The Millau Viaduct, an impressive bridge that connects Northern Europe and Eastern Spain, holds the title of the world’s tallest bridge. With eight spans suspended from seven pylons, the bridge stands at a total height of 345 meters, surpassing the Eiffel Tower in height. The construction of this cable-stayed bridge was completed in just three years and was opened to the public in 2004.
The city of Millau is situated at the confluence of the Tarn and Dourbie rivers, which have carved deep valleys in the old Massif central plateau. Choosing a suitable road alignment was a challenging task, as the motorway needed to pass from a plateau on the north (at an altitude of about 600 meters) to the Larzac Plateau on the south (at an altitude of about 720 meters). Moreover, the presence of unstable clay in the lower portions of the hills further complicated the decision-making process.
After evaluating various options, it was determined that the best solution was to construct a viaduct that would directly span from plateau to plateau, soaring 275 meters above the Tarn river. The pylons supporting the viaduct reach an impressive height of 803 feet, making them the tallest pylons in the world.
Figure-1: Millau Viaduct, tallest bridge in the world
The decision was made to create a slender bridge with a cable-stayed design and a series of equal spans, which would have a majestic appearance when viewed from Millau city. The bridge’s height is so remarkable that it can be seen from the city itself.
1. Geology of the Millau Viaduct Site:
The Millau Viaduct, constructed on two limestone plateaus, is situated in a deep valley formed by erosion from the Tarn River. The site consists exclusively of sedimentary rock, including dolomitic limestone and loosely bonded marls. Local tectonics reveal the presence of old faults to the north of the viaduct, which affect the older horizons of the geological structure but not the more recent ones on the top of the southern plateau.
There are also more recent non-active faults that cut across the viaduct site, particularly around pier P4 and between pier P7 and the southern abutment C8. The strike slips of these faults have posed challenges during construction, necessitating adaptations to the foundations. The rock mass rating (RMR) on the site ranges from 0 to 105, with mean values of 65 for the limestone and 53 for the marls.
The foundation rocks along the viaduct can be categorized into three types. The first type is the Bajocian dolomitic limestone at the northern abutment, which is very hard with an unconfined compressive strength of 110 MPa, but contains karsts filled with clay. The second type is the compacted marls from pier P7 to pier P6, which exhibit slides at the soil surface due to a 2 m thick scree layer over soft clay, with mean values of shear strength determined for the top layer of marls at RMR=45, C = 0.1 MPa, and φ = 300.
The third type is the Hettangian limestone on the two sides of the Tarn River from pier P4 to the abutment, with sub-horizontal bedding on the south side and a 150 angle on the north side. The determined shear strength values for this rock type range from RMR = 65 to 70, C = 2.5 MPa, and φ = 370. Based on these observations, it is evident that the limestone is more resistant than the marls, leading to the use of longer length piles at marls rock locations compared to the limestone rock locations.
Figure-2: Geology of the Millau Viaduct site
2. Bridge Cross-Section
The Millau Viaduct is a bridge that spans 2460 meters in length and consists of eight spans. Among these spans, there are two side spans that are 204 meters long, and six intermediate spans that are 342 meters long. The cross-section of the viaduct is a streamlined orthotropic steel box-girder, which includes two vertical webs as required by the selected erection technique.
Figure-3: Cross-sectional view of Millau Viaduct
The box-girder bridge has been constructed with tri-angulated cross-beams, spaced at 417 m longitudinally, instead of full diaphragms. This design choice was made to accommodate two lanes of traffic in each direction, along with 3 m wide shoulders to increase the distance between the traffic and the bridge edge, thereby reducing the risk of vertigo for drivers. Additionally, the box-girder is equipped with windscreens and fairings. The windscreens are designed to limit the wind velocity on the viaduct to the value at the approach ground level, in order to prevent wind shocks to vehicles arriving on the bridge. The fairings are intended to improve both the aerodynamic streamlining and aesthetic quality of the bridge, in addition to the classical barriers in place.
