A dive under the bridges of Paris, for safety

Since 2015, the Cerema (Center for Studies and Expertise on Risks, the Environment, Mobility and Planning) has been responsible for underwater inspections of the 37 bridges spanning the Seine in Paris.

We don't necessarily think about it, but "a bridge has parts below the water, you have to go and check regularly if any disturbances appear", explains Pascal Berteaud, CEO of Cerema.

Over a six-year cycle, the 37 Parisian bridges, some of which are several hundred years old, are thus scrutinized by divers specializing in the "pathology of engineering structures".

After the cycle which has just ended, "our general conclusion is a good general condition of the submerged part, taking into account their age", assures Cécile Maurel, director of the risks, infrastructures and materials department of Cerema in Ile-de- France.

The new cycle begins, with around ten bridges targeted for this first year, including the Pont des Invalides, built in the middle of the XNUMXth century.

Underwater, with limited visibility, two divers: one films, and the other examines the 5 meters of frame below the water level, connected by a communication cable to the third of the team on the surface , to whom he describes everything he sees.

On the images filmed a few days ago, Cécile Maurel describes "cracks in a stone", evokes "joints washed out" by time, parts of "wooden" foundations, which do not rot because they still remain submerged. A "dislocated masonry does not necessarily have an emergency character", assures the expert. It all depends on its place in the structure.

After the inspections, the Cerema makes recommendations to the town hall, defining the degrees of urgency of any work to be carried out.

"There is an issue of safety of the works as for all the infrastructures of the city of Paris. The bridges are, in addition, part of the Parisian soul", comments David Belliard, deputy mayor of Paris, who underlines the importance of maintaining these "Parisian heritage gems".

In the event of cracks, "witnesses" can be set up to monitor the progress, and additional scheduled inspections.

There is "no sign of concern or risk of sudden rupture, but it is necessary to ensure a follow-up", explains Ambroise Dufayet, person in charge of the works of art at the town hall of Paris.

According to a senatorial mission set up after the murderous collapse of the Italian viaduct of Genoa in 2018, "at least 25.000" French "bridges are in poor structural condition", out of the 200.000 to 250.000 structures in the country. The exact number is not known.