The Transbay Tube carrying BART across the bay is immersed tube. The sections were welded together by divers. The sections were filled with water and then pumped out.
replyFehmarnbelt tunnel sections are concrete. I couldn't find how they are connected by concrete would make sense.
A video posted in another thread says the segments are sealed with bulkheads, floated into position, submerged by allowing water into a ballast section, dropped into place , aligned with pins, drawn to the next segment with hydraulic jacks, and sealed to it with rubber gaskets. Then the bulkheads can be removed. The gaskets also allow for some thermal expansion.
replyI'm curious what the lifetime of those gaskets might be and how you might maintain them.
They are GINA gaskets[0], they were supposed to last 120 years[1], but it has recently been shown that they may deteriorate faster than previously expected due to being under constant compression[2][3]
reply[0] https://www.trelleborg.com/en/marine-and-infrastructure/medi...
[1] https://www.trelleborg.com/marine-and-infrastructure/-/media...
[2] https://www.sciencedirect.com/science/article/abs/pii/S08867...
[3] https://dailygalaxy.com/2026/03/rubber-used-in-undersea-tunn...
The "ballast sections" may act as bilges, so that any leaks will accumulate there and can be pumped out. 100% water-tightness is not essential. Occasional re-grouting/caulking of the joints may be good enough.
replyI figure once you join them, you could also apply waterproofing to the outside as well, no?
replyMaybe. What would that look like, adding more gaskets on the outside? That sounds even harder to maintain since the only way to get access is diving.
replySome kind of goop that would get very slowly squeezed into gaps by water pressure ?
replySealing is really not that difficult if you have access to the high pressure side. The hard part is identifying the location of the leak. In sum, this means that they have to absolutely nail it, on the first attempt, for the bottom part that is resting on the sea floor. If they can to that, the rest of the circumference will also be so good they don't have to even think about fixability.
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replyHere’s a fun video about how one sunken tunnel was built in Vancouver back in the 50s. https://youtu.be/A1igKk8eK0M
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