Millau Viaduct: Construction Features of the World’s Tallest Bridge

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The Millau Viaduct is the world’s tallest bridge constructed to connect Northern Europe and Eastern Spain. With its eight spans suspended from seven pylons, the total height of this cable-stayed bridge is 345 m, which makes it taller than Eiffel Tower. The construction of the bridge was completed just in three years and was opened to the public in 2004.

The city of Millau is located at the confluence of two rivers, Tarn and Dourbie, that cut two deep valleys in the old Massif central plateau. As the motorway had to pass from a plateau on the north (at an altitude of about 600 m) to the Larzac Plateau on the south (at an altitude of about 720 m) selecting a road alignment was not an easy task, and more so, considering that the lower portions of the hills are mainly made of unstable clay.

After eliminating all the unviable options, it was decided to erect a viaduct passing directly from plateau to plateau, 275 m above the Tarn river. The height of its pylons is 803 feet, which is the highest in the world.

The basic idea was to design a very slender bridge. Thus a cable-stayed bridge with a series of equal spans which would also look majestic from the city of Millau was chosen. The bridge is so tall that it is visible from Millau city.

1. Geology of the Millau Viaduct Site:

The Millau Viaduct was constructed on two limestone plateaus. The pillars of the bridge are situated in a deep valley formed due to erosion by Tarn river. Limestone plateaus, a sedimentary basin, was formed during secondary era and it is still very intact and well preserved. The following points describe the geology of the Millau Viaduct site:

1. The Millau Viaduct site consists of only sedimentary rock. The main types of sedimentary rocks present at the site are dolomitic limestone and loosely bonded marls. 
2. A study of local tectonics shows that there are old faults that affect the older horizons in the sequence. The old faults are located to the north of the viaduct but do not affect the more recent horizons of the geological structure of the top of the southern plateau.
3. There are also some more recent non-active faults that affect the whole stratigraphy of the zone and cut across the viaduct site by pier P4 and again between pier P7 and the southern abutment C8 (as shown in Figure-3).
4. The strike slips of the faults, particularly where the pier P4 is located, have caused difficulties for the construction and have required an adaptation of the foundations.
5. The rock mass rating (RMR) varies from 0 to 105. The mean values recorded on the Millau viaduct site are 65 for the limestone and 53 for the marls.
6. There are three different types of foundation rocks along the viaduct. The first one, the Bajocian dolomitic limestone at the northern abutment, is a very hard rock with an unconfined compressive strength of 110 MPa but with karsts filled with clay.
7. At the top of the platform where the raft was placed, an RMR value of 70–80 was determined.
8. The compacted marls from pier P7 to pier P6 constitute the second rock type. Slides are visible at the soil surface due to the 2 m thick scree layer underlain by soft clay above the marls. The mean values of shear strength have been determined in the 15 m thick top layer of marls: RMR=45, C = 0.1MPa, ? =30⁰
9. The Hettangian limestone on the two sides of the Tarn River from pier P4 to the abutment constitutes the third rock type. Its bedding is sub-horizontal on the south side and at a 15⁰ angle on the north side. The determined shear strength values are: RMR = 65 to 70, C = 2.5 MPa, ? = 37⁰.
10. From the above observation, it is obvious that the limestone is more resistant than the marls. Therefore, longer length piles were provided at marls rock location compared to the limestone rock location.  

2. Bridge Cross-Section

The Millau Viaduct is 2460 m in length and it consists of eight spans. Two side spans are 204 m long and six intermediate spans are 342 m long. The cross-section is a streamlined orthotropic steel box-girder with two vertical webs required by the selected erection technique.

Tri-angulated cross-beams, spaced at 417 m longitudinally, were preferred to full diaphragms. This box-girder carries two lanes of traffic in each direction with 3 m wide shoulders to increase the distance between the traffic and the bridge edge, in order to reduce the risk of vertigo.

The box-girder is equipped, in addition to classical barriers, with windscreens designed to limit the wind velocity on the viaduct to the value at the approach ground level, in order to avoid wind shocks to vehicles arriving on the bridge and of fairings intended to improve both the aerodynamic streamlining and aesthetic quality.

3. Foundation Details of the Millau Viaduct

The viaduct was designed by Michel Virlogeux and the authorities defined and designed the foundation systems for the piers and abutments based on his designs.

Although the foundation system is based on the same principles, it varies slightly depending on whether a given bearing is located on limestone or on marls. The marls not only have weaker mechanical properties than the limestone but also show superficial slide which affects the upper part.

