An Immersed Tunnel (IMT) is made up of a number of elements produced in a dry-dock or a shipyard. The elements are then towed to the site, immersed into a trench and connected to form the final tunnel. The picture shows one of the elements under towing out forming part of the biggest IMT ever built, the Oeresund tunnel between Sweden and Denmark. The element weighs 55,000 tons.

Fabricated Shipyards
Osaka South Port

The left picture shows outer steel shells of element being fabricated in a shipyard. Then these boxes are towed out like a ship to a site where concrete is filled and completed. (in the right picture) [Osaka South Port (rail & road combined) Tunnel in Japan] (Kobe Port Minatojima Tunnel in Japan)

Kawasaki Port Tunnel in Japan
Osaka South Port Tunnel in Japan

Left: Kawasaki Port Tunnel in Japan. Right: Osaka South Port Tunnel in Japan. The elements are temporarily closed by bulkheads at each end to ensure that they will become afloat after the water has been released and the basin used for the production of the elements will be full. (Photos are reprinted from a book issued by Japan Dredging and Reclamation Eng. Assocation)

The Immersed Tube Tunnel

The immersed tunnel under the Istanbul Strait will be approximately 1.4 kilometres long, including the connections between the immersed and adjacent tunnels. The tunnel will provide a vital link in the two-track rail crossing beneath the Istanbul Strait, and is located between the districts of Eminönü on the European side and Üsküdar on the Asian side of Istanbul. Both rail tracks will run in the same binocular tunnel elements, separated by a central dividing wall.

During the twentieth century, more than one hundred immersed tunnels have been built world-wide for road or rail crossings. Immersed tunnels are constructed as float-in structures and then lowered into a pre-dredged trench and covered (buried). These tunnels have to have enough effective weight to prevent them from ever floating again once they are in position.

Immersed tunnels are usually made up of a number of tunnel elements essentially prefabricated in manageable lengths, each often 100 m long, that are eventually joined up below water to form the final tunnel. They have temporary bulkheads across the ends of each element to allow them to float with the insides dry. Fabrication is either completed in a dry dock, or the elements are launched like a ship and then completed afloat close to their final location. In most cases, the completed tunnel elements are barely capable of staying afloat unaided.

Lowering Element
Element Towed

Completed immersed tube elements produced in a dry dock or a shipyard are then towed out to the site, immersed into a trench and connected to form the final tunnel. Left: The element is towed in a busy port to a place where final fitting out for sinking is made. (Osaka South Port Tunnel in Japan) Right: The element is towed with catamaran placing barge to a sinking location. (Tama River Tunnel in Japan) (Photos are reprinted from a book issued by Japan Dredging and Reclamation Eng. Association.)

Tunnel elements can and have been towed successfully over great distances. After outfitting near to the Istanbul Strait, they will be attached to winches on purpose-built barges capable of lowering the elements into a prepared trench in the bed. They are then given enough weight to lower them and butt them up against the preceding elements, after which the joint between the two is dewatered.

An element on it's way down

The immersion of an element is a time-consuming and critical activity. The picture shows the element on its way down. It is controlled horizontally by anchor and wiring systems, and the winches on the immersion barges control the vertical position until the element is down and rests on the foundation. (Photos are reprinted from a book issued by Japan Dredging and Reclamation Eng. Association.)

The result of the dewatering will be that the water pressure on the other end of the element will compress the rubber gasket and thereby make the joint water tight. Temporary supports will hold the elements in position while the foundation beneath them is completed. The trench is then backfilled and any necessary protection added over the top. Once the end tunnel elements are in position, the space between the ends and the excavated rock will be filled with material that is more or less impermeable to water. The Tunnel Boring Machines (TBMs) will continue drilling towards the immersed tunnels until they have crossed through this material and have reached the immersed tunnel.

Backfill Operation

The tunnel is covered with backfill to secure stability and protection. The picture shows backfill operation by tremie method from a self-propelled grab barge. (Photos are reprinted from a book issued by Japan Dredging and Reclamation Eng. Association.)

Below is a list of the Immersed Tunnels that have been constructed or are under construction. The list shows the status as of 1997.

  No of Tunnels Percentage
Europe 48 44%
North America 27 25%
Japan 20 19%
East Asia (excl. Japan) 9 8%
Others 4 4%
Totals 108 100%

Two Bores

The immersed tunnel under the Bosporus will have two bores, one for train traffic in each direction. The elements will be totally embedded in the seabed and therefore after construction, the seabed profile will be the same as it was before the construction began.

Cross Sections

One of the advantages of the immersed tube tunnel method is that the cross section of the tunnel can be adjusted and optimized to fit the exact needs of each tunnel. On this illustration you can see different cross sections used around the world.

Sandwich Steel Shells
Sandwich Steel Shells

Innovatory technology in element fabrication.

Immersed tunnels have been constructed conventionally as reinforced concrete with or without outside steel shell and act together with reinforced concrete inside. However since the nineties, innovated techniques in using concrete without reinforcement but sandwiched between inner and outer steel shells with ribs acting structurally as full composite members have been used in Japan. This was possible due to the development of splendid quality of self-compacted concrete. This method can remove processing and fabrication of reinforcement bars and formworks and collosion problem in the long-term with adequate cathodic protection for steel shells. Left: This illustration shows sandwich steel shells with diaphragm partitions. Right: Non-shrinkage and self-compactable concrete is being filled from a warf in between the steel diaphragms (Naha Immersed Tube, Japan)