18 Carefully Crafted Mega-structures

While construction of the foundation was underway, work began at Le Grand Port Maritime de Marseille (GPMM) in Bouches du Rhône, France to manufacture 18 concrete reinforced caissons, which will constitute the protective and structural perimeter of the Portier Cove eco-district. Each caisson weighs 10,000 tons and stands 27 meters high and 28 meters wide, using 3,800 cubic meters of concrete. Each caisson is constructed individually, one after another in a large floating metal structure known as a caissonnier.

Photo: SAM Anse du Portier / TA

Le Grand Port Maritime de Marseille

Manufacturing the caissons requires a specific space with a 22 meter draft, ease of access, and realistic proximity to Monaco where the caissons are delivered post manufacturing. The Phocean city of Marseille was found to be the best suited option for this large industrial project. And, thus, in late 2016 and early 2017, Bouygues TP began preparing the port site for the caisson construction.

Photo: SAM Anse du Portier / TA

Two main sites were necessary. First, 10,000 m2 on the quay to prepare the caisson manufacturing (reinforcement, assembly of the framework), and second, a 32,000 m2 on the sea dike where the caissonnier rests and the caissons are stored before transporting to Monaco. The sites had to be prepared for multiple administrative offices, more than 700 hundred workers, the building platforms, delivery and storage of the raw materials, and assembly and access to the caissonnier. Authorisations and permits were issued; for example the caissonnier meets the requirements of ICPE, which is the French acronym for “facilities classified for environmental protection”. In total, nine months of preparation will have been required before manufacturing of the caissons began in October 2017.

Photo: Bouygues TP MC

Caissonnier: Marco Polo

The caissonnier, named Marco Polo, is a large, floating metal structure weighing 4,559 tons, standing 56 meters tall and 50 meters wide. This unique piece of equipment has been built to specification for this project. It is in this structure that each of the 18 caissons have been built one-by-one. The caissonnier was designed and built in Poland by Bouygues TP and transported to Marseille by sea.

Photo: Photo: SAM Anse du Portier / TA

Concrete is cast into each caisson, increasing the weight of the total caisson-caissonnier structure and slowly sinking it. The caisson is then counterbalanced by the buoyancy of the seawater and rises from the caissonnier. Once the caisson is afloat, it is towed to a dock area a few hundred meters away and a second phase of concrete work is carried out to create the part of the caisson that will be visible from the sea, ensuring the best architectural aesthetic. While the second phase of work is completed on the caisson, the caissonnier resurfaces and work begins to cast the next caisson.

Caisson Construction

The caissons are constructed continuously by 120 person crews who work 24 hours a day, 7 days per week in three eight hour shifts Monday through to Thursday and two twelve hour shifts Friday through to Sunday. Caissons are constructed in these phases:

  • Phase 1: Construction of the raft. This is the foundation, or base, of the caisson. It is built inside a dry caissonnier and measures 700 square meters and is 80cm thick. To create the raft, 600 cubic meters of concrete are poured.
  • Phase 2: Construction of the walls or “sails” which will create cavities inside the caisson. These sails, placed end to end, would measure 280 meters in length. The sails are constructed continuously, using a slipforming technique, with an average progress of 12 cm per hour. This procedure continues until a height of 27 meters is reached on the rear façade of the caisson.
Photos: SAM L’Anse du Portier / TA
  • Phase 3: Construction of “Jarlan” type posts (breakwater). Once the required height is reached, the caissonnier is ballasted and submerged into water until the caisson releases and is afloat. It is then transported to a nearby work area where its upper part, called the Jarlan Chamber is completed.
  • Phase 4: The completed caisson is stored before being prepared for towing to Monaco by sea. The first caisson arrived in Monaco in July 2018.

Worksite and Materials

The crews of workers who manufacture the caissons on-site are easily visible in their yellow and orange safety vests. The work of these crews and their seamless and continuous collaboration on the job site are imperative to a successful manufacturing of the end product.

The jobsite is impressive with a massive work area and forests of up to HA40 steel and equipment making up the caissonnier area, with the caissonnier frame itself consisting of 380 kilos of steel per cubic meter of concrete, compared to 80 kilos for conventional structures and 160 kilos for civil engineering works.

Photo: Bouygues TP MC

The concrete used is composed of specific properties, depending on end use. For this project, the concrete must withstand the permanent aggression of sea salt and remain unaffected by chloride ions that erode materials. The concrete was tested and adapted specifically to guarantee the durability of this project’s construction. The concrete, as with all materials used, is tested daily to ensure it meets the standards set. The concrete also needs to be monitored daily according to weather conditions which can affect the drying time and therefor the process of each caisson’s individual manufacturing. The method in which the caissons are constructed layer by layer in the caissonnier proves a unique challenge when considering all of these elements. The crews prepared for this specific work for weeks leading up to the beginning of the project, using methods of education and training, but also using models to practice and understand the specific specifications of each phase.

Photos: Bouygues TP MC

Sustainable Development

The outer surfaces of the caissons are treated with specific materials that favour the acclimation of living species. Artificial seagrass, rocky areas and cavities will be installed on the sides of the caissons to aide in the regrowth of the natural habitat.
The caissonnier was built specifically for this unique project. However, the designers kept in mind the standards set by the Principality to reuse and recycle materials and thus the caissonnier will be remodeled by Bouygues TP after this project and will be used to manufacture wind turbines in the future.

The generators used to power the caisson manufacturing are equipped with particulate filters to limit air emissions, DeNOX modules are in place to retain the release of nitric oxide, and the oils used to lubricate the caissionner framework and to coat the caissons to delay the corrosion effects of sea salt are composed of plant or synthetic elements to limit any impact of toxins to the marine environment.

Departure to Monaco and Immersion

The caissons have each been constructed in one month’s time. The first caisson, C2, was completed in early November 2017 and was delivered to Monaco in late June 2018. The caissons have been sent to Monaco and moved to their exact location before being sunk into place one-by-one. They are weighted with quarry materials for stability. The entire caisson belt has been installed over a one-year period and completed in July 2019.

Photo: Bouygues TP MC