Immersed tube
An immersed tube is a kind of underwater tunnel
A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers...

 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.


The tunnel is made up of separate elements, each prefabricated
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located...

 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 dewatered and 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
Tunnel boring machine
A tunnel boring machine also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre to almost 16 metres to date...

 to drill the final links to the portals on land. 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.

Advantages and disadvantages

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 often partly dictated by what is possible for the connecting tunnel types)

Disadvantages include:
  • The tunnel is 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 constructed in 1910 to carry the Michigan Central Railroad
Michigan Central Railroad
The Michigan Central Railroad was originally incorporated in 1846 to establish rail service between Detroit, Michigan and St. Joseph, Michigan. The railroad later operated in the states of Michigan, Indiana, and Illinois in the United States, and the province of Ontario in Canada...

 under the Detroit River
Detroit River
The Detroit River is a strait in the Great Lakes system. The name comes from the French Rivière du Détroit, which translates literally as "River of the Strait". The Detroit River has served an important role in the history of Detroit and is one of the busiest waterways in the world. The river...


Other examples include:
  • The Louis-Hippolyte Lafontaine Bridge-Tunnel in Montreal
    Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...

    , Canada
    Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

  • The Drogden Tunnel, part of the Øresund Bridge complex connecting Denmark
    Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

     and Sweden
    Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

  • Marmaray, Istanbul
    Istanbul , historically known as Byzantium and Constantinople , is the largest city of Turkey. Istanbul metropolitan province had 13.26 million people living in it as of December, 2010, which is 18% of Turkey's population and the 3rd largest metropolitan area in Europe after London and...

    , Turkey
    Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

    , the world's deepest immersed tunnel
  • Hong Kong Cross-Harbour Tunnel
    Cross-Harbour Tunnel
    The Cross-Harbour Tunnel is the first tunnel in Hong Kong built underwater. It has become one of the most congested roads in Hong Kong and the world...

  • 63rd Street Tunnel, a four-bore rail tunnel under the East River
    East River
    The East River is a tidal strait in New York City. It connects Upper New York Bay on its south end to Long Island Sound on its north end. It separates Long Island from the island of Manhattan and the Bronx on the North American mainland...

     in New York City
    New York City
    New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

  • Sydney Harbour Tunnel
    Sydney Harbour Tunnel
    The Sydney Harbour Tunnel is a twin-tube road tunnel in Sydney, Australia. The tunnel was completed and opened to traffic in August 1992 to provide a second vehicular crossing of Sydney Harbour to alleviate congestion on the Sydney Harbour Bridge....

     - road
  • Transbay Tube
    Transbay Tube
    The Transbay Tube is the part of BART which runs under San Francisco Bay in California. The tube is 3.6 miles long; including approaches from the nearest stations , it totals 6 miles...

    , a BART
    Bay Area Rapid Transit
    Bay Area Rapid Transit is a rapid transit system serving the San Francisco Bay Area. The heavy-rail public transit and subway system connects San Francisco with cities in the East Bay and suburbs in northern San Mateo County. BART operates five lines on of track with 44 stations in four counties...

     subway rail tunnel under San Francisco Bay
    San Francisco Bay
    San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...

    , California
    California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

  • Detroit-Windsor Tunnel, two-lane automobile tunnel under the Detroit River
    Detroit River
    The Detroit River is a strait in the Great Lakes system. The name comes from the French Rivière du Détroit, which translates literally as "River of the Strait". The Detroit River has served an important role in the history of Detroit and is one of the busiest waterways in the world. The river...

    , leading from Windsor, Ontario
    Windsor, Ontario
    Windsor is the southernmost city in Canada and is located in Southwestern Ontario at the western end of the heavily populated Quebec City – Windsor Corridor. It is within Essex County, Ontario, although administratively separated from the county government. Separated by the Detroit River, Windsor...

     to Detroit, Michigan
  • The Posey and Webster Street Tubes
    Posey and Webster Street Tubes
    Methods Used in the Construction of TwelvePre-cast Concrete Segments for the Alameda County, California, EstuarySubway, in American Society of Civil Engineers, Proceedings, 53 :2675-2692 S. W...

    , connecting Oakland
    Oakland, California
    Oakland is a major West Coast port city on San Francisco Bay in the U.S. state of California. It is the eighth-largest city in the state with a 2010 population of 390,724...

     and Alameda, California
    Alameda, California
    Alameda is a city in Alameda County, California, United States. It is located on Alameda Island and Bay Farm Island, and is adjacent to Oakland in the San Francisco Bay. The Bay Farm Island portion of the city is adjacent to the Oakland International Airport. At the 2010 census, the city had a...

  • The Fort McHenry Tunnel and the Baltimore Harbor Tunnel in Baltimore, Maryland
    Baltimore is the largest independent city in the United States and the largest city and cultural center of the US state of Maryland. The city is located in central Maryland along the tidal portion of the Patapsco River, an arm of the Chesapeake Bay. Baltimore is sometimes referred to as Baltimore...

  • Jack Lynch Tunnel
    Jack Lynch Tunnel
    The Jack Lynch Tunnel is an immersed tube tunnel and an integral part of the N25 southern ring road of Cork in Ireland. It is named after former Taoiseach, Jack Lynch, a native of Cork....

     in Cork
    Cork (city)
    Cork is the second largest city in the Republic of Ireland and the island of Ireland's third most populous city. It is the principal city and administrative centre of County Cork and the largest city in the province of Munster. Cork has a population of 119,418, while the addition of the suburban...

