en.Wedoany.com Reported - Since mid-June, a consortium led by Soletanche Bachy France (lead company), Balineau, Eiffage GC, and Aevia has been responsible for the reinforcement of the Stone Bridge, over which tram lines A and E pass in Bordeaux. The bridge experiences subsidence of 4 to 5 millimeters per year. The project aims to limit this subsidence by installing 16 micropiles in 10 of the bridge's 16 piers to strengthen the structure, with the remaining piers having been treated in the 1990s.
To install the micropiles, the piers are wrapped with cables to prevent bursting during drilling, and the internal chambers of the piers are supported and reinforced. Drilling is carried out through pre-drilled holes in the deck, and the micropiles are installed in marl, 10 to 15 meters deeper than the original wooden piles, requiring drilling to a depth of 50 meters. Julien Rami, bridge works supervisor for the Bordeaux Metropolis, stated that the 16 micropiles bear 25% to 30% of the weight of each pier, reducing the load on the existing foundations, while riprap protection is also applied to each pier. By the end of 2026, four piers are expected to be treated, i.e., 64 micropiles installed, with a total drilling length of 2.8 kilometers. The central part of the piers near the tram line was treated this summer, with 32 micropiles installed, while the ends will be treated in a second phase. Due to heatwaves, the tight schedule was compressed by three days at the end of June and may be further compressed in late summer.
On this 487-meter-long, 19-meter-wide bridge, different work zones are active. Les Compagnons de Saint-Jacques are replacing stones in the internal chambers of the piers, which are hollow to reduce weight, each weighing 5,000 tons. Temporary access passages have been created to facilitate entry, leading to low, narrow corridors connecting the different chambers made of masonry and vaults. Further away, two drilling rigs operated by the Soletanche Bachy team are at work. Floranne Dargent, engineering engineer at Soletanche Bachy, explained that starting from the deck, an electric drill bores through the pier to the foundation, then a second drill intervenes, drilling to a depth of 50 meters to install micropiles between the bridge's wooden piles.
The difficulty of drilling lies in traversing the chambers, piers, existing foundations (deck and wooden piles), gravel layers, and then anchoring in the marl. The work is based on a theoretical plan of the wooden piles derived from archival documents. In 2025, only one micropile struck one of the wooden piles, with the goal being not to disturb the original structure. Rods assembled through drilling guide the reinforcement cage of the micropile, which is sealed after injecting cement grout. Once drilling is complete, the chambers are restored, and damaged stones are replaced.
Médéric Thépot, construction supervisor at the design firm WSP responsible for project supervision, explained that work is not carried out on two adjacent piers simultaneously, as intervention increases subsidence, which only slows afterward; simultaneous treatment could cause rapid subsidence, so the work is divided into three phases. The structure takes 6 months to 1 year to bear the load, and permanently installed sensors in the piers ensure subsidence monitoring. It took seven years from contract drafting to the start of work. 2027 will mark the end of pier repair work, with the reinforcement of the last three piers totaling €50 million, of which 4.6% is funded by the Regional Directorate of Cultural Affairs (Drac). Subsequently, the entire deck will be treated, with the overall project expected to be completed by 2029.
