Civil Engineering Reference
In-Depth Information
is that, at equal curing times, higher vibration
levels are permitted at closer distances.
In critical conditions it is recommended that
vibration measurements and strength tests be con-
ducted to confirm the performance of the concrete
and the relationships given in Table 11.7.
250
50 mm/s safe
structural limit
50
Intolerable
25
Electronic equipment and machinery.
Some
types of electronic/electrical equipment are sens-
itive to vibrations. Studies have been done on
computer disk drives, telecommunication equip-
ment such as relay stations and fiber optic cables,
as well as electrical equipment such as older
model power transformers that have mercury
cut-out switches. Vibrations from blasting, pile
driving, or other construction activities can inter-
fere with the operation of this equipment, and in
the absence of guidelines from the manufacturer
on allowable vibration levels, it may be necessary
to conduct carefully calibrated test blasts.
Human response to blast vibrations.
Humans
are very sensitive to vibrations and can feel the
effects of a blast well outside the potential dam-
age zone. Figure 11.21 shows the relationship
between peak particle velocity, frequency and
the possible human response to vibrations. This
indicates that low frequency vibrations are more
readily felt than high frequency vibrations. The
frequency of blast vibrations is usually in the
range of 50-200 Hz.
Control of vibrations.
The magnitude of blast
vibrations at a particular location is dependent
upon the distance from the blast and the charge
weight per delay. Of the blast design factors dis-
cussed in Section 11.4, the most important are the
use of delays and the correct detonation sequence
so that each hole, or row of holes, breaks toward
a free face.
To control vibration levels at a particular dis-
tance from the blast, it is necessary to limit the
explosive charge detonated per delay according
to the relationship given in equation (11.16).
Calculation of the allowable charge weight per
delay, from either trial blasts or design charts, will
determine how many holes can be detonated on a
single delay. If the charge weight in a single hole
Unpleasant
5
2.5
Perceptible
0.25
11
2
4
6
10
20
40
60
100
Frequency (cps)
Figure 11.21
Human response to blast vibrations
related to particle velocity and frequency (adapted
from Wiss, 1981).
is more than that allowable, then either shorter
holes must be drilled, or a decked charge could
be used. In a decked charge, the explosive load
is separated with stemming material, and each
charge is detonated on a separate delay. The min-
imum delay interval between charges or holes in
order to limit the risk of constructive interference
of vibrations produced by each charge is about
15-20 ms.
Pre-blast surveys.
At locations where there
is a potential for damage to structures due to
blast vibrations, it is usual to carry pre-blast sur-
veys on all structures within the potential damage
zone. These surveys should record, with pho-
tographs and/or video tape where appropriate,
all pre-existing cracks, other structural damage
and settlement problems. The Office of Surface
Mining (OSM, 2001) has drawn up a stand-
ardized method of recording structural damage
which ensures that the survey is systematic and
