Noise and vibration from trains is an ever more serious consideration as new projects emerge, conditions change and citizens get wise.
Rail projects are proliferating worldwide, driven by population growth, urbanisation, social development and the need to cut carbon emissions.
These same pressures are also driving the use of brownfield sites and unused land close to rail infrastructure for residential and commercial properties.
“School classrooms should not be exposed to noise levels exceeding 35 dB to ensure speech intelligibility”
As rail infrastructure comes into ever closer contact with the population it serves, the need to curb the effects of noise and vibration is fast becoming a crucial element in the development of both rail projects and adjacent assets.
On the limits
Noise disturbance can have an adverse effect on productivity, quality of sleep and ultimately health. The World Health Organisation provides detailed guidance on acceptable noise levels for various scenarios.
School classrooms, for example, should not be exposed to noise levels exceeding 35 dB to ensure speech intelligibility, while the WHO recommends noise levels remain below 70 dB in industrial areas, shopping facilities and the road network to prevent problems such as hearing loss.
Additional guidance is provided by standards such as BS 8233:1999, which gives recommendations on the control of noise in buildings undergoing a change of use, or BS 4142:1997, which relates to industrial noise affecting mixed residential/industrial areas.
However, unlike noise, there are no firm regulations on the allowable levels of vibration. Instead, we take into account the dosage of interference to adjacent infrastructure and the presumed tolerance of those affected.
Modern codes such as BS 6472-1:2008 use vibration dose values (a measure of the cumulative vibration over a specified period) to predict a likelihood of adverse effects for residential buildings, offices or workshops.
“Absolute values of vibration are applicable to some hospitals and research facilities where vibration-sensitive equipment such as MRI scanners are in use”
Lower levels are given for residential buildings at night because people are less tolerant of disturbances when at rest.
Absolute values of vibration are applicable to some hospitals and research facilities where vibration-sensitive equipment such as MRI scanners are in use. In these cases, the manufacturer of the equipment will specify the acceptable level of vibration.
Variety of project problems
Of course, different rail projects bring different problems.
Underground metro projects are popular as they increase public transport capacity while minimising the amount of above-ground infrastructure needed.
They produce little direct airborne noise but may present problems of ground-borne noise and vibration. Mitigation is especially important, as buildings are likely to be directly above train activity for much of the metro network.
Overground trains on the other hand, both elevated and at ground level, are associated with greater airborne noise pollution. This can be reduced by noise barriers around the track, but these have a visual impact and their positions may be constrained by emergency evacuation routes.
With high-speed rail, the faster the train the greater the level of noise and vibration. As high-speed services usually link cities rather than facilitating movement within them, the added vibration is rarely a problem in dense urban areas, but the effect on rural infrastructure and communities must be considered.
Light rail/trams sit at the other end of the spectrum, as their slower speeds leading to lower levels of noise and vibrational disturbance. But light rail networks are usually in closer proximity to buildings. Another consideration is the electromagnetic interference that passing trains can have on specialist equipment in hospitals, universities and research centres.
How to mitigate vibration
Vibration mitigation works by minimising or absorbing the movement caused as the trains run over the tracks. This is best done at source by isolating the rail line itself, as the more the vibration dissipates in all directions from the track, the harder it is to control.
For functioning rail lines, rigid track sleepers with resilient mountings can be installed progressively during engineering hours. Further mitigation can be provided through the use of resilient rail supports or floating slab tracks, but these options are best incorporated into new rail projects, as retrofitting them on existing projects would require closing the rail line altogether during refurbishment.
“Less sensitive rooms such as kitchens and bathrooms can be located on the rail-facing side of a building and bedrooms and living areas opposite”
To minimise the effects of vibration, new tracks should be built using welded rails rather than jointed rails, producing a smoother path for passing trains and reducing noise and vibration.
If mitigating the vibration at adjacent assets, then elastomeric bearings can be installed between columns and the building’s foundations.
The layout of buildings can also be modified, with less sensitive rooms such as kitchens and bathrooms located on the rail-facing side of a building and bedrooms and living areas opposite.
‘Box-in-box’ techniques – isolating a defined space through resiliently mounted floors, ceilings and walls – can usually mitigate vibration altogether and are often used for recording studios and cinemas. This technique is often not the answer, however, as it restricts the potential for future redevelopment of the building.
Despite the regulations, standards and precedents that guide the industry, a number of additional factors are adding further challenges to vibration mitigation.
The onus in terms of planning is on asset developers to take account of existing conditions, as well as any changes caused by assets that are already going through the planning process.
There is no legal requirement to futureproof an asset by taking into account all conceivable development that could take place in the area. However, with cities becoming denser and transport networks regularly expanding, it would be wise to build in excess vibration mitigation. This has to be balanced with the client’s desire to keep down costs.
“Citizen groups have grown stronger, aided in part by social media and better understanding of the planning process”
With inner-city land at a premium in many cities, we will also see more over-station developments. This will also be driven by rail owners who seek to capitalise on the value of their land assets.
Transport for London, for example, opened up its HQ above St James’s underground station for redevelopment – part of its plans to raise £3.4bn in non-fare revenue. In these projects there is little opportunity to mitigate vibration at the tracks without disturbing rail services, putting extra pressure on ‘designing in’ vibration mitigation.
Citizen groups meanwhile have grown stronger, aided in part by social media and better understanding of the planning process, which is pushing developers beyond simple compliance with industry regulations and standards.
This should be considered early in the design process, as the developer may need to factor in public consultations and communication at crucial points in the project.
All this falls against the backdrop of changing regulatory environments. While the UK’s laws are unlikely to alter much, in the developing world regulations are likely to become stricter as planning laws catch up with rapid urbanisation.
Julie Dakin is divisional director in the metros and civils division at Mott MacDonald