Civil Engineering Reference
In-Depth Information
14.5.5 Ditch and slope design
rippers. The following are some of the issues that
were addressed during construction:
The two principle design issues for the project
were the dimensions of the ditch to contain rock
falls, and the stability of the slope excavated to
create the ditch.
The blasting was carried out in 4.6 m lifts
using vertical holes. The “step-out” required
at the start of each bench to allow clearance
for the head of the drill was 1.2 m, so the
overall slope angle was 75
◦
. The production
holes were 63 mm diameter on a 1.5 m square
pattern and the powder factor was 0.3 kg/m
3
.
Controlled blasting was used on the final
face to minimize the blast damage to the
rock behind the face. The final line holes
were spaced at 0.6 m and charged with de-
coupled, low velocity explosive at a load
factor of 0.3 kg/m of hole length. The final
line holes were detonated last in the sequence
(cushion blasting) because the limited burden
precluded pre-shear blasting.
The detonation sequence of the rows in the
blast was at right angles to the face in order
to limit the throw of blasted rock on to the
railway and highway, and minimize closure
times.
The track was protected from the impact
of falling rock by placing a 1 m thick layer
of gravel on the track before each blast. This
could be quickly removed to allow operations
of the train.
Near the bottom of the cut it was necessary
to protect from blast damage the masonry
retaining wall supporting the track. This was
achieved by controlling the explosive weight
per delay so that the peak particle velocity
of the vibrations in the wall did not exceed
100 mm/s.
Ditch.
The required depth and width of the
ditch to contain rock falls is related to both
the height and slope angle of the cut face as
illustrated in Figure 12.21 (Ritchie, 1963). These
design recommendations show that the required
ditch dimensions are reduced for a proposed
face angle of 75
◦
, compared to the existing
60
◦
face. Another factor in the ditch design was
the face angle of the outside face of the ditch.
If this face is steep and constructed with energy
absorbing material, then rocks that land in the
base of the ditch are likely to be contained. How-
ever, if the outer face has a gentle slope, they may
roll out of the ditch.
For a 30 m high rock face at an angle of 75
◦
,
the required ditch dimensions were a depth of 2 m
and a base width of 7 m. In order to reduce the
excavation volume, the ditch was excavated to
a depth of 1 m, and a 1 m high gabion wall was
placed along the outer side of the excavation to
create a vertical, energy absorbing barrier.
Slope stability.
The stability of the excavated
slope was examined using Circular Chart No. 2.
The proposed excavation would increase the face
angle from 60
◦
to 75
◦
without increasing the
height of 30 m significantly, and the rock mass
strength and the ground water conditions in the
new slope would be identical to those in the
existing slope. Chart number No. 2 showed that
the factor of safety of the new slope was about
1.3
(
c
/(γH
tan
φ)
0.2
)
.
Figure 14.15 shows the approximate location
of the potential tension crack, and sliding sur-
face with the minimum factor of safety, determ-
ined using Figure 8.11 (
X
=
0.275; tan
φ/
FS
≈
14.6 Case Study V—Stabilization of
toppling failure
=−
0.9
H
;
Y
=
H
;
14.6.1 Site description
b/H
=
0.15).
A rock slope above a railway was about 25 m
high, and the rock forming the slope was a blocky
granite in which a toppling failure was occurring
(Wyllie, 1980). Movement of the upper toppling
block was crushing the rock at the base and
14.5.6 Construction issues
The excavation was by drill and blast methods
because the rock was too strong to be broken by
