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Millau Viaduct: The World’s Tallest Bridge

In the early 1960s, a cynical British commentator remarked that a motorway could be defined as “the shortest distance between two traffic jams”. In the 1980s, France had a slightly different problem. Two motorways from opposite directions, leading to the same traffic jam – Millau.

Situated at the bottom of one of the deepest valleys in Europe, the medieval town, a centre of the ceramics trade in Roman times and with its roots back to the Bronze Age, must always have been a difficult place to get to, through or round. In the 20th century it was on the most popular holiday route in the country – Paris to the Mediterranean – and regularly suffered five- or six-hour traffic jams throughout July and August.

The high-speed routes to the north and south stopped short of the chasm that was the Tarn Valley. How to bridge the gap occupied the minds of planners, engineers and architects for the best part of two decades.

Which way to go?

Four possible routes were considered. The road could bypass Millau to the east, crossing the Tarn and Dourbie rivers on two very high bridges with spans of 800 and 1 000 metres respectively. These posed technical problems but the main objection to this route was that Millau itself would be virtually cut off from the outside world.

The four proposed routes for the new A75 autoroute around Millau
The four proposed routes for the new A75 autoroute around Millau. By Nepomuk, is licensed under CC-BY-SA

A bypass to the west was technically easier but more expensive and 12 kilometres longer. The main drawback to this solution was the adverse impact on the environment, a factor that was particularly important considering the spectacular beauty of this area of the Massif Central.

A third suggested route was rejected because of its possible impact on future plans for the area, leaving the fourth possibility, also to the west of Millau, crossing not only the river but the entire valley of the Tarn, as the preferred route. This could be accomplished in one of two ways: A descent into the valley, followed by a bridge, a viaduct and a tunnel; or the seemingly impossible – a 2.5-kilometre-long viaduct, more than 200 metres above the river.

They chose the impossible.

Ambitious plans

Dr Michel Virlogeux
Dr Michel Virlogeux. By Mossot, is
licensed under CC-BY-SA

Fresh from the success of his Pont de Normandie, at the time the longest cable-stayed bridge in the world, Dr Michel Virlogeux had ambitious plans for Millau. While the Normandy Bridge had cable stays on both sides of the road deck, his concept for the Millau Viaduct had just one central set of stays and would be carried by, not two, but nine piers, right across the valley from the plateau on one side to the plateau on the other.

Unwilling to take a chance on one man’s ideas, the French roads administration announced a competition for architects and engineers to come up with a practical design. By July 1993 the applications by 17 engineers and 38 architects had been whittled down to eight structural engineers and seven architects to study the problem. Between them and an independent panel of experts, they came up with five general design ideas by February 1995.

The competition was relaunched, with five engineering/architectural partnerships doing in-depth studies of the selected approaches to the problem and, in July 1996, the multiple-span cable-stayed viaduct proposed by the structural engineering group Sogelerg, Europe Études Gecti and Serf, with British architect Lord Norman Foster, was selected. Foster had taken Virlogeux’s concept even further into the realms of the impossible, by cutting the number of piers to seven and making them even slimmer. The basis of his thinking was to take an inventive piece of engineering and turn it into a work of art – something that would appear to rest lightly on the incredible mountain landscape.

The devil in the detail

The next two-and-a-half years saw extensive studies being made so the intricate details of the design could be finalised. A geological survey showed that the fractured limestone, coupled with the myriad of caves in the area, might pose a problem in the form of landslides, while an 18-month meteorological study showed that the wind being funnelled through the gorge could gust up to 130 kilometres an hour – hurricane force. Wind tunnel tests led to alterations in the shape of the road deck and some detailed corrections being made to the shape of the pylons but, by late 1998, the final design was approved. The project went out to tender in 1999 and was awarded to Compagnie Eiffage du Viaduc de Millau, working with the architect.

millau Viaduc Valley
Millau Viaduc Valley. By Mike Switzerland, is licensed under CC-BY-SA

Now all they had to do was to build the tallest bridge piers in the world and put a 36,000-tonne freeway on top of them. That was for starters. Then they had to erect seven steel pylons each weighing 700 tonnes and secure the road deck with 5,000 tonnes of pre-stressed steel cabling. And they had to do it in under four years or face $30,000 a day penalties for late delivery. Even some of the engineers had their doubts.

Two weeks after the laying of the first stone on December 14, 2001, the workers started digging the shafts for the pilings, four to each pier, 15 metres deep and five metres in diameter. The footings on top of the concrete pilings took another 2,000 cubic metres of concrete and now progress began to show above ground.

The piers start to grow

Every three days each pier grew by four metres. Then, because of the tapering design of the piers, the 15-tonne mould had to be taken down and adjusted for the next pour. The concrete was being manufactured on site, so a new layer could be poured every 20 minutes and the speed of construction increased rapidly.

It was during this phase that the geologists’ fears were realised. A violent storm caused a landslide and 4,000 cubic metres of rock were dislodged near the first pier. Fortunately, the pier wasn’t damaged but manpower had to be diverted to stabilising the ground – and time was of the essence.

Construction continued, with each team aiming its pier to an exact point in the sky. With no visual reference as to whether the piers were straight, the engineers relied on GPS, using multiple satellite feeds, to pinpont the destination of the build. By November 2003, the piers had reached their full height – a month ahead of schedule and accurate to within two centimetres.

