The 5.4 km long viaduct, known as the "Viaduc 5400", is being delivered today in Reunion Island. This is an opportunity to examine its technical and environmental challenges.
The Nouvelle Route du Littoral (NRL) project, in which our teams have been working as project managers since 2011, is beginning to be finalised. The delivery of the 5400 viaduct is a first step, before its opening to traffic. A look back at this colossal project.
This road in the open sea, which will link Saint-Denis to La Possession, will eliminate the risk of rock falls on the current road and will be resistant to swell and cyclones.
This extraordinary project is crucial for the safety of users and for the economic development of the island. The New Coastal Road will be the new preferred route to link the administrative capital of Saint-Denis to the commercial port of La Possession, on the west coast, replacing the current coastal road, which is considered too dangerous due to the frequent rock falls along the cliffs and the regular onslaught of the swell.
The New Coastal Road viaduct has all the characteristics of a highly technical construction site: the gigantic size and complexity of the structures to be built at sea, far from the cliffs, on a water level of between 5 and 12 metres, in an area highly exposed to cyclonic swells, the strong integration of environmental issues and the scale of the resources to be mobilised on site, both in terms of expertise and know-how, are all parameters that must be taken into account.
Located on the northern coast of Reunion Island, the site is subject to particularly difficult sea conditions. The forces exerted by the swell on the piers, the meteorological hazards (cyclones, trade winds, etc.) and the seismic risks, however slight, must be taken into account from the design stage of the structure. The quality requirements of the structure must be optimal to ensure its durability in a salty maritime environment that is extremely aggressive for the structures.
Immersed in more than 10 m of water, it is the largest bridge at sea in France. It has been designed to withstand, among other things, 100-year cyclonic swells and boat impacts, and can support up to six lanes of traffic, including two lanes for tramway-type exclusive right-of-way public transport, for a period of 100 years.
The viaduct deck carries 2 x 3 lanes and is adaptable to the future development of public transport modes. This type of structure required complex finite element calculations to determine the right shapes and constructional arrangements.
In general, our design was oriented towards simplicity in order to facilitate prefabrication and industrialisation of the construction process. Our aim was to obtain reliable, proven and durable structures. In terms of concrete, the formulas were specially developed for a very aggressive maritime site.
To build the coastal viaduct at a height of 20 to 30 m above sea level, exceptional material resources were used. Among them, a mega-barge named Zourite - octopus in Reunionese Creole - as big as a football field for the construction of the piers at sea, and the world's largest launch beam, 278 m long and weighing 2500 tonnes (⅓ the weight of the Eiffel Tower!), to lay the 1386 segments transported on fardiers for the construction of the deck.
Prior to the start of each phase of noisy work, our teams ensured that there were no cetaceans in the area, which was monitored 24/7. In addition, in order to respect the very strict thresholds imposed, we deployed substantial technical means, such as underwater acoustic screens (bubble curtains) at the level of the noisy sources. As for the acoustic thresholds not to be exceeded, they are based on German regulations, which are considered the most restrictive in the world.
6 piers of the main viaduct have been eco-designed to provide nursery areas and further out, eco-reefs have been laid out to enable sustainable ecological development throughout the project area.
Another major challenge was to protect marine birds from light pollution. This has resulted in a 50-day ban on lighting per year, from December to April, to limit the risk of strandings. For the rest of the year, the lights, which are yellow-orange in colour, are directed towards the ground so as not to attract or disorientate the birds.
© Région Réunion - Jérôme Balleydier / Laurent De Gebhardt / Egis - Guillaume Danan