, then the buttress should
be anchored to the base using steel pins to pre-
vent sliding. Also, the top of the buttress should
be poured so that it is in contact with the under-
side of the overhang. In order to meet this second
requirement, it may be necessary to place the last
pour through a hole drilled downward into the
cavity from the rock face, and to use a non-shrink
agent in the mix.
12.4.6 Drainage
As shown in Table 12.1, ground water in rock
slopes is often a primary or contributory cause
of instability, and a reduction in water pressures
usually improves stability. This improvement can
be quantified using the design procedures dis-
cussed in Chapters 6-10. Methods of controlling
water pressure include limiting surface infiltra-
tion, and drilling horizontal drain holes or driving
adits at the toe of the slope to create outlets for