Meanwhile, the steel company Eiffel, founded by Gustav Eiffel of Eiffel Tower fame, was manufacturing the steel road deck. The 2,200 separate sections, each weighing up to 90 tonnes and some as long as 22 metres, then had to be transported hundreds of kilometres by road and welded together on site. The plan was to slide the two colossal sections across the piers from either side of the valley to meet in the middle.

Pylons come into play

To stop the leading edge from dipping and knocking down the piers, one of the pylons was installed on each section to hold the cable stays supporting the front of the deck. Temporary steel support towers were placed at each halfway point between piers to make the distance between them more manageable. Even so, the road deck would still have to be launched over greater distances than had ever been done before.

Viaduc-Millau pylone
Viaduc-Millau pylon. By Roulex 45, is licensed under CC-BY-SA

Also, simply launching the sections over the edge by pushing them with hydraulic jacks was not going to work in the case of such enormous sections – the jacks would need a considerable amount of help along the way. The engineers designed a novel system of pairs of hydraulically driven wedges, four sets of which were installed on the top of each pier. The upper and lower wedge of each pair pointed in opposite directions. Controlled by a computer so that they acted in perfect unison, the lower wedges would slide under the upper ones, forcing them high enough to lift the road deck off its supports. Both wedges would then slide forward, moving the deck forward. The lower wedges would then return to their starting positions, followed by the upper wedges, leaving the deck 600mm further along its journey. Then the four-minute cycle would be repeated.

No launch had ever been done this way before and there was no chance to test the system. It just HAD to work.

And it did. Everything went smoothly until six months into the launch, when one of the launch systems failed. To make matters worse, the meteorologists were predicting a storm and the deck was in a vulnerable position, with its leading edge hanging in space. The engineers had underestimated the friction between the sliding surfaces of the wedges and the non-stick PTFE coating had worn away. There were no spare parts for this impromptu design, but there were as yet unused pairs of wedges which were destined for the piers that had yet to be reached by the advancing deck. The team hastily stripped them of their coating and repaired the damaged units, while the weathermen chewed their nails and monitored the impending storm.

Disaster had been averted. The deck reached its next support safely.

Heading for the middle

Over the next months to two sections of the deck edged towards each other. As each reached its next support the teams breathed a collective sigh of relief and checked the weather forecast before pushing on to the next stage. Things were going well, but there was still no guarantee that the two sides would meet in the right place. Even the slightest inaccuracy could mean that they had built the most expensive white elephant in Europe. The engineers installed a GPS system on the leading edge of the section that was to make the final push so they could compare the actual position with their calculations. They now approached the most difficult part of the launch – bridging the river itself.

Not only was this the longest span of the viaduct, but it was also the one place where it had been impossible to erect any intermediate supports. The leading edge of the longer section launched across 342 metres of open space and the teams held their breath as the suspense mounted… And the French Prime Minister was due to drop by to see the event! As the edges got closer together, the tension eased. It looked as if it would be a near-perfect fit. A magnum of Champagne was positioned at the point of contact and, as it exploded, other corks popped – celebrations were in order. The discrepancy in the alignment was a matter of millimetres.

Of course, the project was nowhere near complete but the first two major challenges – the piers and the road deck – had been successfully navigated and they were still on schedule.

Because steel is flexible – more so than some of them had realised – the road deck had an undulating appearance at this stage that was a cause for worry. Would the cable stays pull it straight or had they got another unexpected problem?

Making use of an ancient technique

Before that question could be answered, the remaining five pylons had to be erected. These 700-tonne steel monsters had to be raised through 90 degrees and accurately positioned on top of the piers. To achieve this, they borrowed a 2,000-year-old technique from the ancient Egyptians, who had used it to erect obelisks and pillars at Karnak. While the Egyptians would have used slaves as their motive power, the 21st century engineers had the advantage of hydraulics to lift this massive weight.

The principle was straightforward. As Archimedes summed it up: Give me a lever long enough on a fulcrum and I’ll move the world.

On top of the road deck the team put up two enormous towers, secured by cables and equipped with a hydraulic system capable of raising a 1000 tons. As the hydraulics lifted each pylon it pivoted slowly until it was vertical and could be lowered safely onto its anchoring point. With all seven pylons in place, the team attached the cables which support the deck. As the tension on the cables increased, so the kinks in the road deck smoothed out and another challenge had been met.

There just remained the finishing touches. The road surface added 10,000 tonnes to the weight of the deck and, just to be sure it was safe, they drove 36 monster trucks, with a combined weight of over 900 tonnes, onto the longest span. The distortion was negligible. On December 14, 2004, President Jacques Chirac formally opened the viaduct and it opened to traffic two days later. This was almost a month ahead of schedule.

The critics are proved wrong

The construction of the Millau Viaduct broke several records. Two of the piers were the highest in the world; the pylon on top of the second pier was the highest bridge tower in the world; at 270 metres above the Tarn, the road deck was almost twice the height of the previous European record holders.

Critics of the project had said that the technical difficulties would be insurmountable and the whole scheme was doomed to fail. They were proved wrong.

Others said that tourists would avoid the bridge rather than pay the toll fee; the project would never break even; toll income would never amortise the initial investment and the contractor would have to be supported by subsidies. They would be proved very wrong the following summer.

The Millau Viaduct was an instant success and, at the height of the tourist season, carried more than 60,000 vehicles per day. At € 8.30 per vehicle, the viaduct would pay for itself in less than three years.

References:

aboutcivil.org

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