What is vibration & air
over pressure?
How do we
measure vibration?
Regulations and
standards
Human
response to vibration
Consultnet
vibration monitoring services
What is
vibration & air over pressure?
Ground-borne vibration can be
generated by a number of sources, including road and railways, construction activities such as piling, blasting and tunnelling.
Vibration can be defined as regularly
repeated movement of a physical object about a fixed point. The parameter
normally used
to assess the ground vibration
is the peak particle velocity (ppv) expressed in millimetres per second (mm/s).
In order to completely define ground vibration, the amplitude and
frequency of the motion are measured in the three orthogonal directions
generally in terms of velocity which is considered to be the best descriptor for
assessing human comfort and the potential damage response of structures. The
vibration velocity signals are summed (in real time) and the maximum amplitude
of this vector sum is defined as the Peak Vector Sum (PVS).
Vibration can cause varying degrees of damage in
buildings and affect
vibration-sensitive
machinery or equipment . Its effect on people may be to cause disturbance or
annoyance or, at higher levels, to affect a person’s ability to work.
To put vibration levels in context, below is a list of common
tasks and the level of vibration they produce:
|
Activity |
Vibration (mm/s – Peak Particle
Velocity(PPV) |
|
Jumping |
Up to 250 |
|
Hammering nail |
Up to 100 |
|
Sliding door |
Up to 10 |
|
Shutting door |
Up to 30 |
Air overpressure is energy transmitted from
a blast site within the atmosphere in the form of pressure waves. As these waves
pass a given position, the pressure of the air rises very rapidly then falls
more slowly then returns to the ambient value after a number of oscillations.
The maximum excess pressure in this wave is known as the peak air overpressure,
generally measured in decibels, using the linear (or unweighted) scale to obtain
an unfiltered reading of the change in pressure. The unit used is dB Linear.
Vibration generating activities which can
cause annoyance and are typically monitored form an environmental impact
perspective include:
Blast vibration is very dependent on the
sites geological condition, distance to the blast and the blasting technique and
amount of explosives used. It can be designed to ensure vibration and air
overpressure levles are within acceptable limits. Blast design can be modified by:
- Number of blast holes
- Weight of explosive
- Amount of stemming
- Delay timing
Typical levels
measured during construction activities are shown below:
|
Construction Activity |
Typical Ground Vibration Level |
|
Vibratory roller |
Up to 1.5mms @ 25m |
|
Hydraulic rock breakers |
4.5 mm/s @ 5m, 0.4 @ 20m, 0.1 @ 50m |
|
Compactor |
20mm/s @ 5m, <0.3mm/s @30m |
|
Pile driving |
1-3mm/s @ 50m depending on soil conditions and piling technique |
|
Bulldozer |
1-2mm/s @ 5m, 0.1 @ 50m |
|
Truck traffic (smooth surface)
Truck traffic (rough surface) |
<0.2mm/s @ 20m
<2mm/s @ 20m |
How do we measure
vibration?
The measurement of vibration is undertaken using specialist
monitors, such
instrumentation, termed a seismograph, is capable of recording both ground
and airborne vibration. Ground vibration is recorded in terms of peak
particle velocity in millimetres per second in 3 mutually perpendicular
directions (T, V & L). Airborne vibration is measured in terms of decibels (dB).
Seismographs can be left unattended and set to trigger when an
emission level exceeds a predetermined set level.
Below is a typical vibration event report produced using an
Instantel Minimate Plus monitoring blast vibration and air overpressure during
rock removal on a major motorway project.
The ground vibration records shows the variation of velocity with
time. Each trace has a point where the velocity is a maximum (+ve or -ve) and
this is known as the
Peak Particle Velocity (or PPV) which has units of mm/s.
Monitoring usually takes place at the closest vibration sensitive building to current
operations. Where blasting/vibration generating construction takes place in more than one area within a
site then more than one monitoring location may be necessary. In some situations access to a vibration sensitive building
may not be practicable. In this case, consideration can be given to the
selection of a location away from the building in a general line with the area
to be blasted and at which monitoring could be regularly undertaken. Such
locations may be at or just within the site boundary.
British Standard 7385: Part 1, 1990 discusses the measurement
of vibration in buildings in general terms with more specific advice for damage
investigation given in BS 7385: Part 2, 1993 and for human perception in
BS 6472, 1992.
When vibration limits are set,
they usually refer to recordings taken at ground level at
specified properties (usually the closest). BS7385:Part 1
states "Where the purpose is to monitor with regard to
imposed vibration, the preferred position is at the
foundation, a typical location being at a point low on the
main load-bearing external wall at ground floor level when
measurements on the foundations proper are not possible".
The reason for this is that structural damage criteria from
around the world specify vibration limits that apply at
foundation level. Indeed the vast majority of damage noted
in studies does not occur on upper floors in structures but
mainly on lower floors where the strains are greatest due to
the confinement of the foundation.
