Immersed tube

Immersed tube type A, built upon a shallow trench on the seabed.

Immersed tube type B, built under the seabed level.

An immersed tube is a kind of underwater 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

An immersed tube segment of the Marmaray tunnel in Istanbul, Turkey during 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 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 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.

Examples

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 under the Detroit River.

Other examples include:

• The Louis-Hippolyte Lafontaine Bridge-Tunnel in Montreal, Canada
• The Drogden Tunnel, part of the Øresund Bridge complex connecting Denmark and Sweden
• Marmaray, Istanbul, Turkey, the world's deepest immersed tunnel
• 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 subway rail tunnel under San Francisco Bay, California
• Detroit-Windsor Tunnel, two-lane automobile tunnel under the Detroit River, leading from Windsor, Ontario to Detroit, Michigan
• The Posey and Webster Street Tubes, connecting Oakland and Alameda, California
• The Fort McHenry Tunnel and the Baltimore Harbor Tunnel in Baltimore, Maryland
• Jack Lynch Tunnel in Cork, Ireland
• Limerick Tunnel in Limerick, Ireland
• A55 Conwy Bypass Tunnel, Conwy, Wales, United Kingdom - first Immersed Tube tunnel in the UK, opened 1991
• The Ted Williams Tunnel in Boston, Massachusetts
• The Maastunnel in Rotterdam, Netherlands
• The tunnel sections of the Chesapeake Bay Bridge-Tunnel connecting Virginia Beach and the Eastern Shore of Virginia
• The Medway Tunnel in Kent, United Kingdom
• Tingstadstunneln, Sweden
• New Tyne Crossing, Newcastle, United Kingdom
• The George Massey Tunnel, connecting Delta and Richmond, British Columbia
• The Washington Metro Yellow Line tunnel from South Potomac Park heading into Washington DC just after crossing a bridge over the Potomac River.^{[citation needed]}
• 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.^{[citation needed]}

See also

• Archimedes bridge