At the forefront of technology
The construction of the Millau Viaduct solicited most of the latest techniques used in the field of public works. Until then, no construction site had brought together in one single place such a concentration of technologies. Laser technology, GPS, hydraulic rams, climbing formwork, special asphalt, high performance concrete, innovative materials… were all instrumental in the success of a very out of the ordinary structure. |
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Concrete
Piers in time
At 4 per pier, fifteen metres deep and 5m wide, the morocan wells form the foundations and ensure stability. At the foot of each pier, a foundation slab, 3 to 5m thick, reinforces the action of the wells.
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The 2100 m3 of concrete necessary for each widest foundation slab were poured in one continuous operation.
By March 2002, the piers had started to rise. The site soon reached its cruising speed. Every 3 days, each pier rose by 4 m. This performance was largely due to the self-climbing formworks. Thanks to a system of anchoring blocks and rails fixed onto the shafts of the piers, 20 minutes were all it took to gain the height necessary for the pouring of a new layer of concrete.
In parallel with the piers, the abutments were built on the Causse du Larzac and the Causse rouge. These are concrete structures which anchor the deck… to firm ground!
On 9 December 2003, the piers and abutments were finished. A great success for Eiffage TP several weeks ahead of schedule and, on top of that, the world record for the tallest pier for "P2".
The piers in figures…
The piers of the Millau viaduct are numbered from 1 to 7, from the north to the south of the structure. Here are their respective heights:
P1: 94.50 m
P2: 244.96 m
P3: 221.05 m
P4: 144.21 m
P5: 136.42 m
P6: 111.94 m
P7: 77.56 m
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Deck
20 months to assemble 36000 tonnes of steel
The steel deck of the Millau viaduct is made up of 173 central box beams, the backbone of the construction, on which were welded the lateral floors and the side box beams. A 4,20 m height, each one of these central box beams is long 15 to 22 m and can weigh up to 90 t. The width of the deck is 32 m, while its total mass borders the 36000 tons.
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Originating from the Eiffel factory in Fos-sur-Mer, the central box beams were brought, by special wide load vehicles, to Millau via Nîmes and the plateau du Larzac.
Manufactured in Lauterbourg, in Alsace, the lateral floors were transported via Clermont-Ferrand then via the Causse Rouge. Because of the slight curve of the viaduct, each part of the deck constitutes a unique piece of the gigantic puzzle.
Two open-air sites were set up behind the abutments, to the north and south of the viaduct. All the welding and assembly work was done there. 96 % of the tasks were carried out at ground level, thus reducing the risks of working at great height. 1743 metres were assembled on the south side, compared to 717 on the north side. From September 2002 to May 2004, the 150 employees in charge of constructing the deck worked for 20 months.
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Launching
The deck leaps out
Setting the steel deck in place on the piers required a specific launching technique. Section by section– each one the length of half a span, or 171 m –, was launched into space.
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To do this, 64 conveyors were used. Installed on the piers and the temporary piers (gigantic steel crutches used as intermediary supports between two piers), they made it possible to move the 36000 t of the deck.
Each conveyor is formed with a frame supporting the deck. Inside this frame, two wedges are driven by hydraulic rams. The lower wedge raises the upper wedge which takes the weight of the deck. A hydraulic jack then makes it possible to move the whole section 60 cm forward. The lower wedge is retracted and the sliders go back to their initial positions. Each conveyor is connected to a computer-controlled hydraulic system, so that their movements are perfectly synchronised.
At the speed of one every four weeks, eighteen launching operations were necessary to bring the two parts of the deck above the Tarn. At an average speed of 9 metres per hour, each of these operations required 48 hours of non-stop work. The juncture of the deck took place on 28 May 2004 at 2:12pm above the Tarn.
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Pylons
Seven steel masts for one viaduct
From the beginning of the launching operations, a partially stay cabled pylon was placed at the end of each part of the deck to prevent it from "taking a nose dive" when it was pushed from one pier to the next. The setting in place of the other five pylons began after the join over the Tarn, at the end of May 2004. This operation took only 3 months.
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Laid on their sides, they are transported by four self-propelled carts on to the deck at the base of the concrete pier on which they are to be installed. Then caught in a pincer movement slightly above its centre of gravity by two huge steel arms, each pylon (700t and 90m high) was progressively lifted by two hydraulic jacks with a total force of 2000 t.
