Introduction

The Hong Kong – Zhuhai – Macao Bridge (HZMB) is a megaproject in South China and the longest man-made sea-crossing bridge in the world. Megaprojects are defined as ‘large-scale, complex ventures that typically cost a billion dollars or more, take many years to develop and build, involve multiple public and private stakeholders, are transformational, and impact millions of people’.[1]

The HZMB is a 55-kilometer-long bridge-tunnel system situated at the waters of Lingdingyang on the Pearl River Estuary in southern China. Construction of the bridge began in December 2009 and the bridge was officially opened nine years after the construction first began. It is the link connecting the Hong Kong Special Administrative Region to the Macao Special Administrative Region and the Zhuhai Chinese mainland. This long link consists of three cable-stayed bridges, two artificial islands, an immersed tube tunnel with associated viaducts, relevant link roads, and the necessary boundary crossing facilities. This aspiring megaproject has overtaken the Stonecutters bridge in Hong Kong, previously the longest bridge in Hong Kong, becoming the longest bridge in Hong Kong and in the world. It was decades in the planning and cost billions, the actual need for this project was a big discussion point. It provides direct link between the east and west of the Pearl River Estuary and can facilitate the development of industries that rely on fast transportation, for example logistics, hence boosting the development of the three vibrant cities in the Pearl River Delta region economically and sustainably. Besides this there is the political ambition of China for HZMB to be a feat of human endeavour on a historic scale.

Overview

In this essay, I will talk about different sections of the bridge: Tuen Mun Western Bypass, Tuen Mun – Chek Lap Kok Link, the Hong Kong Boundary Crossing Facilities, Hong Kong Link Road and Hong Kong Link Road and Hong Kong – Zhuhai – Macao Bridge Main Bridge, deep diving on the Hong Kong Boundary Crossing Facilities.

Tuen Mun Western Bypass (TMWB)

The TMWB is a 9-kilometre double 2-lane expressway, consisting of twin tunnels, a toll plaza and associated administration buildings in Pillar Point. It links together the Tuen Mun – Chek Lap Kok Link and the Kong Sham Western Highway.

Tuen Mun – Chek Lap Kok Link (TM-CLKL)

The Tuen Mun – Chek Lap Kok Link is a 9.7-kilometre double 2-lane route between the between North Lantau and Tuen Mun South. It is divided into two main sections: Northern Connection and Southern Connection. The Northern Connection consists of a 5-kilometre long sub-sea tunnel deep under the seabed at a water head of above five bars crossing a reclamation area while the Southern Connection includes a 1.9-kilometre long land viaduct and a 1.6-kilometre long sea viaduct.

Viaducts are a form of bridge interconnected in a series of small spans. They are used to connect two areas which are similar in height with the purpose of carrying road traffic whereas traditional bridges are constructed exclusively for crossing physical impediments. Hence, using viaducts over bridges are better as they connect the two land areas of similar heights and help reducing the traffic congestion to minimal. Friction bored piles are employed. The piles were designed as end-bearing piles and the top level of the pile caps are made from reinforced concrete structures.

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For the construction of the undersea tunnel, three Tunnel Boring Machines (TBM) were deployed: one with diameter of 17.6, which is currently the largest TBM in the world and two identical mix-shield TBMs with smaller diameter. A tunnel boring machine is a tube-like machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. With the use of TBMs, a smooth tunnel wall can be produced, making them suitable to use in Hong Kong, a heavily urbanised area. Apart from boring the tunnels, TBMs also provide support. Selecting the TBM tunnelling as the method instead of the traditional Drill and Blast technique was in part due to lower environmental impact as TBM would create a much lower ground vibration than D&B, which requires steps such as explosive charging and blasting. Furthermore, due to the tight schedules of the construction, TBM has a quicker excavation rate which allows the construction to be finished in a shorter period of time.

Hong Kong Boundary Crossing Facilities (HKBCF)

The Hong Kong Boundary Crossing Facilities is situated on an artificial island of about 150 hectares and is served as a transportation hub which provides clearance facilities for passengers and goods. The HKBCF will be discussed in greater detail in the main focus section.

