Pushing engineering boundaries, the Millau Viaduct was built across one of the deepest valleys in France and is taller than the Eiffel Tower. The graceful bridge supported on needlelike piers is a result of rigor, precision and professionalism and received the 2006 International Association for Bridge and Structural Engineering Outstanding Structure Award. Officially opened in 2004, the Millau Viaduct is the tallest bridge structure in the world, renowned for its beauty and size. While many viewed it as an impossible project, the engineers overcame many challenges such as using the tallest bridge piers in the world, putting up a 36,000 ton motorway and a local geology that was susceptible to mud slides due to fluctuating river levels. The viaduct comprises of 8 consecutive cable stayed spans totaling 8,100 feet and rises 1,100 feet above the ground. Its roadway stretches 909 feet above the Tarn River and it is the last link in the A75 motorway from Clermont-Ferrand to Beziers.
Before the bridge was built, the Larzac plateau in southern France was famous for its bad traffic bottlenecks that would clog during the summer holiday migration between Paris and Barcelona. The first sketches for the bridge were drawn in 1987 and four years later the decision was taken to build a high crossing. Detailed studies began in 1993 and after a restricted design competition the solution designed by the Sogelerg consortium which consisted of Michel Virlogeux (a structural engineer) and Norman Foster (an architect) was chosen. Two challenges were identified in designing the structure: crossing the River Tarn, and spanning the huge gap from one plateau to the other. Michel Virlogeux proposed a multi-span, cable-stayed bridge with a phalanx of seven masts supporting cable fans. Norman Foster aspired to design a bridge that was delicate, elegant and almost looked like it was floating. The resulting creation illustrated an epic collaboration between engineering and architecture.
Constructing the Millau Viaduct
Taking a little over 3 years to complete, construction on the viaduct started in October 2001. A concrete factory was created on site as the project used thousands of tons of concrete. Deep foundations were set into the bedrock so that the piers that would support the bridge could shoot up. The heights of these piers have different ranges and were erected by hydraulic telescopic equipment. With the highest pier being 234 meters tall, precision was of paramount importance. To avoid the slightest error, constant GPS checks were carried out, ensuring a final precision of 5mm in all directions on completion. The piers were sunk in shafts of reinforced concrete in a pyramidal shape, being divided in an overturned V, and the shrouds were anchored and distributed in semi harps. Each day the shanks would increase in height by 4 meters due to sliding shuttering that allowed a new layer of concrete could be poured every 20 minutes. Hundreds of high-pressure hydraulic cylinders and pumps were utilised to push-launch the deck spans in place and a PC-synchronised lifting system to lift the auxiliary piers.
The deck was launched in 2003 and was constructed from new high-grade steel. This helped the deck to be pre-constructed in 2,000 pieces at Eiffage’s Alsace factory and be aligned by GPS, 60cm at a time. As the decks could not be craned into position (due to the height and distance between piers deck sections) they were pushed from both sides. To support the deck and reduce the span, temporary piers were erected. Additionally, cable-stayed pylons were used to support the overhanging sections, with a rail like structure installed below the deck. To adapt to the expansion and contraction of the concrete deck, there was 1 meter of empty space at its extremities and each column was split into two thinner, more flexible columns below the roadway, forming an A-frame above the deck level. By April 2004 the last launching of the deck from the northern side was carried out. This left two more launch phases to be carried out from the southern side of the bridge. In total 18 launches were required to position the deck, with the last launch accomplished on May 28, 2004. During the positioning of the decks, wind was a major concern and the engineers would wait for 3-day weather windows to reduce the chance of the deck flying out of control. Side screens were also put up to reduce the effects of the wind by 50%. Furthermore a 16-lane toll station is housed under a structure made of a special concrete. It includes a CCTV connection to the viaduct and the highway and accommodates technical and administrative services. Some of the safety measures that feature on the bridge include provision of an emergency telephones, as well as a 24-hour surveillance system that incorporates video recording of traffic, weather stations, and programmable message boards.
Some subtle details were incorporated into the overall design to enhance its visual impact, making the viaduct appear to float across the valley. Curving slightly, the roadway adds a third dimension of shape to the structure and offsets the boredom of repetition. An unusual elegance is lent to the tall, slender piers that gradually split the column into two before closing up again within the mast above the road deck. The faceted sides catch the light and reduce the apparent visual mass. From the valley below, each pier and mast merges to look like a single needle with an elongated eye. The tall wind barrier along the edges of the roadway is highlighted in a stylish aerodynamic shape that still allows for a fantastic view of the valley beyond. The cables were painted white, vanishing into the sky on overcast days. The piers and masts were illuminated so that they appeared to be bright exclamation points lofted in the night sky.
A huge success, the construction of the viaduct took only three years to complete and was three months ahead of schedule and on budget. It was opened by president Jacques Chirac during a ceremony in which fighter jets soared above. It was viewed by approximately 400,000 visitors before it was formally opened to traffic. Since its completion, the bridge has drawn a major number of tourists, resulting in several new hotels and shops being built. It is intended to last for about 120 years.
To address environmental concerns, a wastewater treatment system was installed during construction to avoid polluting the soil, and rainwater was collected from the viaduct and treated in tanks. The use of steel to build the bridge reduced the time the local environment was subject to the impacts of construction and allowed the structure to merge its surroundings. Additionally, prefabrication of materials meant that there was less equipment on site and less transportation. While the carbon footprint of the project was great, it is estimated that the fuel savings will offset the viaducts carbon footprint.