Here's a similar video for BART's Transbay Tube, which was built in a similar way.[1] The major differences come from building in an earthquake zone. The Transbay Tube is mostly steel, rather than concrete, for flexibility. There are expansion joints. And the Transbay Tube sits on a gravel and sand base rather than hard rock, on purpose.
The Transbay Tube sections were built in the Bethlehem Steel shipyards in San Francisco.
A museum opens this month to commemorate that shipyard. It's in Dogpatch in SF, if you know the area. The shipyard still has a submersible drydock, but it hasn't worked in ten years and will be demolished soon, hopefully before it sinks.
The SF Bay Area once had far more heavy industry than most people realize.
It's not in an earthquake zone, but isn't the Scandinavian continent still rising at a surprisingly fast rate? I wonder if that could affect the engineering of the Fehmarnbelt tunnel, in an "in x years one end of the tunnel will have risen n centimeters compared to the other end" way. It's probably such a small amount it's well within levels where regular maintance will cover it anyway, but I'm still curious.
Today I learned the Transbay Tube is the longest immersed tube in the world. Given that it opened in 1974, it presumably has held that record for 52 years!
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.
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.
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]
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.
The Merced to Bakersfield IOS looks like a bargain on a distance basis. I have no idea of the carbon offset or passenger time saving versus flying of course
Well, if we’re comparing CA infra costs, for a more 1-1 comparison you can look at the $9.7B Los Angeles is spending on building out a long-awaited subway line (phase 1 of 3 opened Friday!) and see how tunneling underwater looks like a bargain in comparison.
The Transbay Tube sections were built in the Bethlehem Steel shipyards in San Francisco. A museum opens this month to commemorate that shipyard. It's in Dogpatch in SF, if you know the area. The shipyard still has a submersible drydock, but it hasn't worked in ten years and will be demolished soon, hopefully before it sinks.
The SF Bay Area once had far more heavy industry than most people realize.
[1] https://www.youtube.com/watch?v=247JT7ctQ_I
[2] https://bethlehemshipyardmuseum.org/
Fehmarnbelt tunnel sections are concrete. I couldn't find how they are connected by concrete would make sense.
I'm curious what the lifetime of those gaskets might be and how you might maintain them.
[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...
8B USD for 11 miles
CACHSR IOS 36B USD for 171 miles.
The Merced to Bakersfield IOS looks like a bargain on a distance basis. I have no idea of the carbon offset or passenger time saving versus flying of course
https://www.latimes.com/california/story/2026-05-07/los-ange...