Civil Engineering Reference
In-Depth Information
(a)
(b)
3
4
6
2
1
3
2
1
2
3
4
5
(c)
Free face
175
192
209
226
243
260
277
Free end
217
234
251
268
285
302
319
17 ms surface delay
42 ms surface delay
175 ms in-hole delay
Actual firing time
of hole (ms)
259
276
293
310
327
344
361
403
301
318
335
352
369
386
403
Figure 11.7
Typical detonation sequences: (a) square “row-by-row” detonation sequence; (b) square “V”
detonation sequence; (c) hole-by-hole detonation using both surface and in-hole non-electric delays
(W. Forsyth).
blasting in strongly jointed rock where near ver-
tical joints strike across the bench at an angle to
the face.
In many cases, blasting results can be improved
by introducing hole-by-hole firing, where every
blast hole is initiated in sequence at a unique time
(Figure 11.7(c)). Where appropriate delays are
selected, hole-by-hole initiation exploits the pos-
itive benefits of blast hole interaction while avoid-
ing most of the negative effects. This can lead to
finer fragmentation, looser muckpiles, less over-
break, lower ground vibration, and better control
over the position and profile of the final muckpile.
Hole-by-hole firing can be achieved by using in-
hole detonators. However, the available range of
commercial in-hole delays is limited and, hence
hole-by-hole initiation is usually achieved by
using a surface delay system to control blast hole
sequencing. If long inter-row or inter-hole delays
are required, a combination of surface and in-
hole delays will avoid downline cutoffs caused
by ground movement during the blast. For the
detonation sequence shown in Figure 11.7(c),
there is an identical in-hole delay of 175 ms in
each hole, together with surface delays of 17 ms
along the free face, and 42 ms delays between the
rows. The diagram shows the actual firing times
of each hole using this delay arrangement.
The required delay interval is related to
distance between holes by the following two
relationships.
For row-to-row detonation:
Time delay between
rows
(
units: ms
)
(
10-13
)
×
(burden)
≈
(
units: m
)
(11.7a)
For example, for a 5 m burden the delay between
rows is 50-65 ms.
For hole-to-hole detonation:
Time delay
between holes
(
units: ms
)
(delay constant)
×
≈
(hole spacing)
(
units: m
)
(11.7b)
