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The water margin

Hong Kong's new rail line to the Chinese border is being built across environmentally sensitive wetlands. Ruby Kitching reports from the Northern Territories.

Don't underestimate the power of the green groups, ' warns KowloonCanton Railway Corporation (KCRC) director KK Lee. He speaks from experience, having spent 10 years negotiating the construction of the East Rail spur line.

The 7.4km route between Sheung Shui and Lok Ma Chau in the Northern Territories crosses the Long Valley area of the Mai Po wetlands, seasonal home to about 60,000 migratory birds. Initial plans were for a viaduct across the valley, following on KCRC's success in building the West Rail extension, 14.4km of which is on viaduct (NCE 8 January 2004). But bird enthusiasts feared that trains charging across the new structure would frighten wildlife away.

The only other option - which was adopted - was to build the line underground.

Lok Ma Chau project is split into four sections: modernisation of the existing Sheung Shui station, the Sheung Shui to Chau Tau twin tunnels, a western viaduct and Lok Ma Chau station.

A footbridge from the station links the New Territories to China across the Shenzhen River. Construction began in January 2003 and the first trains will start running in 2007.

The westbound tunnel, the viaduct and Lok Ma Chau station are complete but the eastbound tunnel has 100m to go. The two tunnels have been driven using an earth pressure balance tunnel boring machine (TBM) for 3.5km under the Long Valley - only the second ever use of a TBM in Hong Kong. Cut and cover construction has been used for the 700m of tunnel adjacent to the viaduct. Both tunnels are lined with 400mm thick precast segments.

KCRC has also reclaimed land to extend the wetlands area in order to compensate for habitat lost to the new Lok Ma Chau station.

'Even the design of the footbridge and landscaping had to consider the birds, ' says KCRC public affairs manager Mabel Wan.

'When we decided to build a tunnel, we asked [the protest groups] to join our environmental committee. We've spent the last 10 years turning them round from being our enemies to being our friends, ' says Lee. He adds that a web cam watches over the site so that the public can check that construction is not affecting the wildlife.

But by far the biggest constraint placed by environmentalists on the rail project has been the absence of a full site investigation of the Long Valley area.

'Because the area is so environmentally sensitive, we couldn't do a sufficient site investigation. The original design was for a viaduct so we took a few borehole samples, but not enough for a tunnel, ' explains KCRC chief resident engineer Edwin Ching. The best the engineering team could do was to use old borehole logs and geological maps which revealed that the upper stratum of ground was fully decomposed volcanic soil with a mixture of hard rock and soft ground below. This flimsy data was used to locate the tunnels which are up to 40m below ground level.

Despite having very little knowledge of the exact ground conditions in the area, stringent thresholds were imposed on settlement during construction.

Only up to 25mm of settlement was allowed before the ground had to be injected with grout, and beyond 40mm of settlement construction would have to stop.

Groundwater levels were also scrutinised.

'The farmers established baselines for the ground water and irrigation channels and we have to monitor them to ensure they don't drop, ' says Ching.

These were hairy days for Ching, who would have to make the call if these thresholds were exceeded. Working closely with French tunnelling contractor Dragages, initial TBM advances were watched closely.

'Dragages went through the soft ground without any problems but had to monitor water levels and ground settlement very closely under the Long Valley, ' says Ching.

The result of careful planning has meant 98% of settlements are less than 25mm -'so we think we've done very well', he says with a note of triumph.

Controlling settlement was not Ching's only headache. They also had to construct three 4.4m diameter, 7.8m long cross passages between the two tunnels under the Long Valley in waterlogged ground without disturbing the ground above.

Although expensive, the only practicable option was to freeze the ground before attempting excavation. Ching claims this is the first time ground freezing has been carried out in Hong Kong.

'It's a very effective system - everything is going perfectly, ' he says.

Freezing has been achieved by installing 32 pipes through each tunnel wall where the cross passages will be. Brine cooled to -35 oC is then pumped through them, forming a 1.2m thick zone of ice around the cross-tunnel's circumference.

Hand held jack hammers are being used to break out material and the tunnel walls are shotcreted to provide temporary support until the whole length of tunnel is complete. A waterproof membrane is then installed and a permanent shotcreted surface is applied. The last cross passage will be completed next month.

Outside the Long Valley a mixture of cut and cover and rock drilling is being used to build the cross tunnels.

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