Civil Engineering Reference
In-Depth Information
effect on the excavation rates. Therefore, careful
evaluation of the blast to determine how improve-
ments could be made to the design is usually
worthwhile.
help to avoid excessive backbreak:
(a)
Choke blasting into excessive burden or
broken muck piles should be avoided.
(b)
The front row charge should be adequately
designed to move the front row burden.
11.4 Controlled blasting to improve
stability
Slope instability is often related to blast damage to
the rock behind the face. Blast-induced instabil-
ity is usually surficial, extending possibly 5-10 m
behind the face for open pit scale blasts, that can
result in rock falls occurring over time as water
and ice open cracks and loosen blocks. It is also
possible that the blast damage can cause larger-
scale instability where, for example, the slope
contains persistent bedding planes dipping out of
the face. The explosive gases can travel along,
and open, the beds resulting in displacement of
substantial blocks of rock.
Control of blast damage to final walls can be
limited by implementing one or both of the fol-
lowing procedures. First, the production blast
should be designed to limit rock fracturing behind
the final wall, and second, controlled blasting
methods such as line drilling, pre-shearing, and
cushion blasting can be used to define final faces
precisely (Hagan and Bulow, 2000). With respect
to production blasting, the following precautions
(c)
Adequate delays and timing intervals should
be used for good movement towards free
faces and the creation of new free faces for
following rows.
(d)
Delays should be used to control the max-
imum instantaneous charge.
(e)
Back row holes, together with reduced
charge “buffer holes,” should be drilled at
an optimum distance from the final face to
facilitate excavation and yet minimize dam-
age to the wall. The length of the stemming in
the buffer holes may also need to be adjusted
depending on the degree of fracture along the
crest of the bench.
11.4.1 Pre-shearing and cushion blasting
On permanent slopes for many civil projects, even
small slope failures are not acceptable, and the use
of controlled blasting to limit damage to the final
wall is often required; an example of controlled
blasting is shown in Figure 11.10. The prin-
ciple behind these methods is that closely spaced,
Figure 11.10
Example of
controlled blasting for rock cut
on highway project (strong,
highly folded gneiss, I-26/US23,
near Mars Hill, North Carolina)
(courtesy: North Carolina
Department of Transportation).