    , Ireland
    Republic of Ireland
    Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...

  • Limerick Tunnel
    Limerick Tunnel
    The Limerick Tunnel is a 675m long, twin-bore road tunnel underneath the River Shannon on the outskirts of Limerick City. The tunnel forms part of the N18 Limerick Southern Ring Road. The tunnel is tolled....

     in Limerick
    Limerick is the third largest city in the Republic of Ireland, and the principal city of County Limerick and Ireland's Mid-West Region. It is the fifth most populous city in all of Ireland. When taking the extra-municipal suburbs into account, Limerick is the third largest conurbation in the...

    , Ireland
  • A55
    A55 road
    The A55, also known as the North Wales Expressway, is a major road in Britain. Its entire length is a dual carriageway primary route, with the exception of the point where it crosses the Britannia Bridge over the Menai Strait. All junctions are grade separated except for two roundabouts — one...

     Conwy Bypass Tunnel, Conwy
    Conwy is a walled market town and community in Conwy County Borough on the north coast of Wales. The town, which faces Deganwy across the River Conwy, formerly lay in Gwynedd and prior to that in Caernarfonshire. Conwy has a population of 14,208...

    , Wales
    Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

    , United Kingdom
    United Kingdom
    The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

     - first Immersed Tube tunnel in the UK, opened 1991
  • The Ted Williams Tunnel
    Ted Williams Tunnel
    The Ted Williams Tunnel, also known as the Williams Tunnel, is the name of the third highway tunnel under Boston Harbor in Boston, Massachusetts, the Sumner and Callahan Tunnels being the other two...

     in Boston, Massachusetts
    Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...

  • The Maastunnel
    The Maastunnel is a tunnel in Rotterdam, the Netherlands, connecting the banks of the Nieuwe Maas. About 75,000 motor vehicles and a large number cyclists, mopeds and pedestrians use the tunnel daily, making the Maastunnel an important part of Rotterdam's road network. Building commenced in 1937...

     in Rotterdam
    Rotterdam is the second-largest city in the Netherlands and one of the largest ports in the world. Starting as a dam on the Rotte river, Rotterdam has grown into a major international commercial centre...

    , Netherlands
  • The tunnel sections of the Chesapeake Bay Bridge-Tunnel
    Chesapeake Bay Bridge-Tunnel
    The Chesapeake Bay Bridge-Tunnel is a long fixed link crossing the mouth of the Chesapeake Bay and connecting the Delmarva Peninsula's Eastern Shore of Virginia with Virginia Beach and the metropolitan area of Hampton Roads, Virginia...

     connecting Virginia Beach and the Eastern Shore of Virginia
    Eastern Shore of Virginia
    The Eastern Shore of Virginia consists of two counties on the Atlantic coast of the Commonwealth of Virginia in the United States. The region is part of the Delmarva Peninsula and is separated from the rest of Virginia by the Chesapeake Bay. Its population was 45,553 as of 2010...

  • The Medway Tunnel
    Medway Tunnel
    The Medway Tunnel is a tunnel under the River Medway linking Strood with Chatham in Kent, England. It forms part of the Medway Towns Northern Relief Road...

     in Kent
    Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...

    , United Kingdom
  • Tingstadstunneln
    Tingstadstunneln is a motorway tunnel under the Göta Älv, connecting Hisingen with mainland Gothenburg. The tunnel was constructed with two parallel immersed tubes with three lanes of traffic in each tube...

    , Sweden
  • New Tyne Crossing, Newcastle
    Newcastle upon Tyne
    Newcastle upon Tyne is a city and metropolitan borough of Tyne and Wear, in North East England. Historically a part of Northumberland, it is situated on the north bank of the River Tyne...

    , United Kingdom
  • The George Massey Tunnel
    George Massey Tunnel
    The George Massey Tunnel is a highway traffic tunnel in the Metro Vancouver region of southwestern British Columbia. It is located approximately 20 km south of the city centre of Vancouver, British Columbia, and approximately 30 km north of the Canada-U.S...

    , connecting Delta
    Delta, British Columbia
    Delta is a district municipality in British Columbia, and forms part of Metro Vancouver. Located south of Richmond, it is bordered by the Fraser River to the north, the United States to the south and the city of Surrey to the east...

     and Richmond, British Columbia
    Richmond, British Columbia
    Richmond is a coastal city, incorporated in the Canadian province of British Columbia. Part of Metro Vancouver, its neighbouring communities are Vancouver and Burnaby to the north, New Westminster to the east, and Delta to the south, while the Strait of Georgia forms its western border...

  • The Washington Metro Yellow Line
    Yellow Line (Washington Metro)
    The Yellow Line of the Washington Metro consists of 17 rapid transit stations from Huntington to Fort Totten. The line terminates at the Mount Vernon Square station during peak hours.Peak hours are 5 a.m. to 9:30 a.m. and 3 p.m. to 7 p.m. Service is extended to the Green Line stations during...

     tunnel from South Potomac Park heading into Washington DC just after crossing a bridge over the Potomac River.
  • The concrete immersed rectangular tubes Tokyo Port Daiichikoro tunnel (Rinkai Tunnel) built in 2001

The immersed tube is the most frequent type applied in the Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

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