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West Rail will be one of the quietest railways in the world.

Noise mitigation dominates the design process in the northern and western part of West Rail and its 13km of viaduct has been designed primarily for its acoustic properties.

'It will be the most silent railway ever built,'says Naeem Hussain, director of structural designer Ove Arup & Partners International. 'You might see it but you won't hear it.'

The form of the structure and noise attenuation make it a first in the world. Individual components are not in themselves breakthroughs in acoustic technology, explains KCRC environmental manager Vic McNally. 'The breakthrough is applying that technology and putting different aspects together.'

There are two problems: airborne noise from the wheels and rails, and structure-borne noise. 'If you are not careful you can still have a problem from this deep rumbling even if you wipe out the airborne noise,' he says.

A dozen different measures are being combined to meet the permitted levels, principally involving the creation of three plena to trap noise, use of resilient baseplates and a floating slab track, and stiffening up the structure.

Hong Kong has some of the most stringent noise criteria in the world for railways, according to McNally. In a rural location the maximum is 55dB - 'like a library', he says. In the more urban environment it goes up to 60dB, but much of the line lies within the stricter area.

These criteria are based on the noise generated over a 30 minute period between 11pm and 7am.

'We put trains into service starting at 5am and are running a lot of trains between 6am and 7am. After 7am, the noise limit goes up 10dB.'

Trains will be run slower in the first hour of the day, which helps a little.

'When we highlighted our problem to the Environmental Protection Department, their initial reaction was that we would need to cover the railway,' says McNally. This simply would not have been possible, he asserts.

High ambient temperatures in the elevated enclosed viaducts would have meant an incredible power requirement for cooling, especially as the trains' own airconditioning would be pumping heat into the enclosure. Temperature would have become a real problem if a train had to stop inside. Smoke containment measures would also have been impractical over the lengths involved.

So for the past six months structural, acoustic and permanent way designers have worked intensively to solve the problem. Railway acoustic consultant has been Wilson Ihrig & Associates, and KCRC's environmental consultant is ERM.

A computer model of noise patterns from a conventional design showed it would be necessary to reduce levels by up to 24dB(A).

Noise plena are the principal measure. The first traps noise right under the train, aided by sound absorptive material and 'skirts' along the trains.

Safety requirements for emergency walkways have helped. 'We realised the potential for the voids underneath to be used as plena,' McNally says.

Noise escaping from the gap between the skirt and the containment upstand is channelled under the walkways. The viaduct safety parapet doubles as a further line of protection.

Noise generated within the structure itself has to be minimised by reducing the vibration of steel wheel upon steel rail. Each track is supported by a single box section. The rails will be mounted on resilient baseplates or a floating slab track to absorb vibrations. This sits on rubber pads on the rigid deck of the viaduct. 'The structural designers then had to create a solid mount for the floating slab to work against,' explains McNally.

The design might look odd to the structural engineer, but it is totally acoustically optimised, he says. The initial idea had been for a very lightweight structure, the most cost effective to build, 'but to kill the noise you need a fairly rigid structure'.

The solution was to stiffen up the section and carefully position the webs and fillets. Deep flanges and inner splays together attenuate the low frequency vibration.

Prestressed concrete segmental construction will be used, with spans from 40m to 90m. Some will be built as balanced cantilevers while others will be assembled span by span on a gantry and stressed together. The deck slab's frequency and that of the box girder simply supported beams are tuned so as not to be too similar to the trains'.

Together, the structure-borne and airborne noise will be less than 64dB at the full speed of 130km/h. Points and crossings generate an extra 10dB, so full enclosure will be needed for four stretches each of about 200m.

The structures are up to 20m high. There is a visual integration between the viaduct and adjacent station designs - the viaduct has a noise covering at the stations and the parapet edge detail is taken through the station. Viaduct architect is Chris Wilkinson Architects, while station design for Long Ping and Tin Shui Wai is by Rocco Design.

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