The Rhône River bridges on the N09 motorway provide the crossing of the Rhône at Saint-Maurice, on a site characterised by the river meander at the foot of the Arzilier hillside. Both structures were designed as cable-stayed bridges with a distinctive configuration, featuring a single pylon installed asymmetrically on the right bank, where the geotechnical conditions are favourable. Integrated into the tunnel portal, this pylon carries the loads from the main span crossing the Rhône by means of stay cables.

Commissioned in 1987, the structures have been subject to enhanced monitoring since 2005 following the identification of several behavioural anomalies and various non-conformities requiring additional investigations. The inspections, condition assessments and analyses carried out have made it possible to identify mechanisms likely to affect their safety and long-term durability.

The investigations indicate that the structures are generally in a deteriorated condition (condition class 3). Structural verifications performed over recent years have identified several critical issues affecting, in particular, the pylons, anchorage systems, bearings and certain elements of the steel superstructure.

On this basis, a targeted rehabilitation project was defined to ensure the long-term preservation of the structures. The selected measures include strengthening and protecting the pylons by jacketing them with ultra-high-performance fibre-reinforced concrete (UHPFRC), as well as strengthening the upper crossbeam by means of a steel structure. The project also includes the installation of new prestressed anchor ties at the anchorage blocks of the backstay cables, the installation of an inspectable pendular prestressing system on the Valais side, replacement of the abutment bearings, local strengthening of the steel superstructure and replacement of the barrier system with a device offering improved containment capacity and reduced deflection.

This intervention strategy aims to ensure operational safety, improve the structural robustness of the bridges and extend their service life.