1. Introduction
This guide is intended for use by anyone who wishes to gain a basic
understanding of how to monitor and quantify ground borne vibration arising
from piling and other activities caused by civil engineering works.
The author has many years’ experience working in the field measuring
vibration in order to prevent damage to structures during building projects.
Why is it necessary to measure ground vibration?
It goes without saying that in order to quantify a problem, or potential problem,
it is first necessary to measure it.
There are basically three reasons why there may be a requirement to
measure vibration during civil engineering works:
a. To ensure that levels of vibration do not cause damage to
b. To prevent annoyance to people by maintaining the lowest
possible levels.
c. To demonstrate compliance with conditions.
It is also becoming a matter of ‘Best Practice’ to carry out ground vibration
monitoring during construction projects as part of company quality controls.
This guide will explain, in an easy to follow and practical manner, the
fundamentals of ground vibration and how to measure it in order to satisfy the
above three points.

2. Units of Measurement
Ground vibration is measured in terms of Peak Particle Velocity (PPV) with
units in mm/s.
It should be noted that the PPV refers to the movement within the ground of
molecular particles and not surface movement. The displacement value in
mm refers to the movement of particles at the surface (surface movement).
The mathematical formula being:
PPV=2πfa where PPV is in mm/s
π = 3.142
f = frequency in (cycles per second) Hz
a = displacement in mm
The formula is true only for sine waves.
3. Instrumentation
Monitoring of ground vibration is carried out using a vibrograph, also known
as a seismograph. It must be understood that this is a very different
instrument to that used for measuring earthquakes. Ground vibration from
civil engineering projects results in frequencies above 4Hz, whereas
frequencies arising from earthquakes are around 1Hz or less.
Worldwide, there are several manufacturers of these instruments and there
are various styles of vibrograph, typically:
a) Basic non-logging with built-in printer – this is the traditional older type.
It has no memory and output is entirely dependent upon the printer.
b) Data logging unit – no printer, data is stored in memory for subsequent
download to a computer.
c) Combined type – has memory and printer, data may be printed out but
is also stored for download to a computer for post processing and
d) Virtual instrument – this is a computer with an interface for the sensors.
A software program effectively turns the computer into a ground
vibration monitor.
Each type has its merits depending upon the requirements of the project.
The combined unit, described in (c) above is the most versatile. If for any
reason the printer fails, the data is stored, conversely if the memory storage is
corrupted or fails the printer may still have produced an output.
A further advantage of this type of vibrograph is that it enables the user to see
the output as it is generated by the on board printer. This gives a complainant
added reassurance that the data is real and has not been altered after being
downloaded to the computer.

3. Ground vibration monitors (vibrographs) usually detect vibration by means of
transducers known as geophones. A geophone consists of a powerful
permanent magnet around which there is a coil made up of very many turns of
fine copper wire. The mechanism is contained in a tubular metal casing.
The cylindrical magnet is held centrally in place, within the coil, by delicate
A geophone operates as follows:
The casing held in contact with the ground (fixing methods are discussed in
the section entitled, methodology) moves with any ground disturbance
(vibration), the magnet remains steady within the assembly. A voltage,
proportional to the movement of the coil is produced. This alternating
(analogue) voltage is fed along a cable to the vibrograph where it is digitised
and sampled. Electronic circuitry then processes the signal to produce
vibration data. Cables may be several hundreds of metres in length without
loss or degradation of the signal.
Vibrographs use three geophones, two in the horizontal plane at ninety
degrees to each other, and one in the vertical plane. This (orthogonal)
arrangement enables vibration from three directions, lateral, transverse and
vertical, to be detected simultaneously. Whilst all three geophones look the
same, the vertical geophone is designed only for use in an upright position
and the horizontal geophones are designed for horizontal use.
The vibrograph is able therefore to measure vibration in three axes and to
compute the vector sum of the three axes (also known as the resultant). It is
important to realise that British Standards refer to maximum plane readings,
the highest level from any one of the three axes, and not the resultant. The
resultant will always have a larger value (up to ten percent higher) than the
maximum plane reading.
Note that the physical movement of the geophones coil, about the magnet, is
in the order of a few millimetres (typically 4 to 6), which with most vibrographs
allows for the detection of peak particle velocities up to around 200mm/s.
Manufacturers instruments vary, some operate on a single scale between
0.1mm/s up to 200mm/s. Others however may have different ranges and
require the appropriate range to be selected.
Vibrographs, almost always, have two modes of operation, waveform or bargraph. These terms vary between manufacturers and may be known as:
impulse or continuous, trigger or histogram (bargraph) respectively.
Waveform mode (may also be called trigger mode or impulse mode) is
generally used for monitoring vibration from blasting in mines or quarries. In
this mode, the instrument records only when vibration exceeds a pre-set
(trigger) le
called the scan time) often user selectable between 1 and 10 seconds. At the
end of this time, the unit goes back to its standby mode ready for another
waveform event to be recorded. The data is processed and stored or printed
out, if a printer is fitted.
Note: This mode of operation is not suited to long-term recording, as is
required when measuring vibration from civil engineering work, for the
following reasons:
Far too much data would result, typically one page per event.
The instruments memory would soon be filled to capacity.
Almost every vibrograph used in this mode, also allows for the measurement
of air pressure using a pressure transducer (often an optional extra), usually a
microphone. However, it must be borne in mind that this pressure
measurement, which might very well use a decibel scale, is not acoustic
noise, but a low frequency (typically 2Hz to 150Hz) peak pressure level
having a linear response (i.e. no weighting filters are applied – not dB(A)).
This feature, usually only inherent in the waveform mode, is to allow for the
measurement of a blast wave during explosive events such as those used in
quarrying activities and must never be used to try to measure acoustic noise.
Note: Acoustic noise measurements are made between 31Hz and 20kHz with
a filter (‘A’ weighting) to replicate the response of the human ear and use a
dB(A) scale.
Bar-graph mode (may also be called continuous mode, bargraph mode or
piling mode) is used for long term monitoring and is particularly useful for
measuring ground vibration during civil engineering work.
In this mode, the vibrograph is constantly storing data that is presented on the
printout as a histogram (bar-graph). The histogram shows peak particle
velocity against time. The output shows the highest level from any axis. This
way there is a continual record of the event which may be of many days
duration, subject to memory constraints, paper roll (on those units that do not
store information) and battery life.
Possible accessories are: Alarms, solar panels, mains adapters, car battery
leads and modems.

