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Changi checks in

Some of the most complex engineering currently under way is on the Changi airport MRT extension, though conscious of its airport's worldwide status, client the Land Transport Authority (LTA) and Singapore's civil aviation authority have been at pains to be discreet.

The $850M line project comprises an 2km long elevated spur from the existing East-West line, running over an existing depot line to a station designed by Sir Norman Foster for Singapore trade exhibition grounds.

Another 4.3km of cut and cover continues the line, running into a 3.5km bored tunnel. This is Singapore's longest drive and particularly sensitive as the twin bores pass directly under a main runway. Mindful of potential problems, the Land Transport Authority preordered two highly specified Lovat earth pressure balance TBMs. Contractor Nishimatsu of Japan has just successfully tunnelled under the runway with just 2mm settlement.

But behind high painted screens and potted plants at the airport are two huge excavations and delicate underpinnings for the air terminal station, which will dominate the space between two existing terminals and a third in planning.

The design by New York architect Skidmore Owings Merrill makes a statement. Two high, sheer, 36m high glass curtain-walled, column free atria will rise from each end, flooding platforms with daylight, or shining at night. Below ground a 200m long curving glass and steel bridge over the platforms - engineered by Ove Arup's New York office - will link to the terminals.

The two 25m deep main excavations are well advanced, the first using conventional soldier piles and lagging, the second comprising three rectangular interlinked spaces inside 35m deep diaphragm walls, says Changi line project manager P Sripathy, of the LTA responsible for the main design works.

But the most difficult work is now under way. Contractor Kumagai Gumi and Singapore's Semcorp must continue the west side of the station box beneath Terminal Two and its multistorey carpark. Heavy underpinning has been installed to keep the building open. The consequences movement are unthinkable.

The LTA designed a raft structure to support the lift and service core. Explains Paul Gasson, deputy project manager: 'We excavated 4m underneath, inserted 35t steel 14m long span needle beams and concreted a slab around.' Once loads were transferred, embedded piles were sliced off to let excavation continue below .

Beyond this, train overrun tunnels will extend beneath the car park, a Skytrain elevated passenger shuttle train, a bus ramp and an airside finger pier. Concrete box tunnels are formed within supporting diaphragm walls, close to and extending beyond the carpark piles.

Grouting and compensation grouting were proposed to cope with possible movement of foundations just 1.5m away.

But the contractor's geotechnical adviser Richard Davies Associates (RDA) proposed an observational method with pre-testing. 'No amount of number crunching can substitute for measurement, ' says Davies. Test piles were made close to where diaphragm walls for the main excavation would go, to see if the ground would support piles as the wall panel hole was excavated. Would the destressing of the ground cause a loss in load bearing capacity?

Results showed piles would stay in place as long as diaphragm wall panels were kept to 2m long and added one at a time. They had to be no closer than 2m to a pile and formed within 24 hours, because the groundwater dissipation weakened the supporting ground.

If a panel took too long, or the building moved, a special 'Zimmer frame' girder could be assembled under the car park floor while the panel hole was backfilled with a weak concrete. 'We had ú1M of instruments watching it, ' says RDA's Alison Norrish.

The walls made, slabs have now been formed 4m down, around piles that sit inside the two parallel walls. These are being cut off once loads are transferred to the diaphragms. Excavation below allows concreting of a 6m deep box for the tunnel. Beneath the live Skytrain viaduct and the bus ramp, a 20m long section of tunnel will be dug by hand because there is no space for diaphragm rigs.

A Hong Kong/British joint venture of Gammon and Balfour Beatty is gearing up for trackwork on the tunnels, having completed much of the elevated section.

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