How can a tunnel be built on land and then sunk to the sea floor?
The world's deepest immersed tunnel links Europe and Asia in Istanbul, and the first of its kind was a sewer pipe in Boston.
βΆ Start the storyYou build it on land and sink it into the sea. An immersed tube is a kind of undersea tunnel made of segments built elsewhere, floated to the tunnel site, sunk into place and then linked together. Each segment is prefabricated in a manageable length, and its ends are sealed with so that it can float, like a giant sealed box. At the same time a trench is on the bottom of the channel and graded to fine tolerances to support the elements.
Once each element has been towed to its place, additional weight is used to sink it into the final location, a critical stage because each piece must be aligned correctly. Then the joint between the new element and the tunnel is emptied of water and made watertight, and the process continues along the tunnel. At the end, the trench is backfilled and protection, such as , is put over the top.
Step 1: Build the elements
Prefabricated segments with sealed ends so they float
Step 2: Dredge a trench
Graded to fine tolerances on the channel bottom
Step 3: Sink each element
Extra weight lowers it, aligned carefully
Step 4: Empty and seal the joint
Made watertight, then repeated along the tunnel
Step 5: Backfill and protect
goes over the top
This sounds strange but it is practical. The main advantage of an immersed tube is that it can be considerably more cost effective than a bored tunnel, which may not be possible due to the geology and activity, or than a bridge. Immersed tubes are commonly used for road and rail crossings of rivers, estuaries and sea channels.
One surprise is how it began. The first immersed tunnel was the Shirley Gut Siphon, a six-foot sewer main laid in Boston in 1893. It was a modest start for a method that now carries trains and cars.
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Recap
Build it on the surface, float it out, sink it into a trench, seal the joints and bury it.
π‘ A trick to remember it Β· Float it, sink it, seal it, bury it: a tunnel in four verbs.
Surprising fact Β· The first immersed tunnel was a six-foot sewer main laid in Boston in 1893.
Connects to
- π How did Brunel's iron shield let miners dig under the Thames when engineers said it was impossible?
- π How did two crews digging from England and France meet under the Channel within 30 centimetres?
- ποΈ How do you dig the world's longest railway tunnel through the Alps?
- Immersed tube
- Marmaray
Sources (1)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.