Civil Engineering Reference
In-Depth Information
possible by the use of appropriate techniques, to
control rocks with dimensions up to about 2 m,
falling from heights of several hundred meters,
and impacting with energies as high as 1 MJ.
Rigid structures, such as reinforced concrete walls
or fences with stiff attachments to fixed sup-
ports, are rarely appropriate for stopping rock
falls.
verify the design by constructing a test structure.
Sections 12.6.2-12.6.4 describe types of ditches,
fences and barriers, and the conditions in which
they can be used.
Benched Slopes.
The excavation of interme-
diate benches on rock cuts usually increases the
rock fall hazard, and is therefore not recommen-
ded for most conditions. Benches can be a hazard
where the crests of the benches fail due to blast
damage, and the failed benches leave irregular
protrusions on the face. Rock falls striking these
protrusions tend to bounce away from the face
and land a considerable distance from the base.
Where the narrow benches fill with debris, they
will not be effective in catching rock falls. It is
rarely possible to remove this debris because of
the hazard to equipment working on narrow,
discontinuous benches.
There are, however two situations where
benched slopes are a benefit to stability.
First, in horizontally bedded sandstone/shale/coal
sequences the locations and vertical spacing of
the benches is often determined by the lithology.
Benches are placed at the top of the least resistant
beds, such as coal or clay shale, which weather
quicker (Wright, 1997). With this configuration,
the more resistant lithology is not undermined as
the shale weathers (Figure 12.20). The width of
intermediate benches may vary from 6 to 8 m,
and the face angle depends on the durability of
the rock. For example, shales with a slake dur-
ability index of 50-79 are cut at angles of 43
◦
(1.33H:1V) and heights up to 9 m, while massive
sandstone and limestone may be excavated at a
face angles as steep as 87
◦
(1/20H:1V) and heights
up to 15 m. Figure 12.20 also shows a bench at
the toe of the overburden slope to contain minor
sloughing and provide access for cleaning.
A second application for benched slopes is in
tropical areas with deeply weathered rock and
intense periods of rain. In these conditions, lined
drainage ditches on each bench and down the
slope face are essential to collect runoff and
prevent scour and erosion of the weak rock
(Government of Hong Kong, 2000).
12.6.1 Rock fall modeling
Selection and design of effective protection meas-
ures require the ability to predict rock fall
behavior. An early study of rock falls was made
by Ritchie (1963) who drew up empirical ditch
design charts related to the slope dimensions (see
Section 10.6.2). Since the 1980s, the predic-
tion of rock fall behavior was enhanced by the
development of a number of computer programs
that simulate the behavior of rock falls as they
roll and bounce down slope faces (Piteau, 1980;
Wu, 1984; Descoeudres and Zimmerman, 1987;
Spang, 1987; Hungr and Evans, 1988; Pfeiffer
and Bowen, 1989; Pfeiffer
et al
., 1990; Azzoni
and de Freitas, 1995).
Figure 12.19 shows an example of the output
from the rock fall simulation program
RocFall
(Rocscience, 2004). The cross-section shows the
trajectories of 20 rock falls, one of which rolls
out of the ditch. Figures 12.19(b) and (c) show
respectively the maximum bounce heights and
total kinetic energy at intervals down the slope.
The input for the program comprises the slope
and ditch geometry, the irregularity (roughness)
of the face, the restitution coefficients of the slope
materials, the mass and shape of the block, and
the start location and velocity. The degree of
variation in the shape of the ground surface is
modeled by randomly varying the surface rough-
ness for each of a large number of runs, which in
turn produces a range of trajectories.
The results of analyses such as those shown in
Figure 12.19, together with geological data on
block sizes and shapes, can be used to estimate the
dimensions of a ditch, or the optimum position,
required height and energy capacity of a fence or
barrier. In some cases, it may also be necessary to
