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.

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]

[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.

I figure once you join them, you could also apply waterproofing to the outside as well, no?