The New Bridge Design: Building Before Building

Leona Frank Leona Frank October 14, 2018

4 min read

Estimated Reading Time: 5 minutes

Colas SA is using BIM for virtual planning and construction of the Rivière des Galets Bridge on Reunion Island. Autodesk software—including ReCap, Civil 3D, Revit, 3ds Max, Navisworks, and BIM 360—is helping the firm and all the project stakeholders better plan, coordinate, and carry out construction.

Rivière des Galets bridge project

Reunion island’s Rivière des Galets rises on the western face of the extinct Piton des Neiges volcano and runs northwest to the Indian Ocean just south of Le Port, the island’s main seaport. As it nears the ocean, it crosses under N1, one of the island’s major highways and the only motorway connecting its two largest cities, Saint-Denis and Saint-Paul. At Le Port, the riverbed itself is very wide (close to 300 meters) but usually only has a few channels of running water. However, during the annual tropical cyclones that are common to this area, the rainfall can completely fill the river to its banks.

dThe existing Rivière des Galets bridge is being replaced to increase traffic flow.

Undersized and aging infrastructure

Nearly 70,000 vehicles a day cross over the river on N1 via two existing bridges: a two-lane concrete bridge carrying traffic south to Saint-Paul, and a two-lane metal carry traffic in the opposite direction towards Saint-Denis. These two-lane bottlenecks can cause heavy traffic backups. In addition, the age of the metal bridge (built in the 1950s) poses safety concerns, particularly during cyclones when the river flow becomes dangerously powerful.

A new bridge over Rivière des Galets

To solve these problems, Reunion Region (France’s regional administrative division that governs the island) commissioned Colas to build a new bridge. The 430-meter mixed metal structure is being built between the two existing bridges to keep traffic moving during construction.

The aging metal structure is a safety concern during cyclones, when the river flow becomes dangerously powerful

The new bridge will have three lanes in each direction, plus a pedestrian and bicycle lane. It will be supported by four piers with foundations in the bedrock below the river’s alluvium sediment. Once the new bridge is in use, the existing metal bridge and its piers will be demolished, as will the entrances to the existing concrete bridge. The project is scheduled for completion at the end of 2020.


Building the new bridge between the two existing bridges means all the various trades have to be carefully coordinatsded, as do the physical movements and storage of equipment and materials. During the dry season, construction is accelerated—creating even tighter movement and space restrictions.
Moreover, the large cranes required for construction are located in the riverbed itself, which during the rainy season can quickly become a raging torrent. As such, water flow in part of the riverbed must be diverted throughout the construction process.
These conditions require precise construction planning and sequencing throughout the project. In addition, there must be close collaboration between the various trades and extended project teams.

“This is Reunion Island’s first major BIM infrastructure project and it has been a huge success, helping us overcome the many challenges related to the bridge’s technical and environmental constraints. All the project stakeholders and contractors have fully embraced BIM because it gives us the ability to virtually build before building!”

– Maud Guizol, Corporate BIM Manager, Colas SA

Model-based design

Throughout design, Colas relied on BIM processes and intelligent 3D models. These models are now the basis for ongoing project coordination, collaboration, and construction planning. The firm created a series of discipline-specific design models for the project. The bridge itself was modeled with Revit, as was the project’s mechanical, electrical, and plumbing (MEP) systems. The earthworks, road, and utilities infrastructure were modeled using Civil 3D. During design, these models were combined in Navisworks for multi-discipline project costing, coordination, and clash detection.
Colas also used cameras mounted on drones to survey the existing conditions of the riverbed and ReCap was used to process this photogrammetry data into a 3D point cloud. The point cloud was used to produce an extremely precise digital terrain model in Civil 3D, helping Colas more accurately design the pier foundations, implement water diversions, and optimize earthwork cut and fill.

Virtual construction

Now in construction, Colas uses Navisworks to link the 3D models to a construction schedule for 4D planning, phasing, and schedule simulation. The models are constantly updated by the individual trades to reflect current project situations and changes, leading to improved project coordination and collaboration.
In addition, Colas performs regular photogrammetric surveys to track construction progress and detect changes to riverbed conditions due to rains—enabling the team to better monitor and manage construction and earthworks. The models and other project data are also uploaded to BIM 360, supporting cloud-based project communication and facilitating better informed decision-making.


BIM software and digital 3D models have already helped Colas optimize its design for the new bridge and produce a more accurate project bid. With construction underway, these same models are now enabling all the project stakeholders to:

• Understand the project more fully,
• collaborate more efficiently,
• make better technical choices,
• control risks,
• and deliver the project more cost-effectively.

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