3. Foundation Details of the Millau Viaduct
The viaduct’s design was created by Michel Virlogeux, with foundation systems for the piers and abutments defined and designed by the authorities based on his plans. The foundation system is based on similar principles, but slight variations are made depending on whether the bearing is located on limestone or marls. Marls have weaker mechanical properties and exhibit superficial slide that affects the upper part.
For abutments C0 and C8, which are founded on limestone, spread foundations were chosen. The foundation system consists of a monolithic set comprising a 1-meter-thick raft foundation for each front abutment, connected to two side footings for each rear abutment. The abutment platforms are located at different levels.
Figure-4: Pile foundation used for the construction of piers of Millau Viaduct
The foundation system for each of the seven piers consists of four reinforced-concrete piles with a diameter of 5 m and a depth of 10-15 m, drilled into the rock and bonded together at the top by a 3.5 m thick reinforced-concrete footing, which is also bonded to the pier. In marls, the footing is thicker and the piles are deeper, with their base diameter increased to 7 m.
Pier-2, which is the tallest pier at 245 m, is founded on limestone, while pier-6, which is founded on marls, has a medium height. The behavior of this type of pier foundation system is complex, as it is a piled raft foundation system where part of the load is transferred to the footing. However, the simplifications made in the design are restrictive, assuming that the footing bears none of the load and that no skin friction is created along the pile shaft, except in the case of tensile stress.
This assumption implies that the bearing capacity depends solely on the ultimate pressure on the rock at the bottom of the pile shaft, and that settlement results only from deformations of the rock at the bottom of the shaft, which makes the foundations more flexible than they actually are. In order to assess the skin friction along the pile shaft, several pile-loading trial tests were conducted in the marly soils. One of these tests on a bored pile with a diameter of 0.80 m showed that the critical load was approximately 5200 kN for a settlement of 5.6 mm.
Despite the uncertainties in assessing the mechanical properties of the rock and the calculation methods used, the design for the pier foundations appears to be reliable.
4. Piers of the Millau Viaduct
The design of the bridge is driven by various structural demands, including the need to balance unsymmetrical live loads in multiple cable-stayed spans and accommodate length variations caused by temperature effects in the box-girder. Additionally, the piers are designed to resist high bending moments due to their considerable height. They are constructed as wide strong box-sections that split into two flexible shafts in the upper 90 meters.
To ensure stability, the box-girder deck is anchored to the pier using vertical prestressing tendons aligned with the two fixed bearings on each shaft. The pylon above the pier takes on the shape of an inverted V. During the presence of unsymmetrical live loads or extreme wind loads, the vertical load on each bearing can reach up to 100 MN.
Figure-5: Pier construction process
To reduce the size of the bearings, spherical bearings coated with a new composite material capable of withstanding stresses up to 180 MPa under ultimate loads were utilized. The piers for the structure have a varying cross-section, designed in a way that allows for ease in construction despite their variations. Four panels have fixed dimensions, while the other four panels change slightly in each segment, including their orientation. This design feature enables the use of external self-climbing forms and classical internal shutters moved by the tower crane during erection. The two tallest piers, named P2 and P3, are 245 m and 223 m in height respectively. The tallest tower crane, used for P2, reaches a height of 275 m. As a result, it was necessary to fix each tower crane to the corresponding pier step by step as per the construction progress. Each pier is supported by a series of four artificial wells with diameters ranging from 4 to 5 m and depths ranging from 9 to 16 m.
Figure-6: Elevation of tallest pier of Millau Viaduct
5. Pylons of the Millau Viaduct
On May 18, 2004, the pylons for the bridge over the Tarn River were transported to the construction site after being fabricated in different factories and assembled behind the bridge abutments. Each pylon was transported one by one onto the deck using two crawlers, and the weight of the convoy reached 8 MN, putting extreme load on the structure and serving as a load test.
Once on site, the pylons were positioned horizontally and then tilted up into their vertical position with the help of a cable-stayed temporary support tower. This marked the final stage of the structure’s construction. The installation and tensioning of the stay-cables were done using the Freyssinet system, completing the construction process.