Spread foundations were chosen for abutments C0 and C8, which are founded on limestone. The foundation system is a monolithic set composed of a 1 m thick raft foundation for each front abutment, connected to two side footings for each rear abutment with the abutment platforms at different levels.

The foundation system for each of the seven piers is composed of four reinforced-concrete piles with a diameter of 5 m and a depth of 10-15 m drilled in the rock and bonded together at the top by a 3.5 m thick reinforced-concrete footing, which is itself bonded to the pier. In marls, the footing is thicker and the piles deeper, with their base diameter being increased to 7 m.

The pier-2 is the highest one (245 m) and is founded in limestone, while the pier-6 is founded in marls and has a medium height.

The behavior of this type of pier foundation system is complex. It is a piled raft foundation system in which part of the load is transferred to the footing. The way that this behavior was simplified is particularly restrictive as it was assumed that firstly, the footing bears none of the load and secondly that no skin friction is created along the shaft except in the case of tensile stress.

This comes down to assuming that bearing capacity depends solely on the ultimate pressure on the rock at the bottom of the 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 really are.

Several pile-loading trial tests were carried out in the marly soils to assess the skin friction along the shaft. 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 the pile with settlement of 5.6 mm.

Despite the uncertainties regarding the assessment of the mechanical properties of the rock and the calculation methods used, the design for the pier foundations seems to be quite reliable.

4. Piers of the Millau Viaduct

The design of the bridge results from major structural demands; to balance unsymmetrical live loads in the multiple cable-stayed spans, as well as to adapt to the length variations due to temperature effects in the box-girder. To resist the high bending moments due to their extreme height, the piers were designed as wide strong box-sections that split into two flexible shafts in the upper last 90 m.

The box-girder deck is tied down to the pier by vertical prestressing tendons in line with the two fixed bearings on each shaft, and the pylon above has the shape of an inverted V. Under the effects of unsymmetrical live loads or extreme wind loads, the vertical load on each bearing can reach 100 MN.

To reduce the bearing size, spherical bearings covered with a new composite material to resist stresses up to 180 MPa under ultimate loads were used.

The piers have a variable cross-section, the shape of which has been designed to allow for ease in construction despite its variations. Four panels have fixed dimensions, and the other four change slightly in each segment, including their orientation. This allowed for an erection with external self-climbing forms and classical internal shutters moved by the tower crane.

The two taller piers are 245 m (P2) and 223 m (P3) high. The tallest tower crane, for P2, reached a height of 275 m. It was, therefore, necessary that each tower crane was fixed to the corresponding pier, step by step, according to the construction progress. Each pier is founded on a series of four artificial wells, 4 to 5 m in diameter, 9 to 16 m deep.

5. Pylons of the Millau Viaduct

After the closure above the Tarn River, on May 18, 2004, the pylons, which had been fabricated in different factories and assembled behind the bridge abutments, were transported one by one onto the deck, each by two crawlers. The weight of a convoy reached 8 MN, producing an extreme load test for the structure. Then the pylon, in a horizontal position, was tilted up with the help of a cable-stayed temporary support tower. The structure construction ended with the installation and tension of stay-cables by Freyssinet system.

6. Launching System Used for Construction of the Millau Viaduct

The steel box-girder deck was launched from both ends, with a final closure made above the Tarn River between piers P2 and P3. Intermediate temporary supports, each in the shape of a tubular truss, were installed in each span except for the closure span. In the intermediate spans, these temporary supports, 12 m by 12 m, were at mid-span with two lines of launching equipment to reduce the launching span to about 150 m. The temporary supports in the side-spans were simpler, smaller and with only one line of launching equipment.

Each of the two launched structures was equipped with its front pylon (without the summit to reduce wind effects during launching, limiting the pylon height from 87 to 70 m) and with six stay-cables in order to reduce bending moments during launching.

Each launching operation corresponded to 171 m and took five days, for the first launching operations which were more complex, to three days for a typical one, under good weather conditions. No launching operation could begin if winds of more than 37 km/h were to be anticipated by the meteorological station during the launching period.

The launching system was highly innovative. Due to the extreme pier height, friction forces had to be directly balanced within each support. Each support was thus equipped with active launching bearings (two per line of bearings). Horizontal hydraulic jacks at the bearings produced horizontal motion with a central command from a computer and sensors to maintain the displacement to be the same on all supports at all times.

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