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Taming the elements

Rail - Taiwan

Earthquakes, flooding and poor ground put heavy demands on Taiwan's new high speed railway. Lisa Russell reports.

The word 'challenging' crops up regularly in any discussion about the high speed railway under construction in Taiwan - and with good reason.

Not only will the lines carry some of the world's fastest trains, but they will run largely on viaduct in an area prone to earthquakes. The ground is poor, the flood plains are wide, site access is difficult and the timescale is demanding.

The £9.4bn Taiwan High Speed Rail Project will allow 300,000 people a day to travel at speeds of up to 300km/h along the island's western corridor between the cities of Taipei and Kaohsiung.

All but 39km of the 345km route will be on viaduct or in tunnel.

Civils construction contracts were let in March 2000.

FaberMaunsell was brought in by a contracting joint venture of Germany's Bilfinger + Berger and Continental Engineering of Taiwan to carry out detailed design on two of these. Its work has covered a total of 48km length of high level viaduct on contract C270 near the middle of the route, and the station guideways which form part of the adjoining contract C260.

Time pressure meant the contractor had to work at several fronts along the length of viaduct, with design almost concurrent with construction. At its height, the UKbased design team was 59 strong.

This section of the railway tracks through low mountain ranges and across wide river plains of weak soils. Frequent seismic activity meant the specification was onerous: peak design ground acceleration is 0.34g for the most part, and higher in the Meishan fault zone which lies within the contract.

'Here the equivalent seismic acceleration could exceed twice this value, ' says FaberMaunsell project design manager Kandiah Kuhendran.

Other factors, such as soil liquefaction and mudslides, made the design even more complex, particularly in the river areas.

The viaduct is up to 28m high, and made up of 35m long precast post-tensioned concrete box girders 3.25m deep, simply supported on columns. The deck units have free sliding mechanical pot bearings at each corner. Steel or concrete shear keys at either end of the span provide transverse restraint with one end fixed longitudinally. Both shear keys are capable of resisting uplift forces under lateral seismic loading.

The structure has to be stiff enough to provide a comfortable ride at very high speeds, and must remain safe even during an extreme earthquake. To achieve this stiffness a span to depth ratio in the order of 10:1 has been used.

The design covers two levels of earthquake. No repairs must be needed following a moderate 'serviceability' earthquake.

Requirements are set out for track-structure interaction, deck displacement and rail stresses.

Some repairs would be expected following the extreme 1 in 950 year ultimate earthquake, but even in this case the track must remain safely in place on the deck.

The specification allows formation of plastic hinges in the columns, thus limiting the loads transmitted to other structural elements.

Seismic response is not the only challenge. The contract crosses four rivers including Taiwan's largest, the Chosue.

The railway passes some 28m above the foundation level, and the design scour depth is 11m. The flood plains are wide - in the dry season, the river passes under about four spans, spreading to almost 1km in the typhoon season.

The most economical pile diameter proved to be 2m. 'In the river you might need six or even eight piles to resist the forces, ' says Kuhendran. Pile length varies up to a maximum of about 60m.

Bedrock plays no part in carrying the structure: it is over 100m down.

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