Hong Kong Link Road (HKLR)

The Hong Kong Link Road includes a dual 3-lane road, comprising of a 9.4-kilometre long viaduct section going from the HKSAR boundary to the Airport Island; followed by a 1-kilometre tunnel section to the reclaimed area formed along the east coast of the Airport Island and a 1.6-kilometre long at-grade road section on the reclaimed area connecting to the HKBCF.

To minimize the visual and environmental impacts to the nearby residents, the majority of the tunnel section was constructed undersea. Pile caps of the viaduct have been buried below the sea bed to minimise the disturbance to the existing current flow of Airport Channel. The substructure construction was comprised of reinforced columns on marine bored piles, surrounded by casings. This helps protect the marine environment against spilling or leaking. The superstructure of the viaduct is composed of a dual-3 lane with hard shoulders. The viaduct required having spans longer than 75 metres for navigation purpose. Three different construction methods were available: the precast segmental method, the precast spans method and the In-situ balanced-cantilever method. Precast segmental methods and In-situ balanced-cantilever methods were adopted. The precast segmental method involves the elevating onto place precast segments of less than 80 metres long whilst the In-situ balanced-cantilever method can span greater distances. It involves constructing on place the different segments of the viaduct and to pre-stress them onto the segments. Some reclamation work was done on the eastern part of the HKLR tunnel. A non-dredge reclamation method which had more benefits over the conventional dredging method, was introduced, eliminates dredging and greatly reducing backfilling material.

Main Bridge

The HZMB Main Bridge consists of a 29.6-kilometre dual 3-lane roadway in the form of bridge-cum-tunnel structure consisting of an immersed tunnel and two artificial islands for the tunnel landings on the western coast of the HKSAR boundary.

Due to the non-uniform soil types which are mainly plastic clays and very hard marine clay, as well as the extensive dual-3-lane road layout that had to be accommodated with an ample operational safety level, the traditional tunnel option was practically not feasible. This was because the foundation would made it intricate to construct machinery with the risk of material changing from sand to clay and so forth. Other reasons behind were that it would require an 80-metre high bridge and a high bridge tower with height of 200 metres, yet the Hong Kong International Airport does not allow anything higher than 88 metres. Owing to the above restrictions, traditional sea-crossing bridge could not be built and immersion was the only option. An immersed tube tunnel is a tunnel constructed directly under a waterway. This underground tunnel is the longest and deepest immersed tunnel ever built in China, as well as the most technically advanced part of the project. It has a length of 6.7 kilometres and is 44.5 metres below sea level. The tunnel comprises of 33 immersed segments of 180-metre long and weighing 76,000 tonnes each, linked by a butt joint in the middle. The segments are made of reinforced water-proof concrete and each has a segmental structure with a cross-sectional profile of two bores and one middle gallery. Unlike the undersea tunnels in TM-CLKL, this tunnel requires a greater depth below water that immersed-tube method is the most preferred method as it is mainly used for constructing tunnels that cross deep water.

Main focus – HKBCF

Traditionally, reclamation requires dredging and dumping a large amount of soft marine mud so that the seawalls are constructed on a firm foundation by replacing the soft marine mud in the seabed by sand fill. As mentioned before, the non-dredge reclamation method was used to build the artificial islands, this was chosen for the purpose of minimizing the ecological impacts caused by dredging. Dredging, disposal of marine mud and bulk filling activities can give rise to many potential impacts on water quality, affecting marine wildlife. Comparing the traditional reclamation method to the non-dredge method, less environmental impacts are created. However, the procedures for construction of steel cellular seawalls are more complex with the use of more technical and advanced equipment when compared with that for the construction of traditional rubble mound seawalls. In conclusion, although the non-dredge method is far costlier than the traditional one, it is certainly a better choice for reclamation in the prospective of sustainability.

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Among several types of non-dredged methods: stone column, sand compaction pile (SCP), deep cement mixing (DCM) and cofferdam type seawall, DCM and cofferdam type seawall were adopted. This was to improve the overall stability by reducing sliding, overturning and bearing failures and global instability. It was also to enhance internal stability by keeping the tension failure of interlocks between sheet pile at the most favourable tension.

