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Hong Kong bridges special: The Macao-Zuhai crossing

The construction on the Hong Kong- Macao-Zuhai crossing is proving to be the most technically challenging in China’s History. Andy Bolton reports.

As construction projects go, the 38km Hong Kong-Macao-Zuhai crossing must be one of the most challenging underway anywhere on the planet.

Aside from the fact that it includes three cable stay bridges, the world’s longest immersed tube tunnel and four major pieces of land reclamation, contractors will have to overcome a raft of logistical and environmental constraints as they work towards the project’s 2016 completion deadline.

“It is one of the most technically complicated projects in the transport infrastructure history of China,” says the Hong Kong Highways Department project manager for the crossing Albert Chen.

For most of its length the crossing will snake across the open sea at the mouth of the Pearl River delta starting at its eastern end on a reclaimed island close to Hong Kong’s international airport at Chek Lap Kok and ending at a border crossing facility on reclaimed land just off Macao.

From there the route splits with roads running to Macao and to the neighbouring city of Zuhai.

Hong Kong - Zhuhai - Macao Crossing

The 5.5km immersed tube tunnel will take traffic under the sea between two reclaimed transition islands as the crossing dips under flight paths for Chek Lap Kok airport. Major reclaimed islands are also to be built at each end of the crossing to house border crossing facilities and park and ride schemes.

A consortium comprising Arup, Cowi and Shanghai Tunnel Design Institute has been hard at work on preliminary designs for the crossing’s bridge and tunnel components for the best part of a year now, and there is an expectation that design and build contracts will start to be let this spring.

Arup’s work covers the three cable stay bridges and viaducts linking them together. Cowi and Shanghai Tunnel Design Institute are concentrating on the tunnel.

Prefabrication is likely to dominate the project, given that it will be built in open sea across a region where construction programmes are likely to be squeezed by extreme weather ranging from typhoons and tornadoes to tropical cyclones and lightning storms.

“The whole concept of the design is influenced by the fact that it is visible from the air”

Naeem Hussain, Arup director

There will also be tough working restrictions to protect marine life in the Pear River estuary (see below), and prefabrication of key elements will help to keep disruption of the seabed to a minimum.

With this in mind, the consultant has decided to recommend orthotropic steel box girders for the 35m wide deck which will span between 75m and 110m between piers on the viaduct sections linking the cable stay bridges and the tunnel section.

“We were looking for systems where we could lift whole spans into place,” says Arup director Naeem Hussain.

Minimising risk

Weight considerations were also a factor. The girders, which are to be positioned using floating cranes, could run aground on the western end of the crossing where they will be working in water as shallow as 2m. There is also a risk that the floating cranes will run aground if heavier concrete or composite steel and concrete girders are used.

The Arup team has attempted to minimise the impact of the bridge on water flows in the Pearl River estuary by resting the bridge deck on single piers rather than on pairs of supports. The bridge also meanders across the estuary in a series of curves, partly to enable piers to be positioned at right angles to tidal flows, further reducing the bridge’s impact.

The need to minimise disruption to water flowing through the estuary has also influenced the design of the innovative cable stay towers for the 296m span cable stay bridge at the Macao end of the crossing.

Pylons for the three cable stayed bridges are located between the twin girders

Pylons are located between the twin girders

These 170m high steel towers will take the form of a vertical bowstrings set in the centre of the deck, with the curved bows adding strength to the tower and negating the need for intermediate anchor piers under the back spans.

Seven tubular steel elements through which seven of the back span cables will pass and will connect the curved bow section with the tower adding extra strength. The tower and bow sections will be prefabricated will be floated to site for assembly insitu.

Aesthetic considerations figure strongly in Arup’s approach to the design of the crossing’s bridges and viaducts. Towers for all three cable stay structures will be monopole structures penetrating the deck.

“It is one of the most technically complicated projects in the transport infrastructure history of China”

Albert Cheng, Hong Kong Highways Department

“The whole concept of the design is influenced by the fact that it is visible from the air,” says Hussein. “One of the things we wanted to do was to have visual continuity between all three bridges, although we still wanted there to be a slight visual distinction between the three,” he adds.

The back to back bowstring towers at the western end of the bridge are also intended to improve its visual appeal, as it will be clearly visible from the Macao shoreline.

The other two cable stay structures will be more conventional but just as challenging. The middle one will have three concrete towers with two 240m main spans, while the one at the Hong Kong end will have two towers and a 460m main span.

Environmental hurdles

Construction sequencing and planning will be a major factor in ensuring that the project is completed on time. Constraints include the weather, environmental considerations and the fact that crossing passes through seven major shipping lanes.

The weather can be extreme and unpredictable. Typhoons strike an average of 1.75 times a year, although in 1999 there were six. “The constraints have been taken into account in the project programme,” says Cheng.

Reclamation work in the sea is a priority right now, and work on the island for the Macao/Zuhai is already underway, having started in December.

“The most critical elements, which have a pressing need to start first are the reclamation of the artificial islands for the boundary crossing facilities and those for the tunnel landings,” says Cheng. “This is because time is needed for the reclaimed land to settle to some extent before the subsequent construction of the superstructures and tunnels can proceed.”

 

Dolphin protection in the Pearl River Delta

The protected white dolphin

Ultimately the authorities claim that the crossing will benefit the endangered Chinese white dolphin, because its fast links between Hong Kong and the area of mainland China across the Pearl River estuary will reduce demand for shipping in the area.

“It is expected that the completion of the crossing would reduce the amount of marine traffic at the Pearl River Estuary, thereby reducing the chances of Chinese white dolphins being hit by ships,” says Cheng.

But while the crossing is being constructed, heavy restrictions will apply to contractors to ensure the endangered dolphin species is not disturbed.

For a start, all work will have to stop if a dolphin is seen within 500m of a worksite. Construction shipping will have also have to respect speed limits and only use designated routes to and from the site.

Silt curtains, suspended from floating booms will also be used to prevent the spread of material disturbed during dredging work.

Percussive piling will face tight restrictions during the April to August mating season.

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