The Trois-Bassins canyon viaduct is on La Réunion, an island in the Indian Ocean and one of the overseas départements of France. The length of this impressive curved pre-stressed structure crossing the Trois-Bassins ravine is 370 m and it spans a valley at a height of 170m. The steel deck weighs 3,560t and consists of 24 box sections to form a multi-cellular orthotropic deck, which is 4m high and 20m wide. The road crosses a high-altitude region with terrain marked by canyons, gorges and cliffs. This means traversing 120 gullies and the construction of 48 civil engineering structures, four of which are large viaducts. By 2015, between 40,000 and 69,000 vehicles are expected to use the road per day.

The Grande Ravine project has to contend with stringent environmental constraints. The site is a refuge for a protected bird species, the puffin. The canyon itself is an environmentally protected site. So, the Réunion regional authority opted for a bridge without fixed cables and with a works footprint limited to a bare minimum to avoid disturbing the site. To cope with this, materials were brought in as work progressed.

ARCADIS’ role after wining an engineering and architecture competition (together with partner Thalès EC) involved supervising the design and construction process, while also providing support to the client for the external consultants.