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EIFFAGE WON the Millau contract by promising it could deliver the structure within 39 months.

A little less than half way through construction, the race is well and truly on, says Tiberghien.

Key to fast, reliable progress was Eiffage's decision to build the deck in steel and to launch it.

Winds howl along the Tarn valley and Eiffage has planned work so as to minimise reliance on craneage, which under French health and safety legislation, must stop operating when wind speeds rise above 72km/h. However, launching a 30,000t, 2.46km long deck between piers spaced at 342m intervals throws up one fundamental challenge: intermediate support.

Specialist US subcontractor Enerpac, responsible for the deck launch, is erecting giant temporary towers mid-way between each pair of piers with the exception of P2 and P3, where the valley is simply too deep.

Towers closest to the abutments are fixed height. , but the rest are relescopic: sections are pushed up from a 12m 2base unit equipped with hydraulic jacks and a rack and 'rack and chock' locking system.

At the end of each 1m stroke, the tower is fixed in position by inserting a chock in the rack and the jacks are moved up for the next 1m push.

Foundations for the temporary towers are of the same design as those for the permanent structure - beefy.

Over the deepest section of the valley, between P2 and P3, Eiffage has been forced to take a different approach.

From the south abutment - where launching is already under way - the launch will extend across piers P7 to P3, stopping mid-way to P2. A shorter launch from the north abutment will cross piers P1 and P2 to meet it.

To prevent the decks sagging into space while the two ends are welded together, the P2 and P3 pylons will be mounted on the deck and stays installed before the deck leaves terra-firma - the P3 pylon is currently being raised in nine 10m sections in the south abutment launching yard.

Eiffage could have saved a huge amount by erecting pylons for the entire viaduct as the launch advanced, were it not for the wind, says Tiberghien. Wind resistance offered by pylons and stays would have dramatically increased risk of interference with the launch programme. The remaining five pylons will be transported complete on to the deck on wheeled transporters and hauled into the vertical with the aid of a pair of temporary truss masts, currently under design.

Construction of the deck is advancing in 171m lengths to span between permanent and temporary towers, with prefabricated sections delivered to the launching yard from steelwork contractor Eiffel's workshops at Lauterbourg and Fos-sur-Mer.

The launch is taking place in 600mm/min increments. To prevent overturning forces generated as the deck slides forward from bending and damaging the piers, Enerpac is installing jacks on each of the pier heads. These can be adjusted vertically to control the angle of the deck, but more importantly they provide longitudinal movement, enabling Enerpac to compensate as the deck is thrust forward.

'The effort is so great we could break piers if the system didn't work, ' Tiberghien frowns. 'We are watching movement of the piers second by second: we have to know how much they are bending to prevent damage. The piers can move a maximum of only 400mm.'

With the deck launch now well under way, Tiberghien is hoping to span between one permanent pier and temporary tower every five weeks. The temporary towers should be struck mid-2004, he says.

INFOPLUS www. viaducdemillaueiffage.

com www. enerpac. com www. eiffel. fr www. peri. de

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