Locale : Global (English)
With a growing population and pressure on land, people are being brought into closer proximity with the transport networks that serve them. That’s a good thing in many respects, but every silver lining has a cloud.

Noises off Dr Julie Dakin

Social media has given the public a louder voice over transport schemes and their likely nuisance issues. Noise and vibration are particular bugbears.

“In the past, noise levels were higher because people generally didn’t comment,” observes Dr Julie Dakin, director of special engineering services at Mott MacDonald. “Now, when we perceive a noise nuisance we have the ability and opportunity to complain, and see others doing so successfully.”

What creates noise and vibration on the main lines and what are the solutions?

Speed of sound

The three noise sources generated by a railway become most perceptible within different ranges of speed. Up to 80kph, the dominant noise comes from the traction system – engine, gears, exhausts, bearings, brakes – together with ancillary equipment such as air conditioning.

Unavoidable rolling noise comes into play at 80-200kph, a function of roughness on the surface of both wheel and rail, forcing the two apart and then back together. Extreme forms of this are rail corrugation – which brings ground vibration and noise level increases of up to 20dB – and wheel flats. The latter cause cyclical thudding. The word ‘extreme’ applies to the relative result; the defects themselves can be almost invisible.

Good quality track and wheels remain the best control measures. Preventative maintenance strategies can include rail grinding to address rolling contact fatigue (RCF), with the happy side-effect of reducing noise levels by 5-10dB. Experience is suggesting that the adoption of harder rail grades – while superficially attractive as they slow the development of RCF – can increase the number of wheel defects.

By improving aerodynamic efficiency, energy loss (and noise) is reduced, hence the characteristic long nose of a high speed train. The speed at which aerodynamic noise becomes dominant is, to some extent, dependent on detailing (connections between the carriages, skirts around the bogies) to limit turbulent airflow, but is typically around 200kph. More difficult to mitigate is the impact of vortex shedding from the pantograph, which inevitably sits proud of the vehicle body.

Mitigation: interwoven factors

While sound barriers represent a simple and cost-effective approach to tackling airborne noise, a crucial factor is the height above the rail at which the noise is being generated. Essentially, barriers only work if they block the receptor’s view of the source.

Differences in population density mean urban and rural areas demand different approaches. The former tends to be where stations are sited, resulting in lower speeds – so traction and rolling noise predominate.

Significant ground-borne noise is rarely transmitted more than 100m, but if a building is within that range it is inescapable. Energy from the railway propagates through the building structure, causing it to resonate. Vibration is converted into airborne noise that affects all rooms almost equally.

Ground-borne vibration can also cause annoyance, not only to people but also by its effect on sensitive equipment, such as that found in hospitals or research centres. The range of this disturbance depends not only on the amplitude of the vibration, but also on the sensitivity of the equipment being used.

As new housing encroaches on railways, designers are looking to incorporate mitigation to filter that part of the spectrum where ground-borne problems are initiated. “For vibration, we’re looking at low frequencies; anything up to 80Hz,” explains Mott MacDonald senior engineer Ben Carlisle. “For noise – depending on the train, track and so forth – it tends to be 40-200Hz.”

Work to mitigate noise nuisance affecting a building can be expensive. Multiply across a street or an estate and the cost becomes very serious indeed. But there is an alternative. “What’s starting to happen is some developers are paying the infrastructure owner to improve the track form, which often turns out to be cheaper,” says Ben.

Known unknowns

As the need to reduce noise increases, the intervention to mitigate it becomes more intrusive, ranging from soft pads beneath the rail to floating slab track. This increases capital cost, driven by the need for specialist components and/or phased construction methodologies. While some efficiencies can be introduced through standardised design elements, there’s no overcoming the realities brought by unique characteristics at every location along the line.

But this doesn’t mean designers and contractors can’t substantially influence capital costs and issues around buildability. Sensitivity analysis considers all the prevailing factors – building type and proximity, adjacent noise sources, ground conditions, trackbed and vehicle design, operating speeds and running hours – and helps to draw out the best-value mitigation.

“And you minimise the areas that need it,” asserts Brian Stewart, Mott MacDonald technical team leader, “so you don’t introduce mitigation along the whole length; you identify critical areas. There’s a realisation now that discussions around this need to happen earlier."

Mitigation methods

Noise barriers: Available in both reflective and absorptive types. Most commonly manufactured in timber, but also metal, acrylic or concrete. Intended to reduce transmitted sound diffracts over the top. They can attenuate rolling noise by 6-10dB with a barrier height of 0.9-1.2m above the rail head. Bunds, earth mounds and cuttings can be constructed instead where space allows. Some barriers are planted to form ‘living walls’.

Soft rail supports/fastenings: By reducing track support stiffness, the amplitude of high-frequency vibrations in the supporting structure decreases. However, increased noise radiation from the rail can result.

Sleeper pads/boots: These provide resilience under the sleepers to increase the track’s participating mass. They further reduce vibration compared to track with rail supports of only medium/higher stiffness. Ballast ‘sharpness’ requires consideration with under-sleeper pads due to high contact pressures on the elastomer.

Ballast mats: An elastomer normally made of rubber, rockwool or synthetic polymer is placed under the ballast. They generally need support from a solid/concrete base to be effective and are most commonly used in tunnel inverts or on bridges/viaducts. Ballast ‘sharpness’ and drainage detailing require consideration to ensure continued performance.

Floating slabs: Highly effective at controlling ground-borne noise and vibration, but expensive. They consist of a concrete slab supported on steel springs, elastomeric bearings or mats. They will only attenuate noise/vibration at frequencies of x1.4 the slab’s natural frequency or above; at lower frequencies they cause amplification.

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