In the south of France, there is a deep 2 km wide valley in the Massif Central Mountain Range at Aveyron divided by the River Tarn. The French government decided an alternative to the nearly saturated Rhône Valley route was needed — as well as a solution to the notorious N9 Millau bottleneck — and so they approved the idea proposed by the owners ‘Compagnie Eiffage du Millau Viaduct’ (CEVM) and the client ‘SETEC’ to cross the Tarn gorges by a viaduct/ road bridge, saving 30 minutes under normal conditions and up to almost 4 hours on some weekends in the summer.
A 40km extension to the A75 (from Paris which runs through Clermont-Ferrand towards Beziers heading eventually for Barcelona), crossing the Tarn would be 60 km shorter than the Rhône Valley route and would enable road hauliers to reduce significantly the journey times for their lorries — 45 minutes on the Paris/Perpignan, Paris/Barcelona or Amsterdam/Perpignan routes.
‘SETEC’ approached the British Architect Sir Norman Foster to design a bridge to last 120 years.
All designs have to satisfy the design brief, but Fosters knew that this bridge posed several novel problems. Tests revealed problems for drivers on such a high, long and thin structure (just two lanes either way) — despite the estimated journey time to be only about a minute by car.
The solution was to incline the bridge slightly (3%) to improve road visibility, and to make the whole structure curved (to lessen the sensation of floating) — even though this would lengthen the bridge to 2.5 km and add to the cost. To prevent drivers from the distraction of the beautiful scenery, the hard shoulder on both sides was increased in width to three metres. Emergency phones were designed for every 500 metres along the full length on each side.
Lord Foster wanted the bridge to look as transparent and lightweight as possible to reduce cost, but also to attempt to minimise the structure in its environment, reduce wind loadings and so forth (the design wind loading was 151km/hr). This implied a multicable-stayed bridge, in sections.
- However, in branding the bridge, instead of the usual two or three columns expected, Michel Virlogeux chose an amazing SEVEN reinforced concrete pylons / columns ranging in height from 75 m to 235 m, six steel decks of 342 m long, and decided to fan the steel cables.
To recoup some costs, Fosters decided to use the absolute minimum amount of material: the masts rising above the road deck and the multi-span cables are all in steel. Even the deck itself is constructed from a new high-grade steel as opposed to concrete.
Three metre transparent high side screens were required to halve the effects of the wind and bring it down to the same wind levels as found at ground level around Larzac and Sauveterre.
The deck was designed to be prefabricated in 2000 sections at Eiffage’s Lauterbourg factory in Alsace and GPS-aligned, 600 mm at a time. The factory was given just 20 months to supply the elements for the deck and the piers of the viaduct.
The decks are supported by multi-span cables placed in the middle. In allowing for expansion and contraction of the deck, there is a metre of empty space at its extremities and each column is split into two thinner, more flexible columns below the roadway, forming an A-frame above the deck level.
The figures began to stack up: nearly 500 workers would be required on site for the million man-hours of estimated work, along with 127000 m³ of concrete, 19000 tonnes of steel-reinforced concrete and 5000 tonnes of pre-constraint steel (cables and shrouds). It was only estimated to cost about 400 million EUR (267.4 million GBP). The bridge is designed to weigh just 290000 tonnes.
‘Terrasol’ were employed as the geotechnical engineers, and it was estimated that three hundred thousand cubic metres of earth moving would be required. The main contractor was ‘Eiffage TP’, with co-contractor ‘Eiffel Construction Metallique’. ‘Enerpac’ supplied the hydraulic system for lifting and pushing the bridge spans and piers for the bridge.
- The height above the River Tarn was going to be about 270m — but Pier 2 at 343 metres would make this the highest bridge in the world. A media buzz started as a result.
Works started in October 2001, and at a ceremony on 14 December 2001, the French minister Mr. Jean-Claude Gayssot laid the first stone, by November the following year, the highest pier had already reached 100m in height. Launching the deck started in February 2003, and was completed by May 2004.
An 18-lane toll station 6km north of the ensures that the bridge’s capital costs would be recouped over the 75 year operating concession granted to ‘Eiffage’. A surveillance and control centre will monitor the viaduct round-the-clock. It will be equipped with high-performance security equipment such as: video surveillance linked to an automatic incident-detection system that will immediately inform the control centre if there is any anomaly in the traffic, automatic video recording, a vehicle counting facility, weather stations, and programmable message boards enabling information to be issued instantaneously.
Road users will also be able to enjoy the countryside and the rest areas which, according to one company director, ‘enable drivers to have some fresh air, where there is plenty of space to relax in a most beautiful environment’.
The Viaduct was officially opened on 14 December at a ceremony by Monsieur Jacques Chirac, President of the French Republic, and opened to traffic on the 17th.