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View from the valley

The latest of France's major projects, the Millau Viaduct, is so big and so daring that tourists are already flocking to see it, two years before it is due for completion. Andrew Mylius joined the throng.

In February a parachutist leapt off the tower crane being used to erect Millau Viaduct's tallest pier. From where he jumped, 150m up, the ground appears vertigo-inducingly distant. But when complete, the viaduct's deck will be an even loftier 245m above the valley floor - it would clear the Eiffel Tower. He is likely to be just the first of many 'base jumpers' attracted to the structure.

In fact the viaduct has already become a magnet for tourists - 70,000 have turned up to watch construction in the past 12 months. The structure will carry autoroute A75, from Paris with Barcelona, across the steep sided Tarn valley in south west France.

Negotiating the valley has in the past forced queues of traffic down a winding route, brakes smoking, into the picturesque town of Millau, before grinding their way up the other side to rejoin the motorway. The 2.46km, $300M Millau Viaduct, carrying three lanes of traffic in each direction, will bypass the town, freeing it from noise, congestion and pollution. By a happy twist, it is also expected to boost the town's economy as people stop off to marvel at the amazing feat of engineering.

The viaduct will be a multiple cable stayed bridge structure, striding across the landscape in 342m leaps on seven sculpted, reinforced concrete piers.

Artists' impressions of the completed project make it appear graceful and relatively insubstantial - and it will be. However, up close, the scale of the towering piers and the first section of deck jutting out over the south abutment is colossal.

A steel deck was chosen over concrete to save on weight - it will come in at 30,000t and 4.2m deep, compared to 200,000t and 7m deep for a concrete alternative. This saving was transferred to the viaduct's piers, which are correspondingly slender. It is all relative, though.

From the ground up, each pier sits on four 5m diameter reinforced C35 concrete piles. Some 16m to 17m in depth, they have been keyed into limestone bedrock at piers P1 and P2 at the northern end of the site, and into stiff clay for the remaining piers P3 to P7. They are capped with a 6m thick, 27m by 18m reinforced concrete slab.

In plan the piers have a hollow polygonal section to 90m below deck level. There, in side elevation, they branch like tuning forks.

The deck, which is being launched incrementally in two sections, one from either abutment, will be anchored fast to each of the forked pier heads.

Bearings will allow for rotational movement caused under live traffic loading. But longitudinal thermal movement, will be accommodated by horizontal flexing of the piers themselves, says Thomas Tiberghien, director of works for concessionaire and main contractor Eiffage.

Piers closest to the abutments, P1 and P7, will flex an estimated 400mm outward and 300mm inward as the deck expands and contracts. At the abutments 1m movement joints are being provided.

Supporting the deck will be towering, 90m tall, A shaped steel pylons, from which the cable stays fan, 22 per pylon. The pylons are major structures in their own right, weighing 600t apiece. Mountings fabricated from 65mm steel are being incorporated into the deck to transmit loads from the pylons through to the pier heads.

But, says Tiberghien, loads will not be high enough to provide the compressive force needed for the pylons to retain structural integrity under horizontal loading. Accordingly, eight prestressing strands are being incorporated into each fork of every pier to give an additional 6,000t of force. They are anchored into a 1m thick reinforced concrete diaphragm at the piers' bifurcation and tensioned from the pier heads.

From 90m below deck level and downwards there is enough concrete and steel bearing on the columns to remove any need for prestressing, Tiberghien adds.

Dealing with the piers' constantly changing geometry has been a tricky challenge, says Tiberghien.

Side-on the piers taper slightly - from 17.028m at the base of the tallest pier, P2, to 16.126m at the 90m mark and 15.5m where the arms meet the deck. In longitudinal elevation the diminution is far more extreme, though - from 25.4m at base to 11m at deck level. Wall thickness also reduces from 1.5m to 600mm over the height of the piers.

With German formwork subcontractor Peri, main contractor Eiffage is working on all seven piers simultaneously. Peri has supplied a self climbing steel formwork system for the piers' exteriors and crane-lifted climbing formwork for use inside the piers and pier arms.

To adjust formwork to the piers' constantly changing shape, Peri custom designed a system of removable 142mm wide 'compensation panels', which are held in place during concreting by telescopic walers.

Concrete is poured in 4m lifts, and panels are removed every four lifts, closing the formwork in, says Tiberghien. Peri was also challenged by the viaduct's architect Sir Norman Foster to produce a near seamless finish:

no sharp edged corners, no horizontal plywood joints, and minimal visible tie positions.

Concrete specification is C60, with additives to reduce water content, enhance flowability, and accelerate curing. Concrete has to be capable of carrying the formwork within 24 hours of pouring: Eiffage is working against a hectic 39 month construction schedule and is aiming to push up the piers at the rate of 8m a week.

Excellent flow characteristics are needed to ensure concrete penetrates into the tight matrix of large diameter rebar. An average reinforced concrete tower might have in the region of 180kg of rebar per cubic metre, says Eiffage technical director Ben Fredj.

'We have an average of 240kg/m 3. Some parts have up to 380kg.' Reinforcing subcontractor Samt is managing to fix steelwork for each lift in just two days, which is pretty impressive. 'It's difficult, ' he shrugs.

Concrete is relayed to the seven piers from a vast site batching plant via an army of trucks. For the first 30m of all of the piers, concrete was pumped, with self-climbing Potain tower cranes taking over and skipping it into place from 30m and up.

The piers' exceptional height is throwing up some unusual setting out challenges, says Tiberghien. They are designed to flex as the deck expands and contracts, but they also bend 'like bananas' in response to differential thermal loads imposed by bright sunshine and shade.

The variation in temperature can be well over 20infinityC, and as the sun moves across the sky, the piers keep track.

GPS and conventional surveying techniques accurate to -3mm are used to monitor the piers' x and y alignment - readings are taken from the corners of each pier every 4m. Their heliotropic behaviour could severely throw out observations, Tiberghien notes.

To compensate, thermal sensors are embedded in each face of the pier, allowing Eiffage's engineers to predict its response to heat and cold.

Project team Architectural design:

Foster & Partners Pier structural engineering:

EEG Simecsol Deck structural engineering:

Greisch Geotechnical engineering:

Terrasol Main contractor and concessionaire for operation: Eiffage Formwork subcontractor:

Peri Steelwork contractor:

Eiffel Construction Metallique Deck launch subcontractor:

Enerpac Cable stay supplier:

Freyssinet

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