Traffic on the stone bridge is slightly revised in August to allow a survey phase to be carried out on the building.
It is a question of comparing two micropile methods, before substantial work which will be carried out between 2024 and 2026.
The stone bridge, which is gradually sinking into the mud, must indeed be strengthened at the level of its foundations.
Its regulars have noticed it since the beginning of August, the traffic on the stone bridge in Bordeaux has been reorganized.
If the tram continues to run there normally, this is no longer the case for buses and taxis which are diverted by the Saint-Jean bridge, while pedestrians and cyclists have been switched to one side of the bridge, before being switched back to the other from August 16.
Traffic will return to normal as of the start of the school year.
These modifications have been made necessary to carry out a new survey phase which will last until August 31, with a view to major reinforcement work on the building which is celebrating its bicentenary this year.
20 Minutes
explains what will happen.
Why this survey phase?
It is a question of determining the technique and methods of intervention for the reinforcement work which will take place in 2024, and which must last two years.
The metropolis is carrying out "life-size" tests for installing micropiles using an innovative so-called "sonic" method, which has the advantage of not reshaping the soil of the bridge foundations, which is made up of silt.
“We are carrying out a comparative analysis between this so-called sonic method and the classic technique for measuring the vibrations induced in the structure using the two methods,” explains Mohamed Mariko, head of the engineering structures department in Bordeaux Métropole.
Concretely, we install two micropiles according to each of the techniques, and we will retain the one that is most suitable for carrying out the work.
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Why do we need to reinforce the stone bridge?
Because it sinks into the mud.
“The stone bridge is a structure that rests on 250 wooden piles, details Mohamed Mariko.
During their construction in 1821, these foundations did not reach the "good" ground [that of hard rock], consequently the bridge, under its own weight, settles, of the order of one to two millimeters per year.
This is why a first reinforcement phase was carried out in the 1990s, for piers one to six of the bridge.
It is now time to consolidate piles seven to sixteen, which are settling more, on the order of two to three millimeters per year.
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Why do you want to apply a new method?
As the head of the engineering structures department explained, the bridge has already been the subject of a reinforcement campaign using micropiles.
"Micropiles are metal rods inside which concrete is poured, which will reach the right ground," recalls Mohamed Mariko.
This reinforcement technique works, and we are going to repeat it, using twelve to sixteen micropiles per pile.
On the other hand, we would like to change the method of execution to avoid going to vibrate in the basement made up of mud where the foundations rest, from where this phase of survey.
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Will the bridge be closed during the works?
The reinforcement work period should last between 2024 and 2026, and the bridge will not be closed to traffic, promises the metropolitan engineer, "even if it is possible that, occasionally, there will be traffic cuts of the tram.
A central link between the two banks of the Garonne in Bordeaux, the bridge is used every day by 20,000 pedestrians and cyclists, and around 40,000 public transport passengers.
It is no longer open to motorized vehicles, apart from emergency vehicles and taxis, since July 2018.
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