With the continuous development of construction techniques and technologies that overcome challenging conditions and minimise risk, immersed tunnels are set to become far more common.
There are only around 200 immersed tunnels in the world, used mainly for transport and utilities, but recent years have seen a marked increase in interest in the technology. Historically, immersed tunnels have been seen as a third option behind the bridge solution and the bored tunnel solution, but that’s changing.
Immersed tunnels are becoming longer, deeper and more competitive than ever, with envelope-pushing projects such as the Swedish-Danish Øresund Link, the Busan-Geoje Link in South Korea and German-Danish Fehmarn Belt Fixed Link commanding global attention.
Immersed tube tunnels are formed by connecting together prefabricated box segments in trenches excavated in waterways. Floating tunnel elements are towed into position before being lowered to the seabed for assembly. The tolerances involved in the complex process are tiny – each element must line up exactly for the watertight gaskets to seal properly.
Immersed tunnels offer many advantages over other crossing types such as bridges and bored tunnels. For example, they are better suited to short distance crossings – where it can be challenging to connect bridge or bored tunnel approaches to existing road or rail networks – and they can be laid in poor ground conditions that would cause major risks to building bridge foundations or driving through a tunnel boring machine.
While bridge and bored tunnel construction techniques are familiar to most engineers, immersed tunnel construction is less well understood and is often thought of as new and specialist construction technology. In fact, it mostly uses established technologies, and has much in common with maritime and offshore engineering.
New technologies are making the construction process faster and safer. For example, on the US$1.6B Busan-Geoje fixed link project in South Korea, the contractor – a consortium of Korean companies led by Daewoo Engineering & Construction – used the innovative ‘external positioning system’ solution from Dutch specialist Strukton Immersion Projects.
This involved fitting steel frames with hydraulic platforms to each tunnel element, like adding legs and feet. Once on the seabed, the hydraulic feet were remotely operated to precisely position each element before connecting it to the preceding section. This greatly reduced the input required from a subsea workforce, improving worker safety and speeding the schedule. Mott MacDonald provided expert advice to Korean construction supervisor Yooshin Corporation.
A proposed immersed tunnel between Denmark and Germany will be the longest ever by some magnitude at 17km. Dredging the 50m-wide trench for the 217m-long precast elements will require reaching down to around 45m in the Fehmarn Belt in the Baltic Sea, and removing an estimated 20M cu m of soil. The tunnel has five planned passageways; two for motor vehicles, one for emergency escape, and two for high-speed trains.
A bridge was the original option for the crossing, but the structure would have posed significant environmental and construction risks: the many bridge piers would permanently disrupt marine life and water currents, and create collision hazards in the busy shipping channel. It was also realised that, because the bridge would have required a double deck and strengthening to accommodate high loads, the immersed tunnel solution is no more expensive.
Headline projects such as the Fehmarn Belt tunnel show just how effective the immersed tunnel solution has become. New markets are opening up, with projects planned in Brazil and the Middle East that will add to other immersed tunnel proposals in Europe and Asia. The immersed tube tunnel will come of age over the next decade.