Edycem's expertise and know-how chosen for the restoration at sea of ​​Fort Boyard

Its owner, the Department of Charente-Maritime, has undertaken an urgent, large-scale restoration which consists of:

• the repair of the berm, a 6m wide belt that limits the erosion of the foundations (ready-mix concrete),
• the reconstruction of the spur to counter the swell and sea currents, and of a landing harbor at the rear of the fort (prefabricated concrete elements).

To carry out this work quickly and in keeping with the original design, the industrial company EDYCEM is leveraging its concrete expertise alongside its client ETPO. Access to the site, constrained by winds, tides, and swells, requires unprecedented logistical resources for both teams. The first two phases of this exceptional project, concerning the berm, have just been completed:

• In September, 20 cubic meters of concrete (equivalent to 3 truckloads), specially formulated by EDYCEM to withstand the extreme conditions of the fort, were transported by helicopter. Over three days, nearly 40 trips were made to fill a difficult-to-access cavity.
• In October, an EDYCEM pump-mix truck from the Rochefort nuclear power plant was loaded onto a barge belonging to the Charente-Maritime Department for an open-sea dumping operation. An EDYCEM driver was specifically trained for this unprecedented operation. This training will enable him to carry out future trips, which will take place until 2027, for a total volume of 300 m³.

The "protection" project at Fort Boyard is due to be completed by mid-2028 to allow it to open to the public during the summer of 2028.

High-performance concrete formulas specifically designed for civil engineering structures

For this project, governed by civil engineering regulations 65A, EDYCEM conducted all laboratory studies: testing, custom formulation, and validation of technical concretes for each type to meet the site's extreme conditions. Among the solutions implemented at the berm:

• PM-ES retarded colloidal concretes for subsea phases,
• C45/45 structural concrete for foundations,
• lean concrete.

Maintaining the rheology (plasticity) of the concrete over several hours is essential, encompassing the time required for loading, flow, and pouring. The concrete must offer the right balance between fluidity and cohesion. All EDYCEM concrete comes from its Rochefort plant, located in the immediate vicinity of the construction site. This was a requirement for the project owner, who primarily used local companies.

A unique construction site between sky and sea

According to Julien Merceron, ETPO Works Director: “EDYCEM’s expertise and the quality of the concrete mixes were crucial in ensuring the safety of the pouring phases in the open sea, beneath the surface of the structure. The emergency repairs to the berm were carried out by helicopter, but the remaining work is planned to be done by boat. These operations require meticulous planning, entirely dependent on advanced weather and ocean forecasts and high tidal coefficients.”

Deliveries by boat follow the rhythm of the tides. An EDYCEM pump-mix truck is loaded onto the barge. The journey between Pointe de la Fumée above Rochefort and Fort Boyard takes approximately 45 minutes. To reach the fort without risk of running aground, the operation takes into account several weather and ocean conditions: a minimum water depth, a low tidal coefficient to extend the working area, winds below 40 km/h, and a reduced swell of less than 50 cm.

Once the barge is stabilized using three mooring blocks and two piles, the concrete mixer truck extends its boom to pour concrete right up to the base of the structures. Underwater, divers (TETIS – a subsidiary of Spie batignolles ETPO) carry out the spectacular operation. When the concrete mixer returns to the dock, a second truck fills it without it needing to leave the barge. This logistics system improves safety and saves time on the subsequent round trip.

For Wilfried Decombredet, Director of BPE region Nouvelle Aquitaine EDYCEM: “Working on Fort Boyard requires mobilizing our entire skill set: preparation, formulation, logistics, and on-site expertise. It is this collective ability to adapt that makes EDYCEM so strong on such a unique project.”

According to Carole Rembaud, HSE Manager at EDYCEM: "In this type of environment, rigor and anticipation are essential. Every action is planned to guarantee safety and environmental protection both at sea and in port."

Prefabricated elements that are aesthetically and technically sound

The design and manufacture of the future prefabricated concrete structures (for the harbor and the spur) are part of a large-scale operation. It will be carried out throughout 2026 in Saint-Nazaire, within the major seaport.

An extraordinary logistical operation will then be put in place to transport the two prefabricated elements by tow to the fort. Weighing between two and four thousand tons, or approximately 2.500 m3 of concrete in total, they will be beached and ballasted on site.

In-depth work on the color of the concrete will ensure aesthetic consistency between the existing parts and the future prefabricated elements of the harbor and the spur.