Immersed Tube Tunnel
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An immersed tube tunnel is a tunnel constructed directly under a waterway. A trench is dredged and prefabricated steel or concrete elements floated to the tunnel site and sunk into place inside the trench. The water is then pumped out from inside the elements. The elements are then rubber sealed together to ensure a watertight tunnel.
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[edit] History
The construction of immersed tubetunnels is not a new concept, but an application in tunneling design that has been in widespread use for well over 100 years. Today, the number of immersed tube tunnels that have been built worldwide well exceeds 150 with the majority being constructed for road and railways.[1]
About 90 percent of the immersed tube tunnels constructed in the U.S. have been, and are still, fabricated out of steel elements or shells. In contrast, in Europe, approximately 100 percent of immersed tube tunnels have been constructed from concrete.[2]
In the United States, the first immersed tube tunnel to be built was the Shirley Gut siphon in Boston, Massachusetts in 1894. The tunnel measured 259 feet (79 m) long and six feet (1.8 m) in diameter. An immersed tube rail tunnel measuring 2,625 feet (800 m) long with two tracks connecting Ontario to Michigan was then built in 1910. The first immersed tube highway tunnel built in 1928 was the Posey Tunnel connecting Oakland to Alameda. It was one of the world’s first concrete tunnels. In 1930, the Detroit-Windsor tunnel connecting Ontario, Canada to Michigan, USA was the first immersed tube tunnel between two countries. It was also a landmark in immersed tube tunnel design because it used an octagonal double shell cross-section. This shape would become the dominant cross-section shape used steel shell immersed tunnel construction in the U.S. During the 1950s, the construction of several immersed tube tunnels in the U.S. occurred and all of these tunnels were fabricated from steel. One of the longest immersed tube tunnels in the world at the time was the First Hampton Roads Tunnel in Virginia with a length of 6,624 feet (2,019 m).[3]
It wasn’t until the 1960s that immersed tube tunnels fabricated and built from concrete elements really took off in Europe. The first concrete immersed tube tunnel in Europe was the Mass Tunnel built in Rotterdam built in 1941 at 1,936 feet (590 m) long. This tunnel set the precedent in the construction of other immersed tube tunnel built afterwards throughout Europe.[4]
In this timeframe, it also became apparent that immersed tube tunnel development was split in two distinct groups—concrete and steel. The next two decades marked further substantial developments in immersed tube tunnel construction. These included larger cross sections, longer prefabricated concrete or steel elements, and immersed tube tunnels also being built at greater depths.[5]
One of the more recent developments in immersed tube tunnels is the idea of a submerged floating tunnel (SFT) which consists of suspending a tunnel within a waterway by tethering a buoyant tunnel section directly to the bed of the waterway or suspending tunnel section heavier than the water from pontoons or columns. The concept, being proposed by a Norwegian company, is in the exploratory stage of development and combines the technical knowledge of immersed tube tunnel and floating bridge construction.[6]
[edit] Process
There are three distinct stages in the construction of an immersed tube tunnel: dredging, element tunnel construction, and tunnel installation.
An immersed tube tunnel is first prepared by dredging a trench at the bottom of a waterway. Since dredging technology has vastly improved in the last several years, underwater material can be removed with relatively low risk or damage caused to the marine environment. Water depths for immersed tube tunnels may range between 16.4 and 98 feet (5 and 30 m). Some tunnel schemes have even proposed depths of up to 328 feet (100 m).[7] Most immersed tube tunnels are built at a shallower depth, however, as this permits construction of a shorter tunnel with a flatter alignment than a bored tunnel.
The elements used for the actual shell of the tunnel are constructed on dry land, usually a dry dock, casting basin, fabrication yard, or ship-like platform or factory. The elements are an average of 328 feet (100 m) in length and are floated to the tunnel site by barge and with the assistance of a crane, lifted and lowered into place in the bottom of the trench. Temporary bulkheads, installed on the ends of the elements, allow them to be floated with the insides kept dry. Each element is sunken into place and lined up next to the element already placed under the water. Water is pumped out of the space between the bulkheads. Water pressure on the free end of the new element lined to an existing section compresses a rubber seal between the two elements engaging the sealing membranes to lock together. This use of robust rubber seals is highly effective in ensuring a high degree of water tightness within the tunnel, more so than a bored tunnel.[8] Backfill is then placed over the trench to permanently bury the tunnel.
There are many advantages of an immersed tube tunnel, and it exists as a feasible alternative to a bored tunnel and at a comparable price. One benefit is that the cross section of an immersed tube tunnel isn’t restricted to a tunnel’s conventional circular shape but can be square or octagonal, making immersed tube tunnels a highly suitable application for wide road and rail tunnels.[9]
Immersed tube tunnels can also be constructed in almost any ground condition, particularly conditions that are not always favorable for bored tunneling such as in soft ground. Immersed tube tunnels are also better designed to withstand the movement and forces of an earthquake. Overall, construction of immersed tube tunnels poses less risk than a bored tunnel and is just as economically viable particularly in densely populated and congested urban areas.[10]
[edit] Equipment Used
[edit] References
- ↑ Gibraltar Crossing. ITA. 2008-09-09.
- ↑ C.R. Ford Institution of Civil Engineers. Immersed Tunnel Techniques. Thomas Telford: 1990.
- ↑ C.R. Ford Institution of Civil Engineers. Immersed Tunnel Techniques. Thomas Telford: 1990.
- ↑ C.R. Ford Institution of Civil Engineers. Immersed Tunnel Techniques. Thomas Telford: 1990.
- ↑ C.R. Ford Institution of Civil Engineers. Immersed Tunnel Techniques. Thomas Telford: 1990.
- ↑ NSFT. 2008-09-09.
- ↑ ITA. 2008-09-09.
- ↑ ITA. 2008-09-09.
- ↑ ITA. 2008-09-09.
- ↑ ITA. 2008-09-09.
