New coastal road

The new coastal road project is designed to provide a safer road connection between Reunion Island’s two main urban centres (Saint-Denis, the largest city, and La Possession). This new road infrastructure is 12.5 kilometres long, and its construction was divided into several work packages. One of the two main work packages was the full-service construction of a viaduct between Saint-Denis and La Grande Chaloupe. The contract awarded to the VINCI Construction Grands Projets consortium called for the design and construction of this 5.4-kilometre viaduct, to be built in the sea along the coastline.

BACKGROUND

The former coastal route (RN1), which was located between the sea and the cliff face, wassubject to multiple natural hazards (rockslides, rain floods, flooding due to high tides); as a result, access to the road was often limited or suspended, intensive maintenance was required, and fatal car accidents occurred fairly regularly.
The new coastal road, which replaces it, is built above the sea 80 to 300 metres from shore in order to elude these natural hazards. This is a unique project in terms of its length (at 5.4 km, it is France’s longest viaduct of its kind), its width (nearly 30 m), its deck featuring a flexible dual 3-lane configuration that can accommodate a future dedicated lane for public transport (tram-train or bus). Given its open marine location, the infrastructure is exposed to particularly harsh weather and sea conditions (cyclones and extremely high tides) as well as sensitive and variable geotechnical conditions, which were taken into account in the design and the construction phases. In addition, the project had to comply with very stringent environmental requirements, especially with regard to marine wildlife.

TECHNICAL OVERVIEW

To avoid having to contend with the tides, we decided to prefabricate all components on land. Accordingly, the structure’s 48 piers were prefabricated at a production plant and transported to the construction site on a huge barge. The same approach, that is, minimising work at sea, was used for the mega-segments resting atop the piers. A set of seven segments, each weighing 2,400 tonnes, and forming the deck portion over each pier was conveyed by barge and installed at the same time as the pierheads. The 1,386 continuous segments that form the rest of the deck of the viaduct were manufactured at a production plant at a rate of three or four a day and transported at night on the current coastal road (RN1) and installed using a 278-metre launch beam. As a result of our decision to maximise on-land construction, we had to adapt our production and handling capacities to accommodate the size of the prefabricated components. The two production plants for prefabricated piers and segments were erected in the harbour zone and feature an extensive set of specific tools. For instance, the segment-production site was equipped with a 350-tonne gantry to move parts and components. The segment-production plant included three concrete-production units (one of which supplied the neighbouring pier-production plant), a system to treat the water used to clean concrete mixer trucks, a cooling system for aggregate materials, and mixers. In addition, there wasan ice-production unit with an output capacity of 45 tonnes a day to help control the temperature of the concrete even during the most intense heat waves. The whole production set-up had the merit of being environmentally friendly so as to meet the contract’s strict requirements in this regard. Work teams, for example, had to protect whales from underwater nuisances created by the worksite and protect nearly coral reefs from the fine dust generated by construction processes.

The viaduct was designed to last 100 years and built to withstand exceptional shocks, including collisions by sea vessels. The underside of the deck is set 13 to 22 metres above the sea – beyond the reach of the highest waves.

IMPACT

One of the major projects on French territory during its construction, the New Coastal Road, which has fully open in early 2023,  now facilitates travel between the capital, Saint-Denis, and the island’s trading seaport some 20 kilometres to the west. It has replaced the former dual 2-lane corniche road located at the foot of the cliff face, which was infamous for being dangerous. The current road was exposed, on the one hand, to sea spray from the high tides (a common occurrence given the absence of a continental plateau) and, on the other, to rockslides despite the presence of safety nets. What is more, the authorities had to close one or two lanes on the road for two months a year on average to protect users. As a result, the use of alternating traffic flow caused major congestion on this road. The New Coastal Road, 12.5 kilometres long, located almost entirely above the sea, and running alongside the coast, is not affected by rockslides or high tides since it is 20 to 30 metres above level of the sea. It is also much larger; in fact, it is a dual 3-lane carriageway that includes a dedicated lane for a future public transport system as well as a dedicated pedestrian and bicycle path.
Just as the grand ravine viaduct, this project endows Reunion Island with an infrastructure designed to reduce traffic congestion and provide users with greater comfort and safety.

Project participants

Client
Région Réunion

Project management
Egis

Key figures

Implementation dates
January 2014 to November 2018

Size of the viaduct

Length: 5.4 km
Width: 30 m
Deck: dual 3-lane configuration

Testimonial

“The new coastal road project is exceptional due to its features and its location above the sea. Because of the natural hazards the project faced, its technical complexity make it one of the world’s top civil engineering projects.”

BERNARD CHIESA, DIRECTOR, CALL FOR TENDERS PROJECT, NEW COASTAL ROAD

Find out more

www.nouvelleroutedulittoral.re

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