The Hong Kong-Zhuhai-Macau Bridge (HZMB), in China, is the longest sea bridge in the world, stretching for approximately 55kmconnecting these three important cities. The work includes an underwater tunnel and two artificial islands to manage maritime and air traffic. To build this colossal infrastructure, which represents a milestone in civil engineering, it took approx 9 years of work and an estimated investment of 20 billion dollars. This bridge embodies the technological progress Chinese and the ability to plan and execute strategic investments of enormous economic significance. In works of this nature, often improperly classified under the single definition of bridges, civil engineering overcomes geographical and environmental constraints, transforming an extreme technical challenge into one territorial development tool on a regional and global scale.
The main features of the Hong Kong-Zhuhai-Macau Bridge
The infrastructure connects Hong Kong to Zhuhai, then rejoins Macau, drastically optimizing previous travel times. The Hong Kong-Zhuhai route, which in the past took about four hours, is now covered in just 45 minutes. The overall length of the link reaches i 55kmmaking the HZMB the longest sea crossing in the world. Its implementation took approx 9 years of workconsidering the design, construction and testing phases (the construction site officially began on 15 December 2009 and the opening to traffic took place in October 2018). A milestone in Chinese civil engineering and among the most ambitious infrastructure projects ever built, in addition to its technical complexity, the work responds to a clear strategic objective: strengthen cooperation between connected regionspromoting financial, commercial and tourism exchanges within the Greater Bay Area.
How the longest sea bridge in the world is made
One of the most interesting aspects of HZMB lies in the coexistence of multiple structural typologies within a single project. In fact, what to a less expert eye may appear to be a single, continuous marine infrastructure, is in reality a complex system composed of different static schemeseach chosen to optimally respond to specific geometric, geotechnical and functional needs.
Structures in the water
The main structures develop in open marine environmentimposing significant challenges from a geotechnical-structural point of view, but also and above all executive. The piles appear founded on water, but in reality they rest on depths foundations a large diameter piles (i.e. greater than 1 metre). The pile system is designed to reach sufficiently rigid and resistant soil layers, capable of withstanding the vertical loads and horizontal actions transmitted by the infrastructure. In most cases, the realization took place in a saturated environment; in some cases, however, prefabricated concrete casings were used as temporary works for water insulation, the positioning of the reinforcement cages and the terminal concrete casting.
The piles are built in high performance reinforced concretedesigned to guarantee high standards of durability in an aggressive marine environment. In these cases, in fact, particular attention must be paid to the protection of the metal reinforcement cages from attack by chlorides, the consequent corrosion of armor and to the wet/dry cycles typical of the interface area with the free surface of the water.
Artificial islands
One of the elements characterizing the entire project is represented by the artificial islandsintended to house the entrances to the underwater tunnel, the transition structures between bridge and tunnel, as well as part of the technological systems and safety systems. These were made through the use of cofferdams, i.e circular steel bulkheads for the confinement of spaces. From a technological point of view, in fact, works in water generally require the use of temporary containment systems which allow the work area to be locally isolated and to create a dry environment provisional within which it is possible to reinforce the elements and carry out the concrete castings in controlled conditions.
Once the bulkheads have been positioned, designed to guarantee resistance to the thrust of sea water, we proceed to the progressive filling with loose materials appropriately selected and mixed, and to subsequent operations soil consolidationaimed at accelerating settlements and ensuring the correct alignment of the tunnel. From an engineering point of view, these operations constitute a particularly significant example of construction on offshore landfill, made even more complex by the need to respect extremely reduced geometric tolerances, essential for the precise positioning of the prefabricated elements of the tunnel
Girder viaducts
Large portions of the route are made up of pre-stressed concrete girder viaductsused in the sections where the subsoil conditions have allowed the creation of close but sufficiently robust foundations. The solution has the advantage of working with simple structural schemes, repetitive elements easy to prefabricate and assemblewith optimized construction phases and a lower economic impact.
Cable-stayed bridges
For them ‘navigable’ main spansor where it is necessary to guarantee large free spans for maritime traffic, the adoption of a different structural typology has become indispensable: that of the cable-stayed bridge. It is a technological system widely used in these contexts, in which the deck is supported by stays (inclined steel cables) which converge on the lateral towers. This configuration allows you to achieve net spans of the order of 450 metres without resorting to intermediate piles, satisfying navigation constraints and at the same time maintaining adequate levels of rigidity and structural safety.
Underwater tunnel
To avoid interference with the naval and air traffic of the delta, part of the connection is resolved by means of a underwater tunnellong 6.7km. This is made up of prefabricated elements in reinforced concreteeach weighing tens of thousands of tons, made on land, transported by sea, sunk and placed inside trenches dug into the seabed. Once in place, the elements are connected to each other and sealed, so as to guarantee the structural continuity and waterproofness of the work.
