Civil Engineering Reference
In-Depth Information
areas of weakness back into the finished wall
and produce overbreak. Therefore, in weak and
highly fractured, or faulted rock, complete stem-
ming between and around individual charges may
be desirable.
the burden. The burden is given by
Cushion blasting: Burden
≥
(
1.3
)
×
(
hole spacing
)
(11.11)
11.4.6 Hole detonation sequence
11.4.5 Spacing and burden
The spacing between blast holes on the final
line differs slightly between pre-shearing and
cushion
As discussed in Section 11.4.1, the main differ-
ence between pre-shearing and cushion blasting
is the detonation sequence. In pre-shearing, the
final row holes are detonated first in the sequence,
and in cushion blasting, the final row holes are
detonated last in the sequence. Furthermore, sat-
isfactory results are obtained by detonating each
hole along the final line on separate delays, and it
is not necessary to use a single delay for the full
length of the final wall blast.
Figure 11.14 shows a possible blast hole lay-
out and detonation sequence for pre-shearing a
through-cut. This excavation is suitable for pre-
shearing because the burden is infinite and there is
no possibility for prematurely displacing the bur-
den. The shear line holes on either side of the cut
are detonated first, followed by the production
blast as a 'V' cut. Theoretically, the length of a
pre-shear shot is unlimited. In practice, however,
shooting far in advance of primary excavation
can be troublesome if the rock characteristics
blasting,
as
given
by
the
following
relationships:
Pre-shearing: Spacing
≈
(
10-12
)
×
(
hole diameter
)
(11.10a)
Cushion blasting: Spacing
≈
(
16
)
×
(
hole diameter
)
(11.10b)
For pre-shearing the burden is effectively infinite,
but as discussed in Section 11.4.1, it should be at
least equal the bench height. For cushion blast-
ing, the burden should be larger than the spacing
so that the fractures preferentially link between
holes along the final wall and do not extend into
Advance
pre-shear
8
7
65 5
6
7
8
Detonating cord
trunkline
7
6
5
4
4
5
6
7
6
5
4
3
3
4
5
6
5
4
3
2
2
3
4
5
Instantaneous
cap
4
3
2
1
1
2
3
4
Figure 11.14
Blasting procedure
for through-cut showing delay
sequence for pre-shearing during
production blasting (adapted
from ISEE, 1998).
Pre-sheared from previous shot
Excavated area
