All over the world, coastal development projects are on the increase. These are either offshore extension projects, aimed at reclaiming space for industrial activities, residential areas, or coastal development projects intended to provide promenades and, in the context of climate change, to protect the coasts. Coasts are then protected from erosion or submersion arising from the combined effect of rising water levels and storms, which are becoming more frequent and intense. In both cases, this type of project requires a high degree of proficiency in the construction techniques of:
- Anti-submersion or anti-overtopping structures: riprap, sea walls, etc;
- Anti-erosion structures: beach sea walls, groynes, T-shaped groynes, detached breakwaters, etc.
Construction techniques for these structures are similar to those used for port sea walls, but their design often requires the integration of specific architectural constraints. The challenge is to limit the number of over toppings as much as possible, while reducing the visual impact of these structures on their environment. Thanks to an active R&D policy in this area, Bouygues Travaux Publics is able to offer its clients innovative anti-overtopping measures. For the Beirut seafront project, “wave traps”, coupled with absorption chambers, were designed and built to trap the mass of water that overtops an initial threshold, thereby reducing the number of over toppings likely to reach the area to be protected. In Monaco, the offshore extension project benefited from the installation of porous chambers with double-perforated walls to limit wave reflection as much as possible.
Construction of the city’s seafront promenade and marina
As the culmination of an ambitious project to rebuild the city centre of the Lebanese capital, the Beirut seafront and marina project comprises two breakwaters with very different profiles, allowing 60 hectares to be reclaimed from the sea. The first was designed to accommodate a prestigious three-level promenade, protected from overtopping that could be caused from swells of up to nine metres. With a length of 1.3 kilometres, this belt is made up of 80 wave-absorbing caissons, equipped with a double absorption chamber with decreasing perforation rates and a “wave trap” that absorbs almost all the overtopping and empties it into chambers. Precast on land, the reinforced concrete caissons were then immersed, ballasted, towed out, and positioned side by side, then filled with sand and finally closed with a one-metre-thick concrete plug. A second 450-metre-long breakwater protects the marina, which can accommodate 400 pleasure boats, including some large yachts. Two staggered crest walls surmount this breakwater to block overtopping without obscuring the view from the neighbouring hotels.
Construction of a sea wall
To support the development of marine renewable energies, the Port of Brest has undertaken the construction of a new terminal dedicated to the handling of goods related to their operation. Several worksites have been launched, including that of the enclosing dyke, the first stage in the creation of a new 14-hectare polder that will house part of the facilities. This contract, awarded to Bouygues Travaux Publics Régions France, includes the construction of an 890-metre-long arched dyke to close off an “enclosure” where the marine sediments dredged from the port will be stored, in other words 1.25 million m³ of sediment. The dyke is composed of 26 circular cofferdams interconnected by curved walls, made of sheet piling 21 to 33 metres long. This solution was chosen instead of a rubble mound as it offered a greater storage capacity for sediments. The dyke also incorporates eco-reefs that retain water at low tide to preserve ecosystems.
Construction of an offshore extension including the access tunnel to the sub sea tunnel
The Tuen Mun-Chek Lap Kok Link five-kilometre twin-tube tunnel connects the New Territories to the artificial island housing the Hong Kong-Zhuhai-Macao Bridge border checkpoint facilities and provides faster access to the international airport. In order to accommodate the launch shaft for the tunnel boring machines and the shaft giving access to the underwater tunnel, an offshore extension of 16.5 hectares was built. This extension, which consists of a concrete block quay wall to the east and a rockfill breakwater to the west, was the starting point for an exceptional project. With a dredging permit limited to the peripheral base of the extension, the consolidation of the central part, crossed by two tunnel boring machines, including the world’s largest earth pressure tunnel boring machine (17.63 metres in diameter), was carried out by installing vertical drains in the marine deposits and accelerated by the use of a preloading fill. Once the offshore extension was completed, the areas created were used for the logistics of the works and then transformed to accommodate the tunnel ventilation building and land areas for port business activities.
Construction of the maritime infrastructure of the new district of the Principality
Both a city and a state, Monaco is a territory that must reconcile its economic and demographic dynamism with considerable space constraints, landlocked between mountains and sea. In the spring of 2013, H.S.H.Prince Albert II. decided to launch a new urbanisation project at sea with the creation of a six-hectare eco-district. The maritime infrastructure, built by Bouygues Travaux Publics, consists of an in-filled area surrounded by a protective belt made up of 18 reinforced concrete caissons placed on an underwater embankment made of quarried material and compacted into seascapes. These caissons, which are 26 metres high and weigh 10,000 tonnes, were built in Marseille in a structure that is unique in France: a 4,600-tonne floating and submersible dock, the Marco Polo. They each have an absorption chamber with a series of openings on the sea side, Jarlan slots, which break wave power and absorb swell energy. Located between two protected marine areas, the Larvotto and the Spélugues reserves, the work on this marine infrastructure project was subject to continuous monitoring and follow-up. Bouygues Travaux Publics developed new methods and technologies to minimise the impact on the marine environment.