This paper considers the opportunities for noise control within the route corridor required for construction of road, rail and other guided transport schemes. It deals with control of noise generation at source, and in the transmission path close to the point of generation. In this way it is possible to control the amount of acoustic power generated, and to absorb part of the radiated power at points of reflection. Purely reflective wayside barriers do little to absorb acoustic energy, merely reflecting it in a different direction. Whilst this has selfish benefits to the receptor in the shadow zone of the barrier, it makes things worse for others on the reflective side of the geometry. The paper therefore considers the options available to the engineer in the design of rolling and sliding interfaces and the use of acoustically absorptive finishes on all surfaces close to the point of noise generation. This includes the running surface itself, structural components, retaining walls, over and under passes, and the inner surfaces of track and wayside barriers.
Keywords: road noise, rail noise, noise control, environmental barriers, acoustic absorption
|How to cite this article:|
Manning C J, Harris G J. Noise control in the transportation corridor. Noise Health 2003;5:43-5
| Highways|| |
The mitigation of road traffic noise from the road corridor is most often achieved by using some form of screening at the roadside. This section of the paper provides an overview of just some sound absorptive treatments that might be used as an alternative means of traffic noise mitigation. The information gathered from a literature search and the results of a noise modelling study were used to develop guidance on the relative benefits of a range of absorptive treatments as part of a study for the UK Highways Agency. It was clear that it would not be possible to provide precise guidance on likely noise reductions for all situations. However, for general guidance, it was considered feasible and useful to illustrate a potential range of noise benefits.
| Carriageway|| |
Quieter road surfaces are well established and documented as a means of controlling road traffic noise and can offer significant noise reductions as a result of their particular texture characteristics [HMSO, 2001]. In some cases, the benefit is enhanced by a degree of acoustic absorption created by the structural characteristics of the material. These road surfaces typically offer operational noise reductions of about 2-3 dB(A) depending on the type of material.
| Verge|| |
The verge offers a substantial area for absorptive treatment that can help to control noise emissions from the road corridor. The degree of absorption and therefore noise reduction will depend on the verge width and the type of treatment provided. Although grassed verges provide a degree of absorption, other types of low planting may give greater noise reductions. The full noise attenuating benefits of vegetation may only be achieved after several years' growth although some ground absorption benefit would be derived earlier as a result of the soft ground conditions associated with planting. The benefit will be greatest if the vegetation is close to the road. Attenuations of up to 5 dB(A) relative to grassland or up to 8 dB(A) relative to hard ground have been reported through 10m of dense vegetation (i.e. a density with optical penetrability of about 0.5m) [Harris, 1986]. A depth of approximately 10m dense vegetation is considered optimum in order to achieve significant attenuation with minimum land use. Dense plantings of spruce or broadleaf evergreens and conifers, interspersed with deciduous leafy shrubs would be expected to be most effective.
| Retaining walls|| |
Generally the outside face of a retained bank is surfaced with concrete slabs or is clad with stone or masonry which would be relatively acoustically reflective. Reinforced earth structures are often planted with grass or other vegetation and may therefore provide some degree of acoustic absorption. Other absorptive materials can be applied to retaining walls to absorb the build-up of reverberant noise caused by reflections between the vertical surfaces. The benefit obtained is related to the ratio of the depth and width of the cutting. As an example, the modelling exercise for this study showed that absorptive treatments to 8m retaining walls either side of a 10m wide road could give reductions of over 10 dB(A) relative to reflective walls. This is based on a receiver height of 1.2m above ground (i.e. retaining wall extending 8m below the top of the cutting) and receiver distances within 100m of the road. The use of absorptive treatment below a level of 1.5m on the retaining wall would probably not be effective.
