A8.3 - Vibration

Vibration

Two types of vibration are evident alongside traffic routes; ground-borne vibrations and low frequency sound which can result in building vibrations.

The primary cause of ground-borne vibrations is the variation in contact forces between vehicle wheels and the road surface. The interaction between vehicle tyres and road surface irregularity can result in the release of significant energy. Therefore, roads with surface irregularities generate more vibrations than new, smooth roads. Once produced, ground conditions markedly affect the way in which ground-borne pressure waves are propagated. Also, distances between the road and dwelling locations will determine how much vibration energy actually reaches nearby properties.

Air-borne low frequency sound below 100 Hz can also induce building vibration. The primary cause of these vibrations is low-frequency vehicle-produced sound, which enters the building and can excite the building structure and/or the contents. This excitation at the natural frequency of the structure being excited is highly dependent upon the type of building structure, and its proximity to the road. In general, air-borne vibration is taken into account in the assessment of noise effects, ie, locations likely to experience significant air-borne traffic-induced vibrations are likely to have been assessed as high noise areas and the impact determined according to appendix A8.2.

Traffic induced vibrations are evident in many parts of New Zealand and variations occur because of sub-soil geological factors such as high water tables, light volcanic sub-soil, or peaty soils. Generally the levels of vibration perceived will be a function of vehicle size, speed, proximity to the road, sub-soil geology, building characteristics, and sensitivity at the receiver location.

Impacts of vibration

The mechanism of vibration disturbance for persons inside a building is a complex combination involving structural vibration and low frequency sound which may be either heard or felt as a body vibration. Both forms of traffic-induced vibration may produce resonance, which is perceived as sound (eg, rattling of windows), or perceived as a body vibration. Such factors as the direction of the vibration, the frequency distribution of the vibrations, and the time history of the vibrations should be taken into account for a comprehensive assessment.

Two main attributes are used to assess vibration, these are peak particle velocity and acceleration. For particle velocity it is generally sufficient to assess the impact of traffic-induced vibrations. This is based on the premise that traffic-induced vibrations are 'event based' and not generally continuous in nature. Where traffic-induced vibrations are of a continuous nature detailed procedures for measurement and assessment are contained in such documents as BS 6472:1992 Guide to evaluation of human exposure to vibration in buildings.

Assessment criteria

The following two criteria are designed for the assessment of traffic-induced vibration for sporadic traffic events such as the passing of heavy vehicles in proximity to vibration sensitive locations (eg, residential housing, schools, hospitals, etc.) If the criteria for level one are met, then this shall be reported and no further assessment is required. If the criteria for level one are not met, a level two assessment is required which will involve a more detailed investigation.

(a) Level one criteria

Traffic induced vibration is assessed as not likely to cause adverse reaction if all the following criteria are met:

(i) The minimum site-back distance between the building location and the nearside edge of the traffic lane conforms to the minimum distance of 12 metres specified in appendix A8.2.

(ii) The road surface is reasonably smooth and meets a set minimum NAASRA count level. In 100 km/h posted speed limit areas a minimum roughness guide is 100 NAASRA counts (3.8 IRI) and in lower than 100 km/h posted speed limit areas a minimum roughness guide is 120 NAASRA counts (4.5 IRI). A check should be made of local road surface conditions in the vicinity of residential areas (or other land uses likely to be sensitive to vibration, eg hospitals). Features such as poorly fitted manhole covers, slumped bridge abutments, or road surface repairs not vertically aligned with the true road surface level (eg by more than 20 mm or more) shall be noted, and a level two assessment carried out.

(iii) The site is in an area not commonly known to experience traffic-induced vibrations. This will require a subjective judgement based on local knowledge. For example, it is known that the light volcanic soils of the central North Island volcanic plateau and the peaty soils (with a high water table) in low lying areas of Christchurch city cause vibration impacts.

(b) Level two criteria

For sites that do not meet the level one criteria a more detailed assessment is required as follows:

(i) Vibration levels shall be measured to determine the level of effect. Vibration measurement equipment usually consists of a transducer or pick-up, an amplifying device, and an amplitude or level indicator or recorder.

(ii) Vibration levels shall be measured at a representative position on the floor level of interest in a room that is normally occupied in a dwelling, or other building in which an assessment is required (eg hospital).

(iii) The peak particle velocity shall be measured during normal traffic conditions, especially during the passage of heavy vehicles past the site. Several recordings shall be made, and the highest particle velocities recorded.

The following guideline levels shall be used in the assessment of vibration effects:

• minor impact 2 to 5 mm/sec
• major impact 5 mm/sec or greater

During measurements an inspection of the building for cracks and other building damage likely to have been caused by traffic-induced vibrations shall be noted and reported.

Mitigation of vibration impacts

There are a limited number of options for reducing the effects of vibration. These include:

• structural isolating houses from concrete driveways;
• the use of effective noise reducing fence designs;
• smoothing the road surface to mitigate wheel bounce and body pitch
• road realignment to increase the distance between the roadway and the building; and
• re-routing heavy vehicles to less sensitive roads or reducing the speed of heavy vehicles.

Reporting of vibration

In New Zealand it is anticipated that the quantifiable disbenefits of vibration will be very much site specific and apply in situations such as roads near historic buildings and to road construction in densely populated urban areas. In general, the number of buildings exposed to significant vibration (and an estimate of the numbers of people affected) shall be identified and recorded on maps.

For a level one assessment the report should include the locations assessed and an explanation of the reasons why the level one criteria has been met.

For a level two assessment the report shall contain a summary of the method, locations, and measurement results together with an assessment of whether either of the minor or major impact levels have been exceeded. Measurement results for one or two locations can be used to interpret the likely impact for other buildings of similar construction, and at similar distances from the nearside edge of the traffic lane.