Each of the 17 piers of the stone bridge weighs at least 5,000 tonnes and by visiting the interior of the piers of the structure, built under Napoleon I between 1810 and 1822, we realize that it is hollow. “The genius of the designer of the bridge is that he made it lighter, by hollowing it out to allow it better stability,” notes Mohamed Mariko, responsible for the engineering works of Bordeaux Métropole, under the masonry vaults of the piers , 200 years old.
However, even relieved of this weight, the bridge is sagging on itself, at the rate of one to two millimeters per year, and deep work will begin in the summer of 2025 to secure and consolidate it. They will continue for at least three years, even four, and will cost 50 million euros. “The first issue is that of security and the work to consolidate it will be particularly spectacular,” Christine Bost, president of Bordeaux Métropole, commented this Friday.
A bridge under close surveillance
“The bridge is monitored, that is to say we follow the movements of the structure, horizontally, vertically and its rotation,” explains Mohamed Mariko. The sensor data is collected by Cerema (the public establishment responsible for evaluating public planning and transport policies). »
Since the collapse of a bridge in Tours in 1978, under its own weight, the movements of the structures have gradually been placed under continuous surveillance in France.
Thanks to this monitoring, experts know that the structure is sinking from year to year. “Theoretically, it has exceeded its life expectancy, it has reached its limits of elasticity,” says Stéphane Pokora, road engineering works director at Bordeaux Métropole, “but don’t worry, it is solid! »
However, there is no question of compromising the safety of the 10,000 cyclists, 8,000 pedestrians and 75,000 tram passengers who use it every day. It was closed to cars by the previous mayor Alain Juppé to “limit its deterioration”, recalls Pierre Hurmic, the current city councilor.
Micropiles 40 meters deep
“The bridge has been sinking under its own weight for many years, the State has reinforced the principle of micropiles on piers one to six (left bank side between 1993 and 2003) and, adds Stéphane Pokora, we will continue this work, which are increasingly necessary as there is continued subsidence of the structure, particularly on pier eight. »
Sixteen micropiles will be installed under each treated pile, at a depth of forty meters, whereas the historic piles have not reached this stable geological layer, called marl. “We need to organize sequencing for the installation of the micropiles otherwise there is a strong risk of the bridge becoming unbalanced,” adds the director of road works at Bordeaux Métropole. The construction site will therefore be moved frequently and will have an impact on traffic (read framed).
Review the waterproofing and make riprap
In the internal gallery of the piles, we can clearly see that the structure is no longer watertight; a few stalactites have even formed in certain places. “During work carried out in 1954, reinforced concrete was glued to stone and this poses problems of waterproofing and homogeneity between the two materials, its repair is fundamental for the sustainability of the work”, points out Stéphane Pokora. The masonry will also be given a nice facelift even though it has shown great resistance to the passage of time.
Another part of the project aims to protect the piles put to the test by the tumultuous currents of the Garonne. Riprap is planned to limit scouring, that is to say digging due to currents.
At the end of the project, the bridge will not be straightened but the settling phenomenon should be stopped and the life expectancy of the structure further extended.