During these operations, a perfectly controlled swinging motion made it possible to position the pylons vertically, just above their anchoring point. They were then firmly fixed in place.
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Stays
1500 tonnes of stretched cables
Each pylon of the Millau viaduct is equipped with a mono-axial layer of 11 pairs of stay-cables placed face to face.
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Depending on their length, they are made up of 45 to 91 steel cables, or strands, each made of 7 strands of steel (a central strand with 6 intertwined strands).
The stay-cables are a result of the technology perfected by Freyssinet. Each strand has triple protection against corrosion: galvanisation, a coating of petroleum wax and an extruded polyethylene sheath. The exterior envelope of the stay-cables is itself equipped along the entire length with a double helical weather-strip. This is to prevent water running down, which, in the event of high winds would cause the stay-cables to vibrate, which in turn would affect the stability of the viaduct.
The stays were installed using a well-tried technique. After threading one strand in the outer protective sheath, it is pulled up on to the pylon to its final location. The strand is then fixed in the upper and lower anchorage points. A "shuttle" then brings the other strands one by one, and they are then stretched to tension. For the longest stays, the overall force applied is from 1200 t.. |
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Surfacing
tested and approved
To cope with any expansion of the deck, a special road surface was perfected by the Appia research teams.
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Flexible enough to adapt to deformations of the steel without cracking, it must nevertheless have sufficient strength to meet motorway conditions (density, texture, adherence, anti-rutting, etc.…). It took two years to find the perfect formula.
Several operations preceded the surfacing. High-pressure projection of steel bearings one millimetre in diameter (shot blasting) removed all traces of rust from the deck. A primary bonding coat was applied on to the bared steel before laying a 4 mm thick layer of bitumen, thermo-sealed at 400 °C. This ensures total protection against all risk of corrosion.
The surface of the Millau viaduct was laid by Appia, from 21 to 24 September 2004. Totally smooth and without a single rut, it covers the steel in a 6,7 cm layer. In total, 9500 t of bituminous concrete were needed to make the driving surface. Two surface production centres of a total capacity of 380t per hour were installed specially for this purpose about twelve kilometres north of the viaduct. Twenty-five articulated lorries ensured the continual supply of the two finishers. No break in the supply was permitted to stop the progress of the machines used to apply the surface..
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Canopy
A tendrilled concrete leaf
The buildings used for the commercial and technical teams of the viaduct and the tollgate are located nearly 4 km north of the structure.
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| The tollgate is protected by a canopy in the form of a leaf of tendrilled concrete. Made up of 53 elements (voussoirs), it is about a hundred metres long and 28 m wide. It weighs around 2500 t.
The construction of the tollgate canopy required the use of a special high-performance concrete, BSI Ceracem®. This contains metal fibres which give it enormous mechanical strength. It had never before been used for a structure of this size.
The voussoirs were poured on a special site located near the north abutment of the viaduct. In 6 months, from October 2003 to April 2004, the 53 elements were produced. A self-propelled truck of 500 horsepower and with no fewer than 120 wheels, was used to transport the voussoirs to the tollgate worksite. These were then taken over by a crane with a capacity of 500 t and positioned in their final location. By the end of June 2004, the tollgate already had its end profile. |
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Instrumentation
A viaduct examined from all sides
Piers, deck, pylons and stays are equipped with a multitude of sensors.
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| These are designed to detect the slightest movement of the viaduct and measure its resistance to wear and tear over time. Anemometers, accelerometers, inclinometers and temperature sensors are part of the array of measuring instruments used.
Twelve optical fibre extensometers were installed in the base of pier P2. The tallest pier of the viaduct, it is therefore subjected to the most stress. These sensors detect movements of a micrometer. Other extensometers – electric this time – are distributed up the entire height of P2 and P7. This apparatus is able to take up to 100 readings per second. In high winds, they ensure permanent monitoring of the reactions of the viaduct in extreme weather conditions.
Accelerometers placed strategically on the deck monitor any oscillations which might affect the metal structure. Movements of the deck at the abutments are monitored to the nearest millimetre. As for the stays, they are also instrumented and their ageing is meticulously analysed.
The information collected is sent by Ethernet to a computer in the IT room in the management building located near the toll gate. |
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> Concrete
"Piers in time" |
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