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Geotechnical News • March 2016
GEOTECHNICAL INSTRUMENTATION NEWS
tures. These types of events provide
very little warning before they happen
and therefore are very hard to predict.
On the other hand, man-made vibra-
tion sources like construction activi-
ties, blasting, mining, pile driving,
dynamic compaction, tunneling, train
and vehicle traffic and people are quite
easy to predict.
Main goals of vibration
monitoring
When it comes to vibration monitoring
the main goals are to protect people
and assets. The more we monitor the
better we understand how these vibra-
tions impact our lives. Monitoring
natural events helps us improve our
predictive models and possibly take
action sooner to reduce their effect
on people. It also helps us understand
these forces which can then be used
to help improve our structural designs
and construction activities. The moni-
toring of man-made vibrations will
also help protect people and improve
our construction activities. However,
in many countries around the world
there are also legal limits that have
been established for man-made vibra-
tions. These limits are generally set to
reduce the vibrations that might have
an effect on people and to prevent
damage to a wide range of structures.
This article will focus on the moni-
toring of man-made vibrations and
present some of the different aspects
of vibration monitoring that should be
considered.
Things to consider
Vibration limits
Before starting any project you must
first understand what the vibration
limits are. The vibration limits will
provide key information on the type
of sensor that should be used on your
project. Many countries have devel-
oped their own general vibration
limits, however some stakeholders of
the project may choose to implement
even stricter limits.
To make sure you understand the
vibration limits of your project, you
will need to answer at least four ques-
tions:
1. Will you be measuring velocity,
acceleration, displacement, strain
or decibels?
2. Will these measurements be peak
or RMS values?
3. What dynamic range is required for
the sensors?
4. What is the frequency range to be
monitored?
Choosing the sensor and data logger
Many software programs today pro-
vide tools to convert back and forth
between velocity, acceleration and
displacement or to calculate strain
and display results in decibels based
on a reference level. Whether you
choose a geophone, an accelerometer
or some other sensor you will need to
make sure the data logger and soft-
ware package will be able to convert
the data into the desired units. If
you choose a geophone and need to
report the results in acceleration you
will need to differentiate the veloc-
ity results to obtain the acceleration.
If this is a manual process and you
have thousands of events to con-
vert, it might be better to choose an
accelerometer to start with. Whatever
sensor you choose make sure the data
is recorded with enough resolution to
be able to convert the results to the
desired units with an adequate resolu-
tion.
When choosing a sensor make sure
it has the dynamic range, resolu-
tion and frequency response to meet
your requirements. Choosing a 500g
accelerometer with a 3000 Hertz (Hz)
frequency response may not make
sense if your limits are 40g and 750
Hz. Generally, you will want to select
a sensor that has a dynamic range and
frequency response that are slightly
larger than your requirements. If your
limits were 40g and 750 Hz then select
an accelerometer that has a range of
50g and 1000 Hz response. Once you
have a sensor in mind make sure the
data logger can provide the resolu-
tion you need. The resolution will
be based on the analogue to digital
convertor (A/D) that is used in the
data logger. This can often be found
on the data sheet for the data logger.
If the data logger had an 8 bit A/D the
best resolution it could provide for a
50g accelerometer would be 0.2g (50/
(2
8
). If the data logger had a 16 bit
A/D the resolution could be as small
as 0.00076g.
What is being monitored?
Now that we understand the vibration
limits and type of sensor we need, we
now need to understand what is being
monitored. This will help to determine
how and where the vibration sensors
can be installed. Monitoring a build-
ing is very different from monitoring a
stained glass window in the building.
There are several methods of installing
the sensors, the most reliable being to
attach the sensor directly to the struc-
ture being monitored. However other
methods like burying the sensor in the
ground next to the structure and some-
times coupling the sensor to a surface
with sandbags can also be used. The
main goal is to install the sensor in
such a way that it will experience the
same vibration as the structure that
is being monitored and not decouple
(move independently) from the struc-
ture. It is also important to understand,
that if the sensor is attached directly
to a structure, where it is attached can
affect the results. Attaching the sensor
in a corner will have a very different
result to attaching it in the middle of
the wall.
The International Society of Explo-
sives Engineers (ISEE) have devel-
oped a “Field Practice Guidelines for
Blasting Seismographs” that can be
found on the Internet. This guideline
contains useful information on the
placement and installation of the sen-
sors.
What frequency response do you
need?
The type of structure being monitored
will also help determine the frequency
response and sample rates that are
required. Generally, you will want to
