Fort Boyard (17): a historic reconstruction project worth 36,6 million euros

An emblematic monument of French heritage, located in the Pertuis d'Antioche (17), Fort Boyard today shows worrying signs of weakening, the consequences of several decades of exposure to naval assaults.

To ensure its longevity while restoring its original appearance, major work will begin in the summer of 2025 to rebuild its original protective structures: the spur to the north, the landing haven to the south, the berm and the protective blocks.

Led by heritage architect Delphine Gramaglia, the BRLi design office and the ETPO company, this exceptional project mobilized, from the upstream phases, cutting-edge maritime expertise, notably that of Groupe Géotec.

To characterize the soils and guide technical consolidation choices, Groupe Géotec carried out complex nautical geotechnical investigations in an environment constrained by weather conditions and water movements.

For Sylvie MARCILLY, President of the Charente-Maritime Department: "By acting to safeguard Fort Boyard, we are making a historic gesture for the next 100 years, both for the people of Charente-Maritime and for our visitors who are so attached to this building."

A project designed to last a century

For many years, Fort Boyard has suffered the combined assaults of the sea and time, gradually compromising its longevity. Initially protected by a spur to the northwest and lateral jetties forming a landing haven to the southeast, the fort is now directly exposed to the swell, its protective structures having been destroyed over time.

This increased exposure is resulting in a progressive deterioration of the structure, several cracks of which are now visible to the naked eye from the outside. While these do not compromise the overall stability of the building, the disappearance of the spur, the berth and the progressive deterioration of the berm constitute major weakening factors. Without the reconstruction of the protective structures designed to mitigate the effects of currents and swell, the ruin of Fort Boyard is inevitable.

Faced with this alarming observation and as owner, in 2020, the Charente-Maritime Department entrusted Artelia with a detailed feasibility study on the protection of the fort against the swell and on the implementation of a secure access solution to the nearby platform. Following the completion of this study, in 2022, the Department launched the project to protect Fort Boyard against the swell.

Designed to ensure the longevity of the structure for the next hundred years, this ambitious conservation program provides for the reconstruction of the historic protective structures, using contemporary technologies and materials, while respecting the original architecture of the fort:

• The reconstruction of the spur to the north aims to protect the fort from swell and sea currents, while avoiding any transmission of mechanical forces to the structure itself.
• The reconstruction of the southern landing stage aims to protect the rear of the fort and its foundations from the action of the swell, while allowing, when nautical conditions permit, direct access to the fort.
• The purpose of the berm restoration is to limit the effects of erosion and scouring generated by currents and waves. Protecting the entire perimeter of the fort is essential to maintain the stability of its foundation.
• The replacement of protective blocks helps dissipate some of the wave energy upstream of the structures, thus acting as a first barrier before the waves reach the reconstructed structures, then the fort.

To meet the major technical challenges of this maritime project, Groupe Géotec will be involved in 2023 to work on the design studies.

In September, the teams carried out a G1 mission, consisting of defining and analyzing the geotechnical model along the existing structures—a key step in understanding the behavior of the subsoil in such a demanding environment. The structural teams also implemented an automated system for monitoring the fort's internal cracks to analyze their evolution and prevent any risks before work begins.

At the same time, a G2 AVP mission is being conducted during the design-build negotiation phase. It focuses on the study of the foundations of future structures: typology, anchoring depth, estimation of induced settlements.

These field investigations, conducted in complex maritime conditions, provide decisive data to guide technical choices and guarantee the reliability of reconstruction solutions.

For Delphine GRAMAGLIA, Heritage Architect, co-contractor of the ETPO/BRLi group: "A project in the footsteps of the ancient works with the most faithful possible restitution of the forms of the old harbor and the old spur now disappeared but with modern materials, in order to propose "the right compromise" between the essential construction of efficient protective works, adapted to current constraints and the necessary consideration of constraints arising from historical analysis. The project of reconstruction of these works beyond ensuring the protection and the sustainability of the fort and its transmission to future generations, is the opportunity to restore this historic monument to its original integrity."

Work on the fort will begin in the summer of 2025 and be completed in 2028.

Geotechnical investigations in the maritime environment: Géotec Group faces the challenges of Fort Boyard

Following on from the G1 and G2 AVP missions undertaken in 2023 to establish the geotechnical model of the site and study the foundations of future protective structures, Groupe Géotec carried out an initial campaign of in situ investigations around Fort Boyard.

This operational phase, essential for validating the design hypotheses, took place in the fall of 2023 and then in the summer of 2024, from the OMER jack-up platform. Six sounding points were initially planned, distributed around the structure. However, the particularly unfavorable weather conditions at this time – strong swell, sustained winds, restrictive tides and shortened days – only allowed four stations to be set up over these two years, two to the north and two others to the south of the fort.

Despite these constraints, initial investigations have enabled the various geological layers present to be precisely characterized: historic embankment, sandbank, and marl-limestone substratum. These data, derived from core and destructive drilling with pressuremeter tests and static penetrometer tests, supplemented by laboratory analyses, have consolidated the geotechnical model and refined the foundation solutions envisaged for the structures to be rebuilt.

Beyond the weather hazards, the site's configuration itself posed a major logistical challenge. The fort's immediate proximity, the presence of exposed rock at low tide, and the limited maneuverability of nautical equipment complicated the platform's positioning operations. Added to this was the impossibility of safely transferring personnel at night.

This first phase of intervention, carried out in a particularly restrictive technical and natural environment, illustrates Groupe Géotec's ability to adapt its methods and organization to the requirements of an emblematic island site, while ensuring the quality and reliability of the data collected.

For Thomas PORTENART, Head of Maritime Services at the Géotec Group: "Fort Boyard is an emblematic site that we spontaneously associate with childhood - ours, and that of our children. A historic place, which we usually contemplate from afar and for which this time we worked, as closely as possible, adapting to the rhythms imposed by the sea. This project also offered us a beautiful symbolism: that of deploying our OMER jack-up platform, for its 10th anniversary, just a stone's throw from its birthplace, in Marennes."

• Client: CD17
• AMO: ARTELIA
• MOE: ETPO / BRLI / Heritage Architecture Design and Construction Group
• Geotec Group service period: September – October 2023
• Project delivery date: 2028
• Amount of the Géotec Group service: €400