Civil Engineering Reference
In-Depth Information
generally accepted that, for permanent slopes on
many civil projects, these savings are greater than
the extra cost of drilling closely spaced, care-
fully aligned holes and loading them with special
charges. The savings are the result of being able
to excavate steeper slopes, and reduce excavation
volume and land take. It is also found that less
time is spent scaling loose rock from the face after
the blast, and the resulting face is more stable
and requires less maintenance during its opera-
tional life. From an aesthetic point of view, steep
cuts have smaller exposed areas than flat slopes,
although the traces of the final line drill holes on
the face may be considered objectionable.
s
H
H
= Excavation
Drill
offset
bench height
s
= Overall
slope angle
f
= Bench face
angle
f
H
11.4.2 Drilling
The maximum depth that can be successfully
excavated by cushion or pre-shear blasting
depends on the accuracy of the hole alignment.
Deviations of more than about 150 mm from
the plane of the holes generally give poor res-
ults due to the uneven distribution of explosive on
the shear plane, and the creation of an irregular
face. While holes up to 27 m deep have been suc-
cessfully cushion blasted, the depths of final line
holes are usually limited to about 8-10 m. Drill
hole alignment can be enhanced with the use of
high rigidity drill steel, button bits and limiting
the down-hole pressure on the bit.
The penetration rates of the drill should also
be considered when determining the depth to be
cushion blasted. If, for example, the penetra-
tion beyond a given depth becomes excessively
slow, it may be more economical to carry out
the excavation in a series of benches in order to
keep penetration rates and drilling costs at accept-
able levels. Also, when laying out final line drill
holes in a benched excavation, allowance should
be made for a minimum 0.3 m offset per bench
since it is not possible to position the drill flush to
the wall of the upper bench. This situation results
in the overall slope of a multi-bench excavation
being flatter than the bench face angle, and this
difference should be allowed for in laying out the
final line holes (Figure 11.12).
When cushion blasting around curved areas or
corners, closer spacings are required than when
Figure 11.12
Alternative explosive placements for
controlled blasting (adapted from ISEE, 1998).
blasting a straight section. Also, unloaded guide
holes can be used to advantage when blasting
non-linear faces (Wyllie, 1999).
In unconsolidated sedimentary formations
where it is difficult to hold a smooth wall,
unloaded guide holes between cushion holes may
be used. Generally, small diameter guide holes are
employed to reduce drilling costs. Where only the
top of the formation is weathered, the guide holes
need be drilled only to that depth and not the full
depth of the cushion holes.
11.4.3 Explosive load
The required explosive load in the final line holes
can be obtained by a combination of steps. First,
the charge diameter should be smaller than the
hole so that the decoupling ratio is at least 2 (see
Figure 11.11). Second, an explosive with a low
velocity of detonation should be used that will
not shatter the rock. Third, the charge should
be distributed along the hole so that there is
