One of the most ambitious projects currently under way in Hong Kong is the Tuen Mun-Chek Lap Kok Link, a new road that will link the New Territories with Lantau and provide a vital alternative route to the airport.
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At present, the only road link to the airport is via the North Lantau Expressway and the bridges that form the Lantau Link, a route that is prone to closure and restrictions during heavy rainfall and typhoons. The Tuen Mun-Chek Lap Kok Link should be more resilient, as it is predominantly in an undersea tunnel between the New Territories and Lantau Island.
The scheme has been planned since 2005, when a feasibility study recommended a 9km dual two-lane road consisting of an immersed tube subsea tunnel and elevated approach structures
Tuen Mun to Chek Lap Kok tunnel
By 2008, consultant Aecom had carried out further investigations and preliminary design, and suggested it should be changed from immersed tube to bored tunnel. This proposal was accepted in 2011, and design and construction have been continuing since, with construction contracts awarded in 2013.
“Immersed tube is the traditional way to build tunnels in Hong Kong,” says Aecom executive director Alan Boden. “But the original design tunnel would have involved extensive dredging; diversion of the main cables to the airport; a lot of impact on busy navigation channels; and considerable environmental issues – including the impact on the endangered pink dolphins.”
Immersed tube is the traditional way to build tunnels in Hong Kong, but the original design tunnel would have involved extensive dredging
Alan Boden, Aecom
A tunnel boring machine (TBM) solution avoids all of these issues, but, says Boden: “Taking forward a TBM scheme shouldn’t be taken lightly. Immersed tube tunnels are done very successfully in Hong Kong, and TBM tunnelling brings risks, including the selection of the TBM, tunnel stability, constructing the cross passages, and how to manage tunnel health and safety when you are 55m deep at a pressure of 5.5 bar.
“You need a contractor with innovative solutions to address these challenges,” he adds.
That contractor is Bouygues subsidiary Dragages Hong Kong, which has been working with Aecom to develop the best – and safest – method for excavating the 14m diameter twin-tube tunnels and the 56 cross passages between them. Construction started in March last year, with the excavation of the approach ramps at the north end of the tunnel. Subsea tunnel boring followed in December, using two 14m diameter TBMs.
More challenges to come
“The biggest challenges are ahead”, says Bouygues Travaux Publics project director Seved Robin. These include working in very high pressures, changing worn out cutting tools, and excavating the cross passages – all while minimising human intervention ahead of the machines.
Two innovations have been specifically developed to reduce the need for manual operations in hyperbaric conditions. The first, “Mobydic”, is a system of sensors incorporated into disc cutters in the heads of the TBMs, that will enable wear on the cutters to be monitored, as well as providing real-time geological mapping of the rock face.
Tuen Mun to Chek Lap Kok tunnel
The second innovation is “Snake”, a remote controlled robotic exploration arm equipped with a high pressure jet that can clean and replace cutters in the TBM heads. “It works in hyperbaric pressure,” explains Robin. “The cycle of cleaning, taking out and replacing is one and a half hours, which is similar to the time taken by divers, but we don’t have to send anyone out in front.”
Robin says the robotic arm will be able to perform about 60% of the disc cutter changes, but will not be able to reach to cutters at the centre of cutter head, so divers will be needed for some of the changes. They will work in “saturation mode”, a technique that is common in the oil and gas industry to avoid decompression sickness.
Bouygues has addressed the challenge of building cross passages every 100m by opting for a TBM solution for the majority of them. “We initially thought of using ground freezing, which is very safe as long as you have enough time, but is very sensitive to ground conditions,” says Robin. “So we decided to use a pipe pushing TBM technique for the cross passages, pushing from one segmental tunnel to another. You get strength from the segments on both sides.”
Construction of the first cross passage took place in January.
South East Asia | Tuen Mun to Chek Lap Kok tunnel