Immersed tube - WikiHQ

An immersed tube (or immersed tunnel) is a kind of undersea tunnel composed of segments, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road and rail crossings of rivers, estuaries and sea channels/harbours. Immersed tubes are often used in conjunction with other forms of tunnel at their end, such as a cut and cover or bored tunnel, which is usually necessary to continue the tunnel from near the water's edge to the entrance (portal) at the land surface.

Construction

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. 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.

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. The oldest immersed tube in Europe is the Maastunnel in Rotterdam, which opened in 1942.

The Marmaray Tunnel, connecting the European and Asian sides of Istanbul, Turkey, is the world's deepest immersed tunnel at below sea level; it is the first rail link crossing the straits. Construction began in 2004 and revenue service began in 2013. The tunnel is long overall, of which were constructed using the immersed tube technique. The HZMB tunnel is set at a depth of below sea level. Its length will be surpassed by with the completion of the Shenzhen–Zhongshan Bridge in 2024. The SZB project includes a immersed tube which also will be the world's widest immersed tube, carrying eight traffic lanes. Prior to the completion of the Marmaray and HZMB tunnels, the Transbay Tube in San Francisco Bay, completed in 1969, was the world's deepest and longest immersed tube, at below water level and long. at an as-designed long. Construction started on 1 January 2021.

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|+ Largest immersed tubes

! Hong Kong–Zhuhai–Macau Bridge

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| 2010 || Pearl River estuary in Hong Kong; Macau; and Zhuhai, China

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! Busan–Geoje Fixed Link

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| 2010 || Busan and Geoje Island, South Korea