The first tunnel made of segments sunk and joined underwater was a sewer completed under Boston Harbour in 1896. But the practice of immersing tunnels is believed to have begun in 1910 with the construction of a twotrack railroad tunnel across the Detroit River between the US and Canada. Since then, more than 100 immersed tunnels of different sizes and lengths have been built worldwide.

The Marmaray Tunnel in Istanbul, Turkey is currently the world’s deepest immersed tunnel at 55m below sea level, and connects the European and Asian sides of the city. At 13.6km long, 1.4km were constructed using the immersed tube technique.

Immersed tunnels are normally constructed using prefabricated segments placed in trenches on a riveror sea-bed. Typically, tunnel segments (usually concrete but sometimes steel) are made in dry docks some distance from the tunnel location. Typically, the segments incorporate temporary bulkheads which are installed at both ends for watertightness. They are then floated and towed to their final location.

Once at the immersion location, the segments are lowered one at a time into a pre-dredged trench (typically using temporary water ballast to overcome the buoyancy) and joined to the previouslyplaced segment using special immersion rubber gaskets. Connecting segments with gaskets is designed to protect against relative displacements of the segment ends which can result from differential settlement, concrete creep, temperature changes and earthquakes.

The design of immersed tunnel cross-sections is usually an iteration and optimisation process and can be influenced by numerous factors other than structural analysis, including but not limited to, the requirements for floatation, internal space proofing, ventilation, smoke discharge and utilities.

Immersed tunnels may not be the best option for every crossing. However, when they constitute feasible alternatives to bridges or bored tunnels, they can offer several advantages:

  1. Immersed tunnel construction involves numerous activities that can be performed in parallel; downtime for one activity may not necessarily impact other operations. Activities such as segment production, trench preparation and segment transportation can significantly accelerate construction.
  2. Immersed tunnels are usually shorter than bored tunnels since the alignment is only several meters below the sea- or river-bed, and approaches can therefore be relatively short.
  3. They are typically more environmentally friendly than bridges (visual, noise and disruption), and have little to no impact on shipping.
  4. In high hydrostatic pressures, immersed tunnels can be advantageous over bored tunnels in terms of constructability, since there is no need for a pressurised TBM. Such tunnels are not constrained by circular cross-sections: rectangular can be particularly desirable for highways and combined road/rail tunnels.
  5. With regards to the potential hydraulic blockage of waterways, immersed tunnels pose little to no impact on water flow. In rivers with large discharges and substantial sediment transport, the presence of obstacles such as bridge piers may result in severe scouring and sedimentation.

Immersed tunnels have a relatively ‘thin’ cover layer which can be damaged by sunken wreckage or ship anchors. Also, being underwater requires more complex waterproofing design around joints and careful design of connections. In rare cases, immersed tubes can influence fish habitats, currents and even reduce the water transparency.

Other tunnels with similarities to immersed tunnels include ‘floating tunnels’ which do not require a trench, nor do they need covering; their final location is somewhere between the surface and bed levels. Two major support concepts have been proposed as ‘floating’ tunnels: buoyant tunnels held down by tethers, or heavier-than-water tunnels on supports, like a bridge.

The Fehmarnbelt Fixed Link (connecting Denmark with Germany) features an immersed tunnel that is currently being constructed to become the world’s longest immersed road and rail tunnel; at the planned 17.6km, it will be the longest ever constructed and will surpass the Marmaray Tunnel.