Immersed tube

An Immersed Tube is a kind of underwater tunnel comprised of segments and/or elements, constructed elsewhere and floated to the tunnel’s site to be sunken into place (often into an excavated trench), with the elements being connected, finished (drained, ballast added, internal works, etc.) and covered up once in location. They have so far been 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 at the land surface.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/seismicity) 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 seismicity
* Safety of construction (for example, work in a dry dock as opposed to boring beneath a river)
* Flexibility of profile (although this often partly dictated by what is possible for the connecting tunnel types)

Disadvantages include:
* Vulnerability to sabotage as it is arguably easier to damage and breach their walls or roof than to achieve the same to a bored tunnel.Fact|date=September 2008
* The tunnel is partly exposed (usually with some rock armour and natural siltation) on the river/sea bed, risking a sunken ship/anchor strike
* Both of these disadvantages should be considered and designed for, although by the nature of these risks are hard to quantify and assess using a risk analysis
* 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

The immersed tube is the most frequently applied type applied in the Netherlands.

Examples

* Hong Kong Cross-Harbour Tunnel
* 63rd Street Tunnel, a four-bore rail tunnel under the East River in New York City
* Sydney Harbour Tunnel - road
* Transbay Tube, a BART rail tunnel under San Francisco Bay
* Detroit-Windsor Tunnel, two-lane automobile tunnel under Detroit River, leading from Windsor, Ontario to Detroit, Michigan
* The Posey and Webster Street Tubes, connecting Oakland and Alameda, California.
* The Fort McHenry Tunnel in Baltimore, Maryland.
* Marmaray in Istanbul
* Jack Lynch Tunnel in Cork
* Limerick Tunnel in Limerick,Ireland
* A55 Conwy Bypass Tunnel, Conwy, Wales,United Kingdom
* The Ted Williams Tunnel in Boston, Massachusetts
* The Maastunnel in Rotterdam, the Netherlands
* The tunnel sections of the Chesapeake Bay Bridge-Tunnel
* The Medway Tunnel in Kent, United Kingdom
* Tingstadstunneln, Sweden
* New Tyne Crossing, Newcastle, United Knigdom

See also

* Archimedes bridge