Immersed tube tunnel

An immersed tube tunnel is built by lowering prefabricated sections into a prepared underwater trench, joining them, and covering them with backfill. Its construction method suits crossings beneath waterways and other submerged routes.

Immersed tube construction: A construction method that places prefabricated tunnel units in a dredged trench and connects them underwater. This is the defining sequence behind the tunnel type.
Tunnel element: A prefabricated structural section designed to be joined with other sections to form a tunnel. Repeated elements make up the immersed tube’s main passage.
Drogden Tunnel: An immersed tunnel forming the Danish section of the Øresund connection between Copenhagen and Malmö. It demonstrates the method in a major fixed link across a strait.
Tunnel boring machine: A machine that excavates a tunnel while installing or supporting its lining. Bored construction advances through ground instead of placing prefabricated sections in a trench.
Underwater noise: Sound that propagates through water from natural or human sources. Dredging and marine construction can disturb aquatic animals through underwater sound.
Dredging: The removal of sediment or rock from underwater areas using specialized equipment. Dredging creates the trench that receives the tunnel sections.
Buoyancy: The upward force a fluid exerts on an immersed object. Construction depends on managing buoyancy while elements are floated and sunk.
Fehmarn Belt Fixed Link: A planned combined road-and-rail connection between Denmark and Germany, including an immersed tunnel beneath the Fehmarn Belt. Its design illustrates the scale of modern immersed-tunnel projects.
Cut-and-cover tunnel: A tunnel built in an excavated trench that is later roofed and covered. It shares trench construction but is generally built in an accessible, drained excavation.
Seabed disturbance: Physical disruption of the seabed caused by activities such as dredging or construction. Preparing and backfilling the trench alters the local seabed.

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