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Current affairs

Bridges

Building a 32km ocean crossing would be challenge enough without 15 knot currents, 7m high waves and tornadoes. Judith Cruickshank reports on China's latest construction extravaganza.

Competition on an extravagant scale has broken out between the Chinese provinces of Shanghai and Zhejiang.

Shanghai is forging ahead with construction of what, in 25 years time, it hopes will be the world's busiest deep water container port. Built on a combination of reclaimed land and two islands 32km out to sea, it will have a mind blowing 22km of quay wall, 50 berths, and an annual handling of around 25M Twenty-foot Equivalent Units (TEU). First phase of construction delivering 6,000 TEU capacity and costing $1.73bn is to be completed by mid-2005. Zhejiang's authorities are extremely unhappy about the project, which will swiftly eclipse its own competing mega-port at Ningbo.

But the this is by no means the only point of contention between the provinces. To link Yangshan deep water port with Shanghai itself construction a new bridge got under way mid-last year. At 32km Donghai Bridge, or the Eastern Sea Bridge, will become the world's longest ocean crossing.

That is, at least, for a while.

For construction of the 36km long Ningbo Hangzhou Bay Bridge got under way in June.

It will link Cixi on the south bank of Hangzhou Bay with Jiaxing on the north bank, cutting 120km by road from the journey between Shanghai and Ningbo.

What really seems to have riled Zhejiang-based project sponsors of the Ningbo Hangzhou Bay Bridge is that, even though they will claim the longest sea crossing crown when the structure is completed in 2008, Shanghai will have worn it for three years before them.

Donghai Bridge is costing $1.42bn. Over most of its length it is designed as a precast reinforced box girder structure with spans of 44.5m. However, to provide clearance for large ships there will be two cable stayed navigation spans - the largest 420m long.

The crossing has been under design since 1994. UK consultant Halcrow, with local firm Shanghai Urban Design & Construction Institute in a supporting role, have been acting as advisor to client Shanghai Tongsheng Bridge Construction Company since the preliminary design bidding phase. They are now working on technical details associated with design, construction, operation and maintenance of the bridge - specifically detailed design review, structural modelling, loading analysis, and corrosion protection methods.

Numerical model analysis and physical wind, wave, ship impact, current model and traffic accident testing has been undertaken. Expected design life of the structure is 100 years.

Construction conditions are extremely testing, says Halcrow project director Dr Ganwei Chen. Apart from the sheer size of the project, Hangzhou Bay is infamous for its high tides. A number of rivers discharge into the sea there, the most notorious being the Quiantang River with a spring tide that roars in at speeds of between 13 to 15 knots in a wall of water reaching heights up to 7.5m, 'foaming and roaring as if it were enraged, ' says Chen.

Daily there are rough seas to contend with, tidal differences of around 5m, long shore currents of more than 2m/sec and wave heights of around 6m or more. Typhoons are common between August and October and high winds between December and February. In total no more than 180 days a year are suitable for working on the water.

And there are pockets of natural gas lying 50m below the surface of the sea bed, says principal director of the project, Wang Yong. Water depth in the bay ranges from between 10m and 20m and ground conditions consist of marine silts and soft clay, clay and a clay/sand mix.

Piers for the crossing are founded on a mixture of driven and bored piles. The mix of precast concrete and steel piles are driven using heavy diesel hammers. The bored piles, which are either 2.5 or 3m diameter, are installed from jackup platforms. Pile caps are precast to ensure quality and to save time working over the water, while also reducing the amount of in situ concrete required for the project. Piers and box girders, which comprise the deck over most of the crossing's length, are also produced in the precasting yards.

Steel is supplied by steelworks from all over China.

Precasting yards have been established and the concrete elements are taken out by barge to where they are needed and installed using shearlegs.

A heavy lift vessel is currently under construction and will be able to lift an entire 70m box girder span weighing up to 2,000t. However, the number of days when this type of operation can be carried out are severely limited by local weather conditions.

Six Chinese contractors are involved in the construction of the bridge which is scheduled for completion by the end of December 2005. The schedule is very tight, but the construction team believes that it is realisable.

And they are doubtless spurred on by that thought that they will have achieved a world record by building a 32km bridge over rough sea in just three and a half years.

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