If transducers are placed on the ground alongside the
building being monitored, the recorded vibrations can be
significantly affected by surface or near-surface features
which may have a very localised affect. At high levels of
vibration which occur at certain frequencies, it is also
possible for transducers to leave the ground. Again BS7385:Part 1
suggests that when monitoring on the ground, a stiff steel
rod should be driven into the ground, through the loose
surface layer, and the transducer attached to ensure close
contact with the ground. Alternatively it can be fixed to a
rigid surface plate such as a well-bedded paving slab. Some equipment
manufacturers suggest placing a small sandbag on top of the transducer if it is
simply placed on a hard surface.
Regulations and Standards
Vibration standards
come in two varieties: those dealing with human comfort and those
dealing with cosmetic or structural damage to buildings. In both instances, the
magnitude of vibration is expressed in terms of Peak Particle Velocity (PPV) and millimetres per second (mm/s).
Guidance relevant to acceptable vibration at the foundation of
buildings is contained within BS 7385 (1993): Evaluation and measurement for
vibration in buildings Part 2: Guide to damage levels from ground-borne
vibration. This states that that there should typically be no cosmetic damage if
transient vibration does not exceed 15mm/s at low frequencies rising to 20mm/s
at 15Hz and 50mm/s at 40Hz and above. These guidelines relate to relatively
modern buildings and are normally be reduced to 50% or less for more critical
buildings. Critical buildings include premises with machinery that is highly
sensitive to vibration or historic buildings that may be in poor repair,
including residential properties.
The German standard DIN4150 provides limits below which it is
very unlikely that there will be any cosmetic damage to buildings. For
structures that are of great intrinsic value and are particularly sensitive to
vibration, transient vibration should not exceed 3mm/s at low frequencies.
Allowable levels increase to 8mm/s at 50Hz and 10mm/s at 100Hz and above.
In Ireland, the National Roads Authority (NRA) have issued
Guidelines for the Treatment of Noise and Vibration in National Road Schemes and have indicated
( Table 2-NRA Guidance)typically deemed acceptable vibration levels in order
to minimise the risk of building damage during road construction as shown below:.
In practice, construction vibration limits may be found in
planning permission conditions for large projects which are likely to generate
potentially significant vibration, limits are usually set which are below the thresholds of structural and cosmetic
damage in order to limit nuisance. In addition EPA IPPC Licenced
facilities such as large quarries and mining operations typically have vibration limits of
8-12mm/s daytime and 4mm/s night time
BS 7385 1993 as referred to above gives guide values with respect damage classifications for residential structures in terms of peak
particle velocity and frequency. These values are based on the lowest vibration
levels above which damage has been credibly demonstrated. Building
Research Establishment (BRE) Digest 353 (July 1990): Damage to structures from
ground borne vibration is also a useful document in vibration assessment.
British Standard 6472, 1992, Guide to evaluation of human exposure to vibration
in buildings (1Hz to 80 Hz) and British Standard 5228: Part 3, 1997, Noise and
vibration control on construction sites, Part 3.Code of practice applicable to
surface coal extraction by opencast methods are also applicable in this
respect.
For additional information standards see the
Acceptable levels on the Good Quarry website
Human Response to Vibration
Human response to blast induced ground vibration is a
relatively complex phenomenon and is dependent upon a range of factors of which
the actual vibration magnitude is only one and not necessarily the most
important. It is well recognised that the human body is very sensitive to the
onset of vibration albeit very poor at distinguishing relative magnitudes.
Although sensitivity to vibration varies significantly between individuals, a
person will generally become aware of blast induced vibration at levels of
around 0.5 mms-1 peak particle velocity, however individuals are very
poor at distinguishing between vibrations of differing magnitudes.
This threshold of perception of
vibration is very much lower than the onset of even cosmetic
damage (plaster cracking at typically least <12mm/s).
Once a received vibration is greater than an individual’s
perception threshold then it is possible for concern to be expressed about the
blasting or construction activity involved. Such concern normally relates to the vibration’s potential for causing
damage to the complainant’s property. Concern may be expressed that damage has
already occurred due to the recent discovery of cracking that may have been
present for some time or have been caused by natural processes. More often,
however, concerns are based on the fear that damage will be caused at some time
in the future as a result of repeated vibration. It is usually the case that
adverse comments are less likely once a neighbour has become accustomed to the
perceived effects of blasting.
Good communication is one of the best ways to help minimise vibration complaints
around a construction site. Keeping neighbours informed of the nature of the
work and progress is a great way to help alleviate unnecessary concern.
Consultnet Vibration Monitoring Services
Our Consultant have been involved in vibration monitoring and
control on Ireland's largest mining and construction projects during the past
twenty years, our services include:
-
Monitor and measure vibration and air overpressure from construction blasting or
piling activities
-
Monitor and measure vibration and air overpressure from mine
and quarry blasting
-
Advice on vibration prediction and mitigation
-
Community interface and complaint management
At Consultnet we use
the Instantel Minimate monitors for
routine vibration and air overpressure
monitoring activities.
See also
Consultnet Noise Monitoring Services
Links to other sources of information on environmental vibration:
Environmental Vibration