AN ACTIVE CONSTRUCTION SITE DAY AND NIGHT
A September morning at the Grand Port Maritime of Marseille. About 150 employees of Bouygues Travaux Publics are working in the bright sunshine right now, but this is in fact a worksite that never sleeps. Eighteen reinforced concrete caissons are being precast here, and when joined together, they will form a protective belt – a rampart against the sea – for the future eco-neighborhood of Anse du Portier in Monaco. “It is a project with a full range of work – maritime, mechanics, civil engineering, and others,” says Geoffroy Broudy, the person in charge of the maritime aspects of the precasting and movement of the caissons. In the port of Marseille, 32,000 m2 of water have been reserved for the floating dock that is used for the precasting of the caissons. Quayside, there is a 10,000 m2 working area for preparing the iron reinforcement, assembling formworks, and adding the final elements to the caissons.
MARCO POLO, A CUSTOM
A giant stands in the port: a floating dock, 51.30 meters wide, 25.50 meters high, and weighing 4,559 tons. Its name is Marco Polo. “This equipment is unique in France. It was designed and built specifically for the requirements of this project,” says Broudy. On this floating dock, each caisson is precast in less than a month. The workers movements amid the forest of iron rods that form the caisson’s framework are precisely choreographed. It is essential that the workers preparing the iron reinforcement and those installing the formworks do their jobs at exactly the same pace as the caisson walls are being poured. “The teams work in shifts, 24 hours a day, seven days a week,” says Eric Cheype, construction manager for the civil engineering in the precasting operations. “With sliding formworks, given the large number of concrete walls that must be made at the same time, it is practically impossible to suspend the precasting once a caisson is started.”
As construction proceeds and the caisson grows heavier (it will weigh 10,000 tons on reaching a height of 26 meters), it gradually sinks to a depth of morethan 20 meters. And then, Eureka! Thanks to Archimedes’ Principle, the finished caisson floats free of the dock. Marco Polo then rises again and work begins on another one. The completed caisson is towed quayside, where the Jarlan concrete posts that will form the breakwater are added at its top. The caisson is then stored until it is towed to Monaco by sea. From start to finish, an unprecedented operation.
After a three-day, 200-kilometer journey along the coast, the caisson arrives at the worksite in Monaco. Starting in September 2018, the caissons have been progressively placed on an underwater foundation made of 1.5 million tons of rock. This is done by filling the caisson with seawater and then ballasting it with quarry materials. “Setting the first caisson in place was a crucial step,” points out project director Christophe Hirsinger. “That caisson marked the culmination of two years of work – from dredging the sea floor to laying down the rock – all of it invisible from the surface.” That “crucial step” took place on an early afternoon in September. The same operation will be repeated until July 2019. “The civil engineering work on the superstructure of the caissons will begin in May 2019, when the access to them is finished.”
Mid-2020, the infrastructure work will wind up and construction of the Anse du Portier eco-neighborhood will get under way (see inset opposite). Located between the Larvotto Marine Reserve and the Spélugues coral reef, the project is being closely monitored in relation to the environmental objectives set by the Principality. A variety of measures are being taken to preserve marine life, maintain water quality, and reduce the noise and vibration caused by the work (see interview, p. 95). The environmental challenge is being dealt with even in the design and construction of the caissons at Marseille, as Broudy explains: “The external faces of the caissons have grooves 15 millimeters deep so that plants, algae, and invertebrates can attach to them. Artificial plant beds and low rock walls will be installed to encourage colonization by various species.” The Monaco extension project is thus being carried out with respect for the marine ecosystem.
Two questions to...
- The sea floor is dredgedto remove silt and expose the bedrock.
- The bedrock is covered with special gravelto serve as a foundation for the caissons.
- The 18 trapezoidal-shaped caissonstowed to the site are put in place.
- An underwater embankmentis constructed to prevent erosion of the foundation by strong waves.
- The area behind the caissonsis filled with 450,000 m3 of marine sand extracted in northern Sicily.
- The sediment is treatedto prevent liquefaction and fill is added.
- Geotextileis installed and a berm is constructed for additional anti-erosion protection.
and 26 m high.
- 10,000 tons
- 3,800 m3 of concrete
for one caisson
- 51.30 m wide
- 25.50 m high
- 4,559 tons