Since the floods of 2021, the departmental road D813 in Port-Sainte-Marie has shown signs of structural instability in its retaining walls, leading to significant deformations. After years of monitoring and occasional repairs, the situation now necessitates a large-scale project.
The Department of Lot-et-Garonne has therefore decided to launch a project of general interest to secure this major axis, crucial for daily commutes and transit. The site presents complex geotechnical and hydraulic challenges, including unstable embankments, degraded walls, the tidal phenomenon of the Garonne river, and immediate proximity to a railway line.
The chosen technical solution involves constructing a self-supporting structure: a reinforced concrete slab comprising 9 sections, each 32 meters long and 10 meters wide, supported by 108 piles, 80 cm in diameter, anchored at a depth of 20 meters. Existing walls will be preserved and connected. Safe pathways for active mobility, with sidewalks of at least 1 meter, will also be created.
This work, managed by the Infrastructure and Mobility Department of the Departmental Council, will be carried out by the consortium NGE Génie Civil / NGE Fondations. The self-supporting pile structure was selected for its resilience to the site's constraints, limiting sensitivity to compressible embankments, Garonne river tides, potential floods, and railway constraints.
The project involved consultation with numerous partners, including Autoroutes du Sud de la France, the DDT 47, the Regional Council of Nouvelle-Aquitaine, SNCF, the Departmental Road Service of Lot-et-Garonne, the SDIS 47, the National Gendarmerie, the Communauté de communes du Confluent et des Coteaux de Prayssas, and the municipalities of Brax and Sérignac.
The total investment amounts to €5,150,000, financed with €4,750,000 by the Departmental Council and €400,000 by the State. Work is scheduled to begin on November 2, 2026, and is expected to conclude in June 2027. A complete diversion of the D813 will be implemented during this period.
