Perched in their nacelle above the Seine, the two technicians inspect every nook and cranny of the top of the Austerlitz viaduct by the light of their headlamps. It is almost 4 am on this Friday, October 28 and the control visit is already over. It is indeed necessary to return the rails to their metros. This is the first of six nights that the detailed inspection of this line 5 viaduct will last. “Inspections of this type are only done every five years”, explains Jean-François Douroux, heritage control manager at the RATP.
And heritage, there is at the RATP: 280 km of engineering structures for the metro and 140 km for the RER. “The first works date from the middle of the 19th century and are mainly located on the RER lines”, specifies Jean-François Douroux. The bridge that interests us, that of Austerlitz, dates from 1904. “It was quite exceptional for the time because, at the request of the river authorities, there is no pile in the Seine, which range of 148 meters, ”recalls the manager. But with the increase in the size of the subways, it was necessary to reinforce the bridge, which was done in the 1930s. total of the bridge, were welded to the structure”, continues Jean-François Douroux.
A note of health for the viaduct
Today, the listed viaduct is more than a century old and, like any self-respecting centenarian, is entitled to close surveillance. “Each inspection gives rise to a health report listing the pathologies of the bridge, such as corrosion or cracks, explains Mathieu Rudinger, head of the inspection of engineering structures. And as we have a database we can follow the evolution of these pathologies. With two objectives, safety and durability of the work of art. »
For these inspections, a large part is left to visual observation, but the RATP also uses drones in the tunnels, robot dogs to access the sub-platforms, satellite measurements to determine any deformation of the structures, etc. According to Mathieu Rudinger, “the inspection of the Austerlitz viaduct is the most complex on the network”. The first checks are carried out during the day, in particular thanks to a nacelle fixed under the structure and which moves under the deck thanks to two pedal systems.
“And at night, the inspection of the top of the structure takes place with a nacelle fixed to a work train which comes from the La Villette maintenance center”, explains the person in charge of the operation. Except that the time to bring back the work train, to secure the area by cutting off the electricity and placing signs on the track, there are only two hours of effective surveillance before having to pack everything up. Without taking into account that in case of strong wind, impossible to take out the nacelle. Fortunately, that night, very little wind and a surprisingly mild temperature. “It is especially at the level of the angles that we see fatigue phenomena with the appearance of small cracks”, indicates Mathieu Rudinger.
“It’s a fragile structure, especially since there’s no ballast and the rails are in direct contact with the structure,” he observes. Every 15-20 years, a reinforcement operation is needed. The last one dates from 2013 with anti-corrosion work and metalwork reinforcement. It is at this price that we can admire the Seine in all serenity.