14.03.16
Rail vibration: An increasing factor in construction
Source: RTM Feb/Mar 16
Mitigating noise and vibration from trains is becoming an integral part of construction projects, says Julie Dakin, divisional director in the metros and civils division at Mott MacDonald.
Population growth, urbanisation, social development and the need to cut carbon emissions are leading to more rail projects worldwide. These pressures are also driving the use of unused land close to rail infrastructure for new residential and commercial developments.
So, as people and rail infrastructure come into closer contact, mitigating the effects of noise and vibration will become an increasingly crucial element of both rail and non-rail construction projects.
The effects of rail noise and vibration
Noise disturbance has adverse effects on productivity, quality of sleep and ultimately health. The World Health Organization (WHO) provides detailed guidance on acceptable noise levels for different environments such as offices, schools, shopping areas and in the home. Additional guidance is provided by standards such as BS 8233 on the control of noise in buildings undergoing a change of use, or BS 4142 on industrial noise affecting mixed residential/industrial areas.
Unlike noise, there are no firm regulations on allowable levels of vibration. Codes such as BS 6472 use vibration dose values – a measure of cumulative vibration over a specified period – to predict the likelihood of undue disturbance for residential buildings, offices or workshops. Lower levels are given for residential buildings at night because people are less tolerant of disturbances when at rest, while absolute values are applicable to some hospitals and research facilities where vibration-sensitive equipment such as MRI scanners are in use.
Rail typologies and associated problems
Underground: These are popular solutions which increase commuter capacity with minimal disruption to above-ground infrastructure. They produce little direct airborne noise but may present problems of ground borne noise and vibration.
Overground: Above ground there are greater problems of airborne vibration. Noise barriers around the track can mitigate the problem but have a visual impact and may be constrained by the need for emergency evacuation routes.
High-speed rail: In general, the faster the train, the greater the level of noise and vibration. High-speed rail is mostly used for intercity travel, and hence the added vibration is rarely a problem in dense urban areas, although the effect on rural communities must be considered.
Light rail/trams: Trams travel at slower speeds, so have lower levels of noise and vibrational disturbance. However, light rail networks are usually in closer proximity to buildings. An added consideration is the electromagnetic interference that passing trains can have on specialist equipment in medical, educational or research centres.
Mitigating vibration
Vibration is best mitigated at source by isolating the rail line itself. 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 means temporarily closing the line. For new tracks, the use of welded rails, rather than jointed rails, produces a smoother path for passing trains, thus reducing noise and vibration.
If mitigating the vibration at adjacent assets, then elastomeric bearings can be installed between columns and the building foundations. The layout of buildings can also be modified, with less sensitive rooms (i.e. kitchens and bathrooms) located on the rail-facing side of a building.
‘Box-in-box’ techniques – isolating a space through resiliently mounted floors, ceilings and walls – can usually mitigate vibration altogether, and are often used for recording studios and cinemas. However, this technique restricts the potential for future redevelopment of the building.
Further challenges to mitigation
Planning: There is no legal requirement to ‘future-proof’ 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.
Over-station projects: More over-station developments will be driven by the need to make the most of inner city land where available. 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 for the over-station project.
Local residents: Strong citizen groups and the effects of social media are pushing developers beyond simple compliance with industry regulations and standards. Public consultations and communication with local people are an essential part of the construction process.
Changing regulatory environments: While the UK’s laws are unlikely to change much, in the developing world regulations are likely to become stricter as planning laws catch up with rapid urbanisation.
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