The first of 90 tunnel elements has been immersed for the Fehmarnbelt Tunnel, an 18 km immersed tunnel connecting Denmark and Germany set to be the world’s longest.
Key Takeaways
At 18 km, Fehmarnbelt will be 3x longer than the current record holder, the 5.8 km Transbay Tube in San Francisco (opened 1974).
90 total immersion operations: 79 Standard Tunnel Elements (217 m each, 33,000 m3 concrete, displacing 75,000 tons of seawater) plus 10 Special Elements and 1 closing joint.
Trench dredging is complete; elements are prefabricated on land, transported via basin to a working port, then towed and lowered 45 m below sea level.
The tunnel carries a 2x2-lane highway and 2x1 railway, designed to shift freight from lorries to rail and complete the Stockholm-Hamburg high-speed corridor.
Three separate civil contracts cover portals/ramps (TPR), dredging/reclamation (TDR), and the immersed tunnel itself (TUX), with RAT Joint Venture (Ramboll, Arup, TEC) advising since 2008.
Hacker News Comment Review
Commenters flagged a real gap: the German landside rail linkup is stuck in bureaucratic limbo, risking a finished tunnel with no domestic connection, echoing the Brenner Base Tunnel bottleneck pattern.
The Transbay Tube comparison sparked technical context – BART’s tube uses steel not concrete for seismic flexibility, sits on sand intentionally, and was built in shipyards; Fehmarnbelt’s concrete design reflects a calmer seabed environment.
The “high-speed Stockholm-Hamburg rail” claim was challenged: Sweden has not committed to building the high-speed segment, so the corridor framing overstates current plans.
Notable Comments
@jedberg: Notes the Transbay Tube has held the world immersed-tunnel length record for 52 years, which puts the scale of Fehmarnbelt in sharp relief.
