Video: The amazing installation of the protective belt of 18 caissons
In September 2018, the 18 reinforced concrete caissons began to be assembled in Monaco. It’s been an amazing work ended in July 2019, marking out the future Anse du Portier district. Caissons act as a bulwark against the effects of swell by absorbing the waves that hit the new coastline in damping chambers. Trapezoidal in shape, each caisson is 28 metres wide, 27 metres high, weighs 10,000 tonnes and requires more than 3,800m3 of concrete for its construction. The final approach manoeuvre to the designated location is carried out with two multicat vessels placed at the front and rear of the caisson. These ships have azimuthal propellers that rotate 360°. They are therefore extremely manoeuvrable, which allows to manually adjust the positioning of the caisson, and offers both stability and manoeuvrability. This is decisive for adjusting its position to the nearest centimetre. This is how the caisson is brought to its installation area, practically to its final location. At this stage, the caisson undergoes a first ballasting to bring it less than one metre above the backfill. The final installation is managed by six hydraulic winches placed on the caisson. Six people are on board to deploy six cables all around the caisson to be immersed, which are then docked on the adjacent caisson already laid. The caisson is equipped with topographic monitoring equipment. Like the driver and co-driver in a rally car, the topographical manager reads the data and gives, in real time, the position of the caisson to the operations manager who moves it by managing the tension of the cables. Once the caisson is definitively stabilised, balanced 50cm above its final position, it is completely ballasted to make it descend onto the backfill. In the event of a difference between the target position and the actual position, as the ballasting is liquid, all it takes is to discharge the ballast water from the caisson to return it to float and resume the operation. When the installation is validated, the water is replaced by 0/20 grain sand to make the caisson even more stable. This is the solid ballasting phase. To limit the risk of turbidity, the water is pumped out as solid ballasting progresses, respecting settling times to discharge clear water into the sea. After this operation, the final weight of the caisson is about 24,000 tonnes.