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Beware of the wolf

Cover feature Bridges

It's all hands on deck in the race to complete an ambitious new Portuguese viaduct. Jon Young reports from Porto.

Iberian wolves stalk the forests and vineyards that blanket the hills and valleys north of Porto, Portugal. And their presence has caused a seven month delay in the construction of a key part of the motorway from Viseu to Vila Verde da Raia in the north of Portugal.

Portuguese environmentalists wanted reassurance that, as it crossed a wide valley near Vila Pouca de Aguiar, the road would not interfere with the wolves.

After consideration of three options a 1.34km long, 100m tall, twin deck viaduct was finally agreed on.

Work is now racing ahead so that it can be completed in time for the road's scheduled opening, in 14 months' time.

The £360M A24S is being built for the Portuguese highways agency by Norscut - a joint venture of Eiffage, Contracto, CDC IXI, Egis Projects, SEOP and Solusel (NCE19 June 2003) - under a design, build, finance and operate contract. The 155km motorway crosses endless hills and valleys and its construction has involved vast quantities of cut and fill, numerous small viaducts, and a handful of major structures.

This viaduct is the largest of all.

Its construction has been subcontracted to a joint venture of French contractor Spie Batignolles and local firm Tamega.

Grand master of Portuguese bridge design, Professor Armando Rito, has provided construction economies for Norscut by dropping the height of the central spans: The deck will swoop across the valley, falling to its mid-point and climbing again at a 5% gradient. This allows Norscut to save on construction materials and time. Rito has also developed an unusual design for the piers of the six highest spans, says Norscut technical director Didier Payerne.

The piers are square in section, but at the bases their corners are extended to form buttresses. These taper back into the square section as the piers rise. But, says Payerne, the corners provide the principal load paths to the piers' full height, with the pier walls acting only as stiffening diaphragms.

This has enabled wall thickness to be reduced to a minimal 300mm.

The piers are set on pad foundations that sit directly on solid rock; there was therefore no need to pile. They are cast using a climbing formwork system and 15,500m 3 of concrete and some 7,700t of steel will go into their construction.

The post tensioned twin decks are split into three sections. At the northern end of the bridge, the first 280m is being cast using a top-mounted travelling falsework system. At the southern end, meanwhile, the first 260m is using a bottom mounted system.

Originally the plan was to use top mounted falsework for both ends. However, time constraints and the non-availability of the falsework meant the contractor has been obliged to use both systems, explains Andrew Else, director of Capita Symonds which is acting as checking engineer for the financing banks.

Balanced antilever construction is being used for the central 808m of the viaduct.

Payerne plans to advance the deck at a rate of 5m a week.

As the deck advances, guy cables will be used to restrain the piers against the overturning forces, exerted by the growing weight of the decks: Their curved plan means that until the spans are complete they will exert considerable eccentric loading.

Some 30,000m 3 of concrete and 1,220t of steel will be used in the construction of the deck and it will take 1,300 men working two shifts and using five sets of cantilever system formwork to complete the viaduct on time.

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