DCM has never been adopted in Hong Kong due to the lengthy process to carry out field trial and the high mobilisation cost. DCM involves the process of ground heaving which is to dredge and dispose the upheaved soil, uplifting it to the surface. The biggest concern of using this method is that it can impact the marine environment significantly.  It can lead to cement slurry leakage from the slurry pump and mixing shaft, as well as seepage during slurry mixing. Moreover, it can cause long-term deterioration or soil mix and dissolution into seawater is not well-defined. Lastly, during the process, the heat emitted from the chemical reaction is undefined that the possible impacts on marine is difficult to be controlled. For the cofferdam type seawall, a series of circular steel cylinders which were 55-metre high and weighed 550 tonnes, were vibrated directly into existing seabed without dredging. They acted as a massive gravity retaining structure when backfilled to its top level. Part of the seawall of the artificial island was formed by sinking large steel circular caissons through the soft marine mud. It was innovative to use a combination of different non-dredge approaches. However, it would be even better if sand compaction pile was used instead of DCM. SCP is one of the most environmentally friendly non-dredge methods compared to the others that it requires lesser rockfill and it is also a similar process to the DCM. However, the major drawback to this method is that the working height is limited. The height of SCP plant can be modified to the minimum which is 40metres, by cutting out the top frame if height is a problem.

The largest building of the HKBCF, the Passenger Clearance Building (PCB) has a construction floor area of over 90,000 square metres. The PCB is designed to be aesthetically pleasing, energy efficient and sustainable. The roof is designed in wavy form and is supported by tree-like structural columns with few interior structural columns. Due to the large-scale construction, the roof was prefabricated in vast modules at the prefabrication yard to speed up the construction progress. The roof modules were composed of structural steel frame and also the pre-installed building service works, architectural builder works and finishes, including smoke vents, baffle ceilings, aluminium claddings, drainage and lighting system. A horizontal launching method was adopted during the delivery and installation of such massive roof components. It involved 4 sets of self-propelled modular transporters and horizontal hydraulic jacks pushing the modules into position to the top of PCB in which the components were then installed one after another. Using this method, the accuracy of installation was greatly improved so that the connection of all modules was made smoother.

A megaproject can’t be built without overcoming serious physical challenges, the HZMB project is of no exception because the project was so ambitious. The HZMB is designed to have a service life of 120 years. This is the first ever time for such an extended period of time has been specified for infrastructure in China, given that it has to have the ability to withstand magnitude-eight earthquakes and super-hurricanes. Problems were mainly encountered during and after the construction of the artificial island. Two massive problems were encountered. One was the consequence of adopting the non-dredging reclamation method. Part of the artificial island moved 20 metres and movements of up to 7 metres had also been observed in several parts of the island. This was a result of the tight schedule of given to the construction, the reclaimed land should have been left to settle for between 5 and 15 years in order to obtain a firm foundation before building on it. As the soft mud varied in thickness from 10 to 30 metres, the caissons settled at different rate therefore reached the hard stratum at different time. Since the caissons are still settling, they can distort and move, and consequently, the sea wall may not hold the proposed shape. Another challenge was that the wave-absorbing concrete blocks, which are called ‘dolosse’, surrounding the artificial island appear to have drifted away and become partly submerged. The propose of the dolosse is to prevent the island from being eroded by the sea. However, if the blocks are submerging and some are already below water, how can they absorb waves effectively? Each dolosse weighs 5 tonnes, which is too light, it would be more secure if 30-tonne blocks were used.

Conclusion

Whether or not it is considered to be successful, the Hong Kong – Zhuhai – Macao Bridge is certainly a milestone in the structural and civil engineering’s history. Overall this marvellous piece of engineering delivers on its keys promises. The connectivity brought by the HZMB has connected the Pearl River Delta region and its neighbouring provinces, resulting in better economic integration and competitiveness. From the political point of view, it has brought China enormous prestige. However, it is not performing as it was intended to due to the factors discussed above in which the majority of problems are concerned with the artificial island. Nevertheless, the innovative methods that were developed to build HZMB will have a strong positive impact on future constructions. There are even more megaprojects being planned already, taking engineering to another level. Has the race of building the world’s longest bridge just begun?

Bibliography