Episode 01 · The built world
How do you build a dry passage beneath a body of water? The answer begins with two very different ways of building a tunnel.
Beneath the water
Explore how tunnels are built beneath the seabed or assembled from sealed sections lowered into the water.
AI-generated illustrative scenes. Simplified visualizations, not documentary footage.
Watch on YouTube ↗ (opens in a new tab)Read the episode summary ↓The key idea
Underwater tunnels are built either through ground beneath the seabed or from sealed sections lowered into a prepared trench. Bored tunnels use a lining and water-management systems; immersed tunnels use temporary bulkheads and sealed joints to keep water out during assembly.
Inside the engineering
A bored tunnel passes through ground beneath the seabed. The Channel Tunnel is an example: its route runs below the seabed, rather than resting directly on it. Tunnel linings and water-management systems help maintain a usable passage.
Large tunnel elements are made on land, temporarily sealed at both ends