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Sound principals

West Rail Environmental design

West Rail trains whisper into stations.

And quiet has been the watchword for designers and contractors ever since work began in earnest.

Noise is an issue on any urban rail project, but in Hong Kong the problem of noise sensitivity is greatly amplified. The city's dense population means people are pressed right up against the West Rail alignment, often in tower blocks.

Hundreds of homes front onto the rail line. Where in other locations engineers can elevate track, lifting the noise emitted by trains above low-rise housing, in Hong Kong, Kowloon and the new towns, raising track on piers merely brings the noise source nearer to high-rise residents.

The whine of electric motors and the clatter of wheels on tracks echoes off cliff-like building facades.

'From the word go we were looking for people vunerable to disturbance, ' says KCRC environmental manager Richard Kwan. 'For high rise population centres like many in Hong Kong, running on viaduct was a bigger challenge than going underground. For the low rise villages found along West Rail, raising the noise source 15m-25m above houses shields them. But when we reached the urban cores of new towns like Yuen Long, Long Ping, Tin Shui Wai and Tuen Mun the noise problem could have been immense. We also had to anticipate that low rise villages will in time grow into dense new towns in their own right.'

'Noise has been a holistic theme uniting track bed, rolling stock and station design, ' confirms West Rail director Ian Thoms.

Noise levels are measured at 25m from trackside using a train travelling at 130km/h. Huge effort has been put into the design, aiming to reduce noise from 88-92dB(A) - average for a modern commuter railway - to below 81dB(A). Noise from the purpose-built West Rail trains has been reduced to 76dB(A), and muffling on the viaduct has achieved an impressive 64dB(A).

It was feared the viaduct's hollow box section deck could act like a drum, resonating to the impact of wheels. As a result the deck has a deep, narrow section, which stiffens it and limits radiation of noise from the underside. Fixed track bed, which offers higher tolerance construction and quieter running than ballasted track, has been used everywhere apart from at the maintenance yard at Pat Heung.

Rails have been continuously welded to iron out the clicketyclicking of wheels over joints, and are mounted on rubber pads to damp the natural resonance that can be set up between wheels, rail and track bed. 'We had to design the softness of the rubber base pads carefully to achieve precisely the right effect, ' Kwan says.

Trains have been equipped with specially designed 'quiet' traction and air conditioning units. These minimise mechanical noise and are fitted with 'skirts' which, in conjunction with low-level trackside screens, help baffle noise and contain it beneath the carriage. They have aerodynamic front and rear ends that pierce the air smoothly and externally mounted air conditioning units are small and flushfitting to keep the sound of wind resistance to a minimum.

The track is wrapped on either side by a solid concrete parapet wall, which rises to the level of train windows and further helps to catch noise. For maximum silencing, West Rail's viaduct could have been entirely enclosed, ventures Kwan. 'We didn't want to build a noise enclosure, though, because it would have created a major heat problem - it would have behaved like a greenhouse.

We might have been able to combat discomfort for passengers with air conditioning, but the heat would have created unsolvable problems for the signalling system.' Complete enclosure is provided only at points where up and down lines cross over.

Passengers have not been forgotten: they are protected from track noise inside the train by rubber underfloor insulation and from the sound of rushing air by perfect seals around the doors and windows.

Caring for the past 'Hong Kong doesn't have a long history but people were living in the New Territories before HK and Kowloon were developed. There are old buildings, ' says environmental specialist Viola Tong.

Special effort has been taken to preserve the 500 year old Tsui Sing Lau pagoda, which stands just 40m away from Tin Shui Wai station. Its foundations were in bad condition and Tong was worried that piling work might have triggered settlement. 'We were concerned that groundwater might be drawn into piles, and that changes in the groundwater regime could end up affecting the foundations, ' she says.

The structure has therefore been monitored 24 hours a day for the last three years - no movement has been detected yet and attention is now focusing on environmental improvements around the pagoda such as tree planting and paving.

Environmental pioneer Conservation has not historically been a big issue in Hong Kong. In 1998 Hong Kong's Legislative Council introduced new environmental ordnance obliging environmental impact assessments for all new projects and environmental mitigation where ecosystems or habitats would be disturbed.

'Conservation suddenly came into the limelight, and West Rail is the first major project here to have taken it on board, ' Kwan says.

Pat Heung maintenance depot sits in the middle of an agricultural landscape that has been gradually reverting to nature. The area is home to a wide range of insect, reptile and amphibian life.

In an attempt to replace habitat lost to the depot and to create new breeding conditions for species long ago lost to the area, 12ha of wetland have been created beneath the viaduct at a cost of HK$87M (US$11.2M). 'Our target species is the Greater Painted Snipe, which is a shallow water bird found in only two locations in Hong Kong. We also want to attract little and great egrets, and herons, ' says Kwan. It is hoped the wetland will also become a haven for the rare narrow mouthed frog, as well as butterflies and dragonflies. 'We are very excited, because already this year we have found some dancing dropwing dragonflies, which have never been seen in Hong Kong or the New Territories before.'

Construction of the wetland got under way as soon as the viaduct was completed in 2001. The land has been profiled to create varying depths of water for wading and diving bird species, and to create a permanent, sustainable marsh environment. 'There was already a water storage reservoir in the area , which now provides a secure resource for the project, and we have also built smaller storage and supply ponds, ' says Kwan.

A bund has been built to protect the freshwater habitat from the influence of salty tides that flow in and out of a nearby nullah.

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