4. British Standards
The British Standards that provide guidance for measuring ground-borne
vibration are:
BS:7385 Evaluation & Measurement for Vibration in Buildings
Part 1 – Guide for Measurement of Vibrations & Their Effects on
Part 2 – Guide to Damage Levels from Ground-borne Vibration
BS:5228-2:2009+A1:2014 Code of Practice for Noise & Vibration Control
on Construction and Open Sites – Part 2: Vibration
BS:6472 Evaluation of Human Exposure to Vibration in Buildings
Note: BS:7385 and BS:5228 are concerned with levels of vibration that may
cause damage, whereas BS:6472 is a guide for nuisance levels. The latest
revision of BS:5228, BS:5228-2:2009+A1:2014 now contains an addendum,
Table 9.4: Guidance on Effects of Vibration Levels.
Vibration levels that cause damage depend upon the peak particle velocity
and the frequency at which it occurs. Damage to property is likely where peak
particle velocity is high when its frequency is low.
For example, due to technical difficulties there are few, if any, vibrographs that
measure frequency as well as ppv in bar-graph (continuous) mode and so a
worst-case situation is adopted when monitoring.
Piling using a drop hammer is considered to be of an intermittent nature. The
ground vibration has settled between hammer drops. To prevent any damage
levels of ppv must be kept below 15mm/s.
Piling with vibratory rigs leads to continuous ground vibration. To prevent any
damage levels of ppv must be kept below 7.5mm/s.
It should be noted that the above values are used for buildings of normal
construction and in a stable condition. For historic buildings, gas pipes, oil
pipelines and cast iron water mains, different criteria may apply and the
above levels must not be relied upon. Maximum levels of vibration may be
specified within the plans for the particular works.
The British Standards provide full information, whereas this guide is written to
help ‘the man on the ground’ with little prior knowledge to be able to carry out
a sensible regime of monitoring and acquire accurate results whilst monitoring
at the most commonly found structures.
Much of the data used to formulate monitoring standards derives from years’
worth of data collected by the former United States Bureau of Mines (USBM).

The analysis of data compiled provides very accurate indicators as to levels
where even the most slight on-set of cosmetic damage may result from
ground-borne vibration.

5. Methodology
Ground vibration measured on a building or structure should, unless
specifically stated otherwise, be measured outside the structure and at ground
Almost all buildings will have some cracks in plasterwork due to shrinkage,
temperature changes or some other cause. In some cases it may be a
sensible precaution to carry out a crack survey before the commencement of
any work. This would normally be a visual check backed up by photographs
showing any cracks or other deformations.
A vibrographs sensor is provided by the tri-axial geophone pack. It is crucial
that this is fixed (or coupled) properly to the structure being monitored.
Methods of fixing include; bolting, gluing (using epoxy resin), spiking into the
ground, sandbagging or burying.
Try to site the geophone pack on or at the structure nearest to the vibration
source (the piling rig, for example).
The geophone pack must be level (within 10 degrees). There is normally an
arrow on the geophone pack indicating the linear axis. It is a convention to
orientate the arrow so that it points to the vibration source.
The geophone pack must never be used by placing it on its side, nor upside
down due to the construction of the vertical sensor.
If a sandbag is used it must be loosely filled and placed so that its sides touch
the ground around the geophone pack, whilst ensuring that the geophone
pack remains level.
Never site the geophone pack on a paving slab, as this may independently
move giving rise to higher readings than the adjacent structure is being
subjected to.
Do not place a brick, or other heavy object, on top of the geophone pack.
There is a very strong chance that it will lead to higher readings. Its centre of
gravity will be raised and the object may well move independently of the
geophone pack giving spurious readings.
Prevent the geophone pack from being knocked or disturbed by objects,
people or animals and thus giving false readings.

6. Daily Checks
If possible, remove the paper record or download the instrument after each
days monitoring session and ensure that it is safely stored or given to the
relevant person. Ensure that levels have not been exceeded.
Clear the memory and check that there is sufficient paper for the next
monitoring session. Ensure that the instrument is clean and dry before
storing. Check the battery and charge if necessary.

7. Calibration
It is important that vibrographs are in calibration.
Vibrographs are supplied with a calibration certificate traceable to national
standards. Manufacturers recommended that vibrographs are serviced and
re-calibrated annually, even if they have not been used. The geophone
sensors contain permanent magnets, whose magnet field strength reduces
slightly year on year due to natural decay. Variations in magnetic field
strength are compensated by calibration adjustments.
Annual service and calibration may also highlight other problems, some of
which may be transparent to the user.
In the event of litigation, accuracy is determined by the possession of a
current calibration certificate.