| Tunnel portals|| |
Although tunnels provide a highly effective means of containing traffic noise, the build up of reverberant sound inside can lead to increases in noise around the tunnel portals. To control the reverberant component, sound absorptive material can be applied to the walls and roof of the tunnel. A length of two to three times the tunnel diameter should be treated with absorptive material in order to achieve a significant noise reduction [Woehner, 1992]. As an example, for a typical tunnel portal in cutting with a 4 degree incline to grade, flanked by retaining walls, noise reductions up to 10 dB(A) could be achieved at a receiver 1.2 metres above ground. However, the benefit would be significantly less at greater receiver heights.
| Railways|| |
Mitigation of railway noise has become an increasingly important issue as new railways are being built after years when closure was more common. Most existing surface railways already have ballasted track with soft verges providing the equivalent absorption that has been discussed for roads. For new long distance railways ballasted track and soft verges will continue to be normal practice, but in urban areas and light rail schemes, slab track or street running finishes will be acoustically reflective and therefore absorptive treatments may be of benefit. Wayside barriers, both reflective and absorptive, have an important part to play in the design of new railways, and are particularly effective because the relatively narrow transportation corridor, compared to roads, means the barriers can be located close to the source, where they provide the most screening. This section of the paper discusses examples of noise control on new railways.
| Channel Tunnel Rail Link (CTRL)|| |
The CTRL was subject to an Environmental Assessment as part of the planning process, which also required an Act of Parliament. One of the specific conditions of the consent was that the environmental impact of the as-built railway should be no worse than that in the Environmental Assessment. The Local Planning Authorities were granted powers to decide on the actual form of the barriers, a Planning Forum was set up between the project and the authorities to agree on the barrier design strategy route-wide. As a result, the wayside barriers for the country sections were agreed to comprise horizontal timber boards set between H section steel uprights with the option to add acoustically absorptive cassettes on the inner face. Wherever practical, planting was also provided to soften what would otherwise be an unnatural horizontal line across the undulating landscape of Kent. This bespoke design provided a consistent appearance across tender packages that could be acoustically tested before installation.
Elsewhere, where land-take permitted, soil arising from the tunnelled section was deployed as combined noise and landscape bunding. For elevated structures, for example the viaduct and bridge over the Medway, the reference design incorporated a 2 metre high solid concrete parapet. By demonstrating to the Local Authorities that the noise attenuation could be achieved by alternative designs, a low-level absorptive barrier design was incorporated into the bridge deck. This permitted a narrower section with significant weight saving and an aesthetically more pleasing profile, whilst saving money over the reference design! Acoustically absorptive linings were incorporated into the tunnel portals not only to reduce reverberant noise build up, but also to soften the sharp rise time when a high speed train emerges from a tunnel.
| Sunderland Direct|| |
This project was an extension of the Newcastle Metro light rail system as far as Sunderland. This is an inherently quieter design, but at one point the alignment on the route of a former disused railway came very close to a group of newly built houses. There was particular sensitivity in this area, because these properties had only just been occupied at the time of the project going public. The disused railway had also been converted to a cycle route and the design had to re-instate a cycle route with a tight cross section. The resulting solution was to use a low level retained wall system topped with a 2 metre high close boarded fence, to provide a degree of low level noise absorption and good geometric screening attenuation.
These two recent projects illustrate that noise control can be integrated into the design at no significant cost penalty. Also, the guidance described above relating to the use of absorptive surfaces to control highway noise, can be adopted to railway or other guided transportation projects.
| Acknowledgements|| |
The work relating to highway noise was carried out for the UK Highways Agency. The contributions of Chris Field and Colin English in the modelling work and formulation of this advice respectively, are gratefully acknowledged. The support of Union Railways is also acknowledged.
| References|| |
|1.||Harris R. A. (1986) Vegetative Barriers: An Alternative Highway Noise Abatement Measure. Noise Control Engineering Journal / July-August |
|2.||Highways Agency et al (2001) Design Manual for Roads and Bridges - Volume 7. London: TSO |
|3.||Woehner H. (1992) Sound Propagation at Tunnel Openings. Noise Control Engineering Journal/July-August |
C J Manning
Arup Acoustics Cambridge, St Giles Hall, Pound Hill, Cambridge CB3 0AE
Source of Support: None, Conflict of Interest: None