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Chapter 26
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Orlylnel Free Fece
Orlaln•I Free Fece
Figure 26-E. Greater rellel on holes of 7th perbd tlelay pmtlucetl by tiring mulflpl• my'
pattern on a 2:7 elope using twice the delay time between rows as between holes In a mat, •,
• Use a blast design that produces the maximum relief practical,
Explosions in boreholes which have good relief, i.e., those having
nearby free faces, produce less ground vibration. The use of delay
blasting techniques establishes internal free faces from which com-
pressional waves produced later in the blast can reflect. By properly
designing the delay patterns, maximum relief can be retained.
In general, when blasting multiple-row patterns, greater relief
can be obtained by using a greater delay time between rows than
between the holes in a single row as illustrated in Figure 26-E. A
delay time between holes in a row otat least one millisecond-per.foot
of burden is recommended for the necessary relief and best fragmen-
tation.
• Use the proper powder factor. An excessive powder factor can
increase both vibration and air blast and may cause excessive bur-
den displacement or throw rock. On the other hand, an insufficient
powder factor can increase vibration by delaying and reducing the
effect of rarefaction waves reflected off the free faces. The optimum
powder factor must be determined by experimentation at any given
blasting site and used.
• Use aspacing-to-burden ratio equal to or greater than one, if
possible. The presence of weak seams or irregular backbreak may
dictate the use of aspacing-to-burden ratio less than one.
• Control drilling of blast holes as closel}~ as possible. Establish
bench marks for use in setting out hole locations of the next blast
before the blast is made help avoid possible errors because of irregu-
lar backbreak.
• -~eep the amount of subdrilling to the minimum required to mltin-
[ain good floor conditions. A typical amount of subdrilling is 0.3
times the burden at floor level. Tape each borehole and match it with
the face height. If hole depth is more than the intended amount,
b'ackfill with drill cuttings or crushed stone. Excessive subdrilling
432
Vibration and Air Blast
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Timer Setling Duration of
+ Shot - me
p 1026
100
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42
1 1 1 1 1 1 1 1 1 1 1 1 1 r l 1
Vedleal Scale: .le Ips/cm F--~ 1 cm Horizontal Scale: 100 ma/cm
26•F. Hecortlings of longitudinal component of two similar single-row quarry
initiated with timer. Shots had 8,400 pountls of explosives distributed in 27 holes.
can increase vibration because of high confinement and lack of
nearby free face to create rarefaction waves.
•Use a•arious techniques to reduce charge-weight-per-delay and,
in turn, the peak particle velocity.
1. Reduce hole depths -lower bench heights.
2. Use smaller diameter holes.
3. Subdivide explosive charges in hole by using inert decks and fire
each explosives deck with an initiator of a different delay period.
4. Use electronic or mechanical timers to increase the available
number of periods of delay electric blasting caps and to increase
timing flexibility. Nonelectric delays coupled with surface delay
connectors can provide similar flexibility.
As an example of the use of timers in conjunction with delay
electric blasting caps, Figure 26-F shows two recordings of the lon-
gitudinal components of ground motion recorded at the same loca-
tion from blasts on the same face of a limestone quarry having the
same geometry, number of holes, and explosive type and weight. The
timer used to initiate these blasts was the electronic Sequential
Timer developed by Research Energy o(Ohio initially for use in coal
stripping. Both shots used two explosives decks of 200 pounds each
separated by a four-foot deck of crushed stone. Shot A was program-
med to give 25 milliseconds between explosives decks and 50 mil-
liseconds between holes. Shot B was programmed [o give 75 mil-
liseconds between decks and an average of 21 milliseconds between
holes. The total shot duration for shot B was 503 milliseconds, which
was less than one-half of the 1,025 milliseconds used in shot A.
While the peak particle velocity and frequency content were similar
for the two shots, the kinetic energy in the ground motion for shot B
was only two-thirds that of shot A. In addition, the duration of the
ground motion was substantially less. For the same peak particle
433

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