“The size of this project is remarkable — the viaduct is one of the largest in the world,” says Olivier Tricoire, Nouvelle Route du Littoral deputy director of operations.
The ‘Nouvelle Route du Littoral’ is currently under construction in Réunion Island’s — a French department — Indian Ocean and is set to be one of the biggest projects ever undertaken by the French government.
The 12.5km highway, parallel to the coast, will connect Saint Dennis with La Grande Chaloupe and replace the existing coastal road between Saint Dennis and La Possession.
The existing road is exposed to sea spray when there’s heavy swell — frequently occurring due to the absence of a continental plateau — and to the falling of rocks. The 80,000 people who frequently use the road are often stuck in traffic jams, as the authorities have to close one or two lanes for a period of two months to a year. The new coastal highway will eliminate these issues, as it will be 20-30m above sea level — expected to be above the highest waves of the biggest storms.
The €1.7bn development has been awarded to two joint ventures. The construction of the 5.4km viaduct section — €715M of works — is under the responsibility of the viaduct joint venture, which comprises Vinci Construction Grands Projects, Dodin Campenon Bernard, Bouygues Travaux Publics and Demathieu Bard.
The second contract, worth €530M, has been awarded to a joint venture of Vinci Construction Terrassement and local companies SBTPC and Grands Travaux de l’Océan Indien, which will be responsible for the 6.7km of embankments and La Possession interchange.
The dual carriageway will have two lanes in each direction, and has been designed to accommodate a rail line later on. Tricoire says: “The Regional council chose to not increase the personal vehicle capacity and instead to promote collective transport. The project includes lanes only for bus — and later tramway — and also pedestrian and bicycle ways.”
The viaduct’s construction
The 48 viaduct piers and sections are being put together in two prefabrication factories onshore.
“We decided to eliminate the risks associated with swell by building everything on land,” explains Thierry Duclos, responsible for production drawings at Vinci Construction Grands Projets.
Francis Guinchard, director of the viaduct construction joint venture, adds: “Building the viaduct over the sea by working as much as possible on land is one of the main features of this project. It eliminates the need to take sea conditions into account, reduces the worksite’s impact on the marine environment, [and] enables the project to move forward under good quality and safety conditions.”
Each pier is made up of two elements: a lower part, weighing up to 4,650t, and an upper part, weighing between 2,300 and 2,420t.
“Each pier will start at ground level at the sea with a big foundation, a big concrete footing, which is a 20-23m diameter disc of concrete,” says Guinchard.
“These are very big and massive heavy concrete precast elements and that’s where the beauty of the system stands.”
According to Guinchard, the construction team is precasting more than 1,000 segments for the deck onshore, to be assembled using the balanced cantilever method.
The big precasted elements will be transported and installed in the sea by a self-propelled and self-lifting mega-barge — 107m long and 44m wide with a lifting capacity of 4,800t — built by the Crist shipyard in Poland.
The vessel, once on station, will stabilise itself by lowering its eight retractable legs on the seabed, delivering each parcel accurately.
Guinchard says: “I would say that our vessel is a new kind of fish. It has been built for the specifications of this project, and yet it can be used in other projects to be found in the meantime.”
Even though the construction work is still in its beginnings, the project’s learning process has or is about to finish.
“We have more or less finished the learning period and the precasting yard is now working full speed or about to work full speed, but it is almost two years of work to precast everything,” says Guinchard.
According to him, 15% of the works have been completed and the marine works — excavations to be done in the seabed at the location of each pier — are “at the very start”.
“We will start next month, early September, the transport and installation of the big precasted elements for the piers. We are in the final preparation for such installation,” he says.
Even though the construction team have still a long way ahead, Guinchard is certain that the viaduct will be completed on time. “We should finish our viaduct sometime by the end of 2018 or early 2019. As far as our piece of the work is concerned the construction of this viaduct will be on time.”
A project with this dimension and specifications will certainly create a few challenges.
The concrete was one of the biggest challenges the team had to face. Tricoire says: “One of the biggest challenges of this project was to design concrete able to resist to the naturel elements for 100 years.”
Besides having to resist to storms, big waves or even cyclones, the concrete has to be dense enough to withstand gas and salty water.
Guinchard adds: “To make it very dense you have to use some specific elements and chemical actions.”
This concrete, according to Guinchard, is at the top of what is available in the market today in terms of concrete formulation, but it did take some learning. “It’s more or less a one-year process. We faced difficulties to find our concrete mix, because once it works well in the lab it may not yet work as expected in real life.
“So, we had to do some adaptions on our line of production of concrete by modifying some equipment, by improving as well our control of the raw materials to reach the right mix. That was definitely an issue that hopefully is now behind,” he says.
The impact of the construction on the marine life is another challenge that the construction team has and is taken into consideration.
Guinchard says: “The client definitely did numerous environmental studies to minimise the impact of the project itself and the construction works in the sea life.
“By precasting everything onshore we are minimising the volume of our activities in the open sea, so when we go on the water it’s for limited periods of times and in limited areas.”
While everything is being precast on land, the installation still has to be done on the sea. Precautions are being taken to reduce its impact on corals, whales, dolphins and turtles, among other fishes.
“We have to check before we work on a certain location that the location by that time is free of whales, dolphins, turtles, because they have priority. If they are there we have to wait until they go.
“So far we haven’t had any major environmental incident or constraints,” says Guinchard.
Moreover, hammering or the use of explosives in the seabed is completed banned, as it is any spillage.
Additionally, the light pollution has to be diminished, conditioning any work during the night, due to young Barau’s petrels — an endemic species of sea bird — being attracted to bright lights.
If the petrels hit a worksite lighting mast and fall to the ground, they have low chances of survival, because they can only take flights from the surface of the sea or by throwing themselves off a high cliff. Lights therefore have to be switched off until the young have grown enough.
Other constraints include logistics and mobilisation of equipment as well as the fact that the island is within a cyclonic area. “If the logistics and equipment are not available on the island, it won’t be here before another month minimum — time to bring it from somewhere else,” says Guinchard.
He adds: “The fact that we are on a very remote place makes the organisation of the works more difficult. They have to be planned in more detail. The cyclonic conditions may hit as well during construction, meaning that all our equipment and solutions have to be made safe if by any chance the works have to stop for a few days.”
Until now all the challenges are under control and construction works are on schedule — at least until the cyclonic season arrives. As Guinchard says, the project is “a big adventure in a relatively remote place in the middle of the Indian Ocean.”
* Images: Sébastien Marchal