The toll bridge opened to traffic in 1988 and was built by balanced cantilever construction. Structural continuity over its 27 spans relies on the use of both internal and external prestressing cables, each formed of 19 strands of seven high-strength steel wires with cement grout inside a standard HDPE sheath.
- Name of the owner and clientDepartment of Charente-Maritime
- Delivery date of the projectFebruary 2019
- Partners of the projectGeneral contractor: Freyssinet France, Sixense Systems
Design office: STRAINS
The 3 km-long Île de Ré Bridge is the only fixed link between the island and La Rochelle and it carries not only highway traffic, but also essential services. The bridge consists of a series of six prestressed concrete box girder viaducts. A routine inspection in 2018 revealed that one of the external tendons in the second viaduct from the island’s shore had ruptured, requiring an emergency response. Speed and weight restrictions were imposed on the traffic using the viaduct, while the owner launched an urgent tender for repair work.
However it was not safe to simply install a new cable – there was a very real risk that replacing the broken cable without considering the wider implications of this structural intervention might cause another to rupture. Some cables are 250 m long and a sudden failure could damage the bridge or even cause a fatality. The repair team needed to establish the condition of the cables in each span – and take any necessary steps to make them safe – before entering the box girder.
The solution put forward by Freyssinet and Sixense was to start from the Île de Ré abutment and progress along the bridge with care, towards the span with the broken cable. While investigations were being carried out, the existing cables were secured with high-strength straps to prevent them retracting violently in the event of any rupture.
At each pier location Sixense’s USCAN diagnostic equipment – originally developed for testing stay cables on bridges – was used to check the condition of individual wires in each tendon close to the anchor. From this data a picture of the overall condition of each tendon – and hence also the cable – could be built up.
Rating the condition of the tendons gave engineers the necessary information to help them to decide whether to replace tendons at risk of imminent rupture or install restraint straps for safety before proceeding. Following the USCAN diagnosis, and securing of the cables, Eversense acoustic sensors were installed to record any subsequent wire breaks and allow the team to fully quantify the number of broken wires in each tendon.
- 282Acoustic sensors installed
- 27Cameras installed
- 12,768Strand wires inspected with USCAN
- 250Length of longest prestressing cable (m)
Access to the anchors in the congested box girders was one of the biggest challenges of the project, along with the need to work quickly across the two viaducts. This initial contract, which focused on the first two viaducts, was successfully completed within three months.