alt Hacker NewsThe 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.
Fehmarnbelt 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.
I'm curious what the lifetime of those gaskets might be and how you might maintain them.