Immersed tube

The tunnel is made up of separate elements, each prefabricated in a manageable length, then having the ends sealed with bulkheads so they can be floated.[1] At the same time, the corresponding parts of the path of the tunnel are prepared, with a trench on the bottom of the channel being dredged and graded to fine tolerances to support the elements. The next stage is to place the elements into place, each towed to the final location, in most cases requiring some assistance to remain buoyant. Once in position, additional weight is used to sink the element into the final location, this being a critical stage to ensure each piece is aligned correctly. After being put into place, the joint between the new element and the tunnel is emptied of water then made water tight, this process continuing sequentially along the tunnel.[2]

The trench is then backfilled and any necessary protection, such as rock armour, added over the top. The ground beside each end tunnel element will often be reinforced, to permit a tunnel boring machine to drill the final links to the portals on land.[2] After these stages the tunnel is complete, and the internal fitout can be carried out.

The segments of the tube may be constructed in one of two methods. In the United States, the preferred method has been to construct steel or cast iron tubes which are then lined with concrete. This allows use of conventional shipbuilding techniques, with the segments being launched after assembly in dry docks. In Europe, reinforced concrete box tube construction has been the standard; the sections are cast in a basin which is then flooded to allow their removal.

The main advantage of an immersed tube is that they can be considerably more cost effective than alternative options – i.e., a bored tunnel beneath the water being crossed (if indeed this is possible at all due to other factors such as the geology and seismic activity) or a bridge. Other advantages relative to these alternatives include:

• Their speed of construction
• Minimal disruption to the river/channel, if crossing a shipping route
• Resistance to seismic activity
• Safety of construction (for example, work in a dry dock as opposed to boring beneath a river)
• Flexibility of profile (although this is often partly dictated by what is possible for the connecting tunnel types)

Disadvantages include:

• Immersed tunnels are often partly exposed (usually with some rock armour and natural siltation) on the river/sea bed, risking a sunken ship/anchor strike
• Direct contact with water necessitates careful waterproofing design around the joints
• The segmental approach requires careful design of the connections, where longitudinal effects and forces must be transferred across
• Environmental impact of tube and underwater embankment on existing channel/sea bed.

Tubes can be round, oval and rectangular. Larger strait crossings have selected wider rectangular shapes as more cost effective for wider tunnels.

The first tunnel constructed with this method was the Shirley Gut Siphon, a six-foot sewer main laid in Boston, Massachusetts in 1893. The first example built to carry traffic was the Michigan Central Railway Tunnel constructed in 1910 under the Detroit River. The Marmaray, Istanbul, Turkey, is the world's deepest immersed tunnel. The Fehmarn Belt Fixed Link connecting Denmark and Germany is expected to be completed as the world's longest immersed tunnel[3] in 2021. Currently the longest completed immersed tube tunnel is the 4.2-mile-long (6.7-kilometre) tunnel portion of the Hong Kong-Macau Bridge.[4]

• Submerged floating tunnel

5. "Foundation of a tunnel by the sand-flow system", Tunnels and Tunnelling, July, 1973 by A. Griffioen and R. van der Veen

• An immersed tunnel under Söderström on YouTube, by Stockholm City Line