The caisson belt: a rampart of almost unprecedented design

2019-10-22T12:09:40+00:00October 22, 2019|Monaco, News|

The caisson belt is a 500-metre long sinuous superstructure. It was composed, like a puzzle, between July 2018 and July 2019 and consists of eighteen concrete pieces, the ‘caissons’, assembled one by one 200 metres from the Monaco coastline.

Each caisson is towed from its manufacturing site at the Grand Port of Marseille and positioned in its specifically defined location. Photo © SAM L’Anse du Portier

Each of the caissons is a massive trapezoidal structure with imposing dimensions: 28 x 33 metres wide, 26 metres high and weighing 10,000 tonnes when ‘empty’.

Once ballasted in its final position, the caisson exceeds 20,000 tonnes.
Underwater, 20 metres below the surface, the caisson belt rests on a backfill and emerges six metres above the water.

The assembly of the caissons forming the belt was completed on 18th July 2019. Photo © Bouygues TP MC

The shape and dimensions of this work are the result of studies that include two factors.

The natural environment: The strength of the swell and waves, the particular currents in this geographical area, the effects of ongoing climate change and the different scenarios of a rising sea level.

The constraints of the architectural choices: For its designers, the future Anse du Portier district is an urban maritime structure. Contrary to what is generally done in offshore construction, all onshore buildings will be as close as possible to the sea (six metres above sea level) and only ten metres from the coast1.

When designing this caisson belt, it was therefore necessary to implement a radically different approach to the usual rules and practices in maritime construction in order to prepare for the construction of this belt2. The structure must counteract hydraulic effects to protect the district and pile-based buildings.
Its pioneering and innovative design pushes all known techniques in offshore construction to their extreme limits.

Absorbing the power of the sea to protect the eco-district

Today, from the open sea the assembled caissons look like the keys of a gigantic piano: we can see an alternation of light concrete surfaces and black shutters.

The caissons and their ‘Jarlan’ openings, now hidden by these black shutters. Photo © SAM L’Anse du Portier.

Until the civil engineering work is completed, these shutters block the entrance to the Jarlan chambers where wave power is dissipated. When removed, they will allow the caissons to fulfil their function by revealing their true structure.

The objective is to break the energy of the swell and waves.
To fulfil this function, the facade of the caissons facing the sea is pierced with vertical and rectangular openings. They begin three metres below sea level and continue four metres above sea level. They open inside the caissons to reveal the Jarlan chambers.

The ‘Jarlan’ openings on the front of the caissons. Photo © Bouygues TP MC.

These chambers are used to limit the overtopping of waves and the reflection of swell energy.

In the case of overtopping, if the caisson walls were closed the wave would hit it, using it as a springboard, to jump over the caisson. This could have consequences for the safety of nearby people and the buildings on the other side of the wall.

In terms of reflection, when hitting a solid wall the water would rebound, reflecting some of its energy back to the sea. In this case, the consequences would be felt in the two surrounding protected marine reserves, at the entrance to Port Hercule and Larvotto beach.

The Jarlan chambers are seven metres high and occupy the upper part of the caissons. Photo © SAM L’Anse du Portier.

How the Jarlan chambers work

Water enters the Jarlan chambers thanks to the openings in the caissons.
The wave is instantly reduced when it passes through these openings, losing a significant part of its energy.
It then passes through the two successive attenuation chambers by a piston movement: the wave gradually fills an empty chamber, then the other, a process known to reduce the power of water.

The absorption function of the caissons makes it possible to treat 100m3 of water per linear metre. Over the entire belt, 45,000m3 can be absorbed for each wave.

Finally, for the water that manages to climb up the caissons, another device has been implemented: wave-return walls. These are concrete parts with a specific shape that surmount the caissons. Any water that might have flowed up the wall is flushed out by the spoiler.

Without these devices, the wave reflection coefficient of the caissons would be 0.9.

The Jarlan device reduces the wave reflection coefficient to 0.35.

The future seaside promenade with the wave-return wall to the right Photo © Valode et Pistre Architectes.
1. Bouygues engineers use as an example the case of Beirut (Lebanon) where the construction company has developed an urban structure at sea. A caisson belt had been constructed, but the buildings were located between 50 and 80 metres behind it.
2. For all the studies carried out and the sizing of the project, see themed feature 16 on hydraulic studies.