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:
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
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.
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:
|Allowable vibration velocity (Peak Particle Velocity) at the closest part of any sensitive property to the source of vibration, at a frequency of|
|Less then 10Hz||10 to 50Hz||50 to 100Hz (and above)|
|8 mm/s||12,5 mm/s||20 mm/s|
|Allowable vibration during road construction in order to minimise the risk of building damage.|
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 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.