Immersedtunnel

Immersedtunnel, more commonly referred to as an immersed tunnel, is an underwater tunnel built by placing prefabricated tunnel elements into a trench dredged on the bed of a water body and then sealing and joining them to form a continuous passage. The method is used for road or rail crossings of seas, rivers, or lakes and is often considered when bored tunneling is impractical or less economical for shorter undersea spans.

Construction typically involves several stages. Engineering surveys determine route and depth, and tunnel elements are manufactured on land. A precise trench is dredged along the alignment, and the elements are floated to the site, lowered into the trench, and aligned. Joints between elements are sealed, and the assembly is grouted and waterproofed to achieve watertight integrity. After the main structure is completed, permanent ventilation, electrical, and emergency systems are installed, and the trench backfilled as needed. Shore approaches are connected through shafts and portals.

Materials used include reinforced concrete sections or steel elements, with concrete linings and joint seals designed to resist hydrostatic pressure and potential leakage. The method permits relatively rapid construction and avoids some risks associated with deep boring, but it requires stable seabed conditions, careful dredging, and precise alignment. Potential challenges include seabed settlement, scour, joint leakage, and maintenance of underwater joints and seals over time.

Notable applications include an Immersedtunnel section of the Drogden Tunnel, which is part of the Øresund fixed link between Denmark and Sweden, and the undersea portion of the Tokyo Bay Aqua-Line. These projects illustrate the method’s viability for mid-length undersea crossings.