ROAD TO SECURITY
On one side, towering green cliffs and the constant threat of rockslides; on the other, huge waves rolling in from the Indian Ocean. And squeezed between them, a highway on which 50,000 vehicles travel each day. It is a vital artery, connecting Reunion’s main city Saint-Denis and its airport, to the north, and the community of La Possession and its port, to the west. Each year, falling rocks and giant waves that regularly batter the coast provoke many accidents and require the partial or total closure of the highway to traffic. As a result, the Regional Council decided in 2010 to replace this hazardous four-lane highway with a 5.4-kilometer viaduct – the longest ever built in France – running along the coast.
Its deck will be between 20 and 30 meters above the water, compared with six meters for the present road. The companies in the coastal viaduct consortium are Vinci Construction Grands Projets, Dodin Campenon Bernard (a subsidiary of Vinci Construction), Bouygues Travaux Publics (a Bouygues Construction subsidiary) and Demathieu Bard Construction. In the opinion of Jean-Luc Bouchet, it was urgent to undertake this project:
“The highway had become too dangerous, and the cost of maintaining it too high. It was time to deal with that situation. Here, it’s not a matter of easing congestion, but of making travel along the coast safe.”
THE VIADUCT PROJECT IN FIGURES:
“And here’s another bridge segment ready to be erected.” Opposite Luis Da Costa, a Bouygues Travaux Publics site worker, stands an immense concrete structure, one of the 1,386 segments, including 210 with specific characteristics, that will make up the viaduct deck. At Port Est, a few kilometers from the worksite, the consortium has constructed two open-air precasting plants, one for the segments and the other for the piers, on a nine-hectare site facing the ocean. Da Costa, who is in charge of concreting the former, explains: “To produce each segment, we pour 100 cubic meters of concrete into formworks that weigh 250 tons.”. Light rain is falling as Luis inspects the newly fabricated segment. With him is a coworker, Aderito Nogueira, who marvels, “I’ve been doing this job for twenty years, and I’ve never seen a project as colossal as this one.” Three large trucks are used to move the segments to the worksite. Each one has 216 wheels, measures 35 meters long and 5 meters wide, and has a payload capacity equivalent to the weight of a three-story building. The segments are hauled along the coastal road at night to disrupt traffic as little as possible.
Elsewhere at the plant, gigantic concrete trunks rise toward the sky. These are baseplates and pier heads. When assembled, these two elements weigh about 7,000 tons and stand roughly 20 meters high. Later, a mega bridge pier segment, a structure consisting of seven segments from which the deck is built out, will then be placed at the top of each pier. To transport the piers and mega bridge pier segment to the offshore worksite, the consortium uses... a giant “octopus”.
THE ZOURITE IN ACTION
Bright sunshine, little wind, a calm sea... optimal conditions for hoisting a mega bridge pier segment onto a pier of the viaduct. At the dock of the precasting site, this mega segment is loaded onto a barge the size of a soccer pitch. Its name is Zourite, meaning octopus in the local language, a reference to its eight legs, which anchor it to the sea bottom at the erection site. “We wanted to limit the amount of concrete poured at the site offshore to minimize the impact of the work on the marine ecosystem", explains Xavier Loye, sector leader, offshore installations. “The Zourite is both a transport barge and a worksite. We install the baseplates, which weigh 4,800 tons, and the 2,400-ton pier heads with a precision of a centimeter. These elements are then joined with 40 cubic meters of concrete and 8 tons of steel, all in an environment by definition unstable.”
It is also an environment new to most of the personnel, who are accustomed to working on dry land. The project is a novel experience for the barge crew, too. The captain, Fabien Géreux, immediately understood the task that lay ahead. “Piloting this barge is an exciting challenge. How fast we progress depends on the weather conditions. A bit of swell, and we’re stuck at the dock”. In the cabin, walkie-talkie in hand, he looks skyward and eyes the rain clouds.
Adds Yann Dieulesaint, deputy sector leader, offshore installations: “If rough seas keep the Zourite from going out, the pier can’t be erected, so the segments can’t be installed. In any case, the highway will be closed because of those same rough seas, so the segments can’t get from the plant to the barge. That results in a backup of segments, and production at the precasting plant is halted. Sailors, construction workers, engineers...everyone’s work depends on the weather.” And because of the island’s rugged landscape, the weather here can be fickle. When a depression or cyclone passes through, it can bring torrential rains. During the cyclone season, Reunion sometimes receives as much rain in a few days as Paris does in an entire year. But in this month of April, the weather is beautiful, and so is the teams’ morale. A twenty-eighth pier out of the total of forty-eight has been erected. On each side of the mega bridge pier segment, a launching gantry puts in place the segments hauled from the plant by those enormous trucks. Like the Zourite, this launcher is big – it is almost as long as the Eiffel Tower is high (278 meters, compared with 300) – and was built specifically to meet the requirements of this exceptional project. The viaduct will be completed during the second half of 2019, meaning that for motorists on Reunion, the danger from rockfalls and unruly seas will be no more than a distant memory.
A PROJECT SENSITIVE
TO ITS ENVIRONMENT
Reunion is home to thousands of protected species.
Using the barge Zourite reduced the amount of concrete poured offshore and thus limited the project’s impact on the marine ecosystem. An underwater noise barrier was installed to attenuate noise from the construction. The coral reef is protected by filter screens, and water acidity and suspended matter are regularly measured. Six of the viaduct’s piers have been fitted with eco-design modules to serve as refuges for marine life. Lighting at the worksite is turned off at night between December and April to minimize the risk of seabirds being stranded on land.
THE NEW COASTAL ROAD
The Nouvelle Route du Littoral consists of this 5.4-kilometer viaduct as well as 6.7 kilometers of embankments. GTOI, the Reunion-based Colas subsidiary, is carrying out this part of the project in a consortium with the Vinci Construction subsidiary SBTPC and Vinci Construction’s excavation division.
The six embankments will connect the seven viaduct deck sections to the land. This consortium is also doing the La Possession interchange project.