Figure-7: Pylons construction process
6. Launching System Used for Construction of the Millau Viaduct
The steel box-girder deck was launched in a unique manner, starting from both ends and meeting above the Tarn River between piers P2 and P3 for final closure. Intermediate temporary supports were installed in each span, except for the closure span, in the form of tubular trusses measuring 12m by 12m. In the intermediate spans, these temporary supports were positioned at mid-span and had two lines of launching equipment, effectively reducing the launching span to approximately 150m. On the other hand, the temporary supports in the side-spans were simpler, smaller, and featured only one line of launching equipment.
Each of the two structures that were launched was equipped with its own front pylon. However, during the launching process, the pylons were designed without their summits to minimize wind effects. As a result, the pylon height was limited to a range of 70m to 87m. Additionally, six stay-cables were incorporated into each structure to reduce bending moments during the launching process.
Figure-8: Launching system used for construction of the Millau Viaduct
The launching operations for the project were conducted in increments of 171 meters, and the duration varied from five days for the initial complex launches to three days for typical ones, assuming favorable weather conditions. Launches were only permitted if the meteorological station forecasted wind speeds below 37 km/h during the designated launching period.
The launching system utilized innovative technology to account for the extreme height of the piers. Active launching bearings were installed on each support, with two bearings per line. These bearings were equipped with horizontal hydraulic jacks that could be controlled by a central computer command, along with sensors to maintain consistent displacement across all supports in real-time. This allowed for precise balancing of friction forces within each support during the launching process.
FAQs
Who designed the Millau Viaduct?
In 1990, the Millau Viaduct was designed by Michel Virlogeux.
What was the cost of construction of the Millau Viaduct?
The Millau Viaduct, a renowned bridge located in southern France, was constructed with a total expenditure of €300 million. This structure is considered one of the most significant engineering feats of modern times and spans across the Tarn River Valley, connecting the cities of Clermont-Ferrand and Beziers.
The construction of the bridge was completed within the allocated budget of €300 million. The Millau Viaduct was opened in December 2004 after three years of construction, and it now stands as a testament to the innovative engineering and design capabilities of its creators.
The bridge itself is an impressive feat of modern architecture, standing at a height of 343 meters and spanning 2.46 kilometers in length. The Millau Viaduct’s construction was a significant undertaking, involving the collaboration of engineers, architects, and construction workers.
Despite the scale of the project and its associated costs, the Millau Viaduct has proven to be a vital transportation link, reducing travel time for commuters and boosting economic development in the region. The bridge has also become a popular tourist attraction, drawing visitors from around the world to marvel at its breathtaking design and remarkable engineering
When was the Millau Viaduct completed?
The Millau Viaduct was opened on December 14, 2004, just 38 months after construction began. This impressive feat of engineering was quickly made available for public use, as it opened to traffic only two days later on December 16, 2004.
The Millau Viaduct, located in southern France, is a cable-stayed bridge that spans the Tarn River Valley. It was designed by the French engineer Michel Virlogeux and British architect Norman Foster. The viaduct is considered a masterpiece of modern engineering, as it stands 343 meters tall, making it the tallest bridge in the world at the time of its construction.
The construction of the Millau Viaduct was a major undertaking, involving many workers and extensive planning. The goal was to create a structure that could withstand strong winds and other natural elements, while also providing a safe and efficient transportation option. The result was a bridge that not only met these goals but also became a new symbol of France’s engineering prowess.
Today, the Millau Viaduct remains an impressive example of modern architecture and engineering. It is a popular tourist destination, offering visitors stunning views of the surrounding landscape and a glimpse into the remarkable achievements of human ingenuity. Its speedy construction and impressive design have made it a source of pride for the people of France, and a testament to what can be accomplished with hard work, dedication, and a commitment to excellence.
What is the type of foundation used in the construction of the Millau Viaduct?
The abutments for a construction project were founded on limestone, and spread foundations were selected for this purpose. In total, there were seven piers, and for each pier, the foundation system comprised of four reinforced-concrete piles. These piles had a diameter of 5 meters and a depth ranging from 10 to 15 meters. The piles were drilled into the rock and bonded together at the top using a 3.5-meter-thick reinforced-concrete footing.