China Daily Global Edition (USA)
Soft clay on seabed posed biggest challenge for engineers
The complex geology of the Pearl River estuary in South China was a driving force behind the technological innovation that makes the Hong Kong-Zhuhai-Macao Bridge stand out from similar projects around the world, according to civil engineers.
They point to the estuary’s variable soil conditions, with thick layers of soft alluvial clay that subject its undersea tunnel section, at a depth of about 50 meters, to high hydraulic pressure, and say the entire 55-kilometer structure may be the most complex and challenging sea crossing ever built — in addition to being the world’s longest.
Hans de Wit, managing director of Tunnel Engineering Consultants, a leading tunnel construction company based in the Netherlands, said complicated infrastructure projects like the bridge typically stimulate innovation.
De Wit, who has nearly 30 years of experience in immersed tunnel projects, was the lead consultant for the design and construction of the HZMB’s island and tunnel project.
The size of the bridge project and its immense budget allowed for study and innovation, he said. “Additionally, these large infrastructure projects very often contain technical challenges beyond the current state of the art. This is also the case with the HZMB.”
Seabed battleground
The bridge’s 6.7-kilometer tunnel, the world’s longest immersed tunnel for road traffic and China’s first offshore immersed tunnel, was the most technologically challenging part of the project.
The tunnel sits in a 20-meter-deep trench along the seabed, and the soil conditions there determine the stability and solidity of its foundation.
There are 33 sections in the HZMB tunnel, with dozens of joints between each section. Unexpected subsidence of a near 80,000-metric-ton tunnel section due to an unstable foundation could destroy the whole undersea structure.
Unlike similar projects in South Korea and Denmark, where the tunnels cross straits with relatively stable geology, the loose sediment of the Pearl River estuary is easily affected by waves and tides, making the seabed unstable.
As a result, the construction team decided to install an extra man-made rock block layer that is 2 meters thick under the tunnel base, in addition to the traditional 1.3-meter gravel bed.
De Wit said the additional rock block will make the tunnel’s foundation more uniform and keep subsidence within an acceptable range.
Internationally, settlement of an immersed tunnel needs to be within 20 centimeters. But that of the HZMB has been controlled within 10 cm, according to its chief engineer, Su Quanke.
Summer storms
The frequent typhoons and rainstorms that hit South China in summer add to the challenges of offshore construction, and each day saved reduced risks to the project.
The 55-km bridge-islandtunnel complex withstood a lashing from Typhoon Hato in August last year, when the project was still under con-