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All quiet on the West Rail front

Work on the innovative, low noise viaduct sections of Hong Kong West Rail is now in progress. David Bennett reports.

From Mei Foo Station on the southern fringes of the New Territories of Hong Kong to Tuen Mun in the west is a journey of 35km. Kowloon Canton Railway Corporation's new West Rail will open up this rapidly expanding area of Hong Kong. For half its distance the route is tunnelled but the last 10km is on viaduct.

Island platforms to the elevated stations along the viaduct split the twin track box girder sections into two single tracks.

The elevated route has been carefully chosen to run through countryside to minimise disruption to traffic and the urban infrastructure. However, where the viaduct passes close to residential buildings in the districts of Yuen Long and Tin Shui Wai, noise pollution control becomes a sensitive issue. Under Hong Kong regulations the noise level of a train with nine cars travelling at 130kph between the hours of 2300hrs and 700hrs, for example, must not exceed 64dBA at a distance of 25m from the track.

The average noise generated by such a train travelling at this speed is around 88dBA. To reduce wayside train noise by 24dBA the different sources of noise created by a train must be analysed.

Train noise is down to a combination of factors - wheel to rail interaction , propulsion system, air conditioning, and 'structure radiated' noise from vibrations transmitted through the track.

'Air conditioning noise is relatively easy to reduce but it's only a small part of the overall noise, ' says structural engineer Robert Benaim & Associates Hong Kong technical director Nick Southward. 'So the big reductions have to be made on airborne and structure radiated noise.'

For airborne noise generated by wheel-rail interaction the best solution on a viaduct is noise barriers along both parapet walls together with an under-car sound absorbing plenum, and a sound absorbing panel to the underside of the access walkway.

To minimise structure radiated noise, twin concrete box girders were preferred for the viaduct superstructure by West Rail consultant engineer Arup.

Southward adds: 'The box girder sections designed by Arup were made wide enough to provide stability to the superstructures from the overturning forces acting on the high sided noise barriers during typhoon conditions. Flange and web panel thickness would have ensured the structure absorbed sufficient noise energy to satisfy noise radiation criteria.'

However, Benain was commissioned by joint venture contractor Maeda-Chun Wo to develop a much leaner alternative box girder design. This won the jv two viaduct contracts on West Rail worth a total of £182M.

During design development Benaim discovered it was not just the mass of the structure that effectively controlled the radiated noise from the viaduct, but the disposition of the mass with regard to the track.

Studies showed that noise radiating from the deck critically depended on the web dimensions and the relationship between web location and the source of the noise. 'The closer the webs are located to the centrelines of the floating slab track bearings, the greater is the mechanical impedance of the structure at that point, ' says Southward. 'And consequently the lesser is the noise radiation from the deck, even when the web thickness is reduced.'

However, placing the webs directly beneath the floating slab bearings leads to a box girder which is too narrow to be stable against the overturning wind loads - unless the box girder can be made monolithic with the supporting piers.

'Such a solution can transfer the overturning loads from the superstructure into the pier foundations, which have plenty of capacity, ' says Southward.

The alternative RBA design consists of monolithic twin, narrow box girder spans with rectangular leaf piers and no bridge bearings. An insitu concrete connection stitches the precast concrete span to the insitu pier supports.

The piers were designed to be sufficiently slender to allow thermal, creep and shrinkage movement of the precast span and rigid enough to sustain the applied forces.

'The viaduct we designed has the same noise reduction as the conforming design while achieving material savings of 40% in the superstructure, 10% in the piers and 40% in the pilecaps and piling, ' says Southward. 'The result is a cost saving of several hundred million Hong Kong dollars.'

Construction on both contracts began in July 1999. Piling and groundwork is substantially complete, with pier support and precast box girder erection operations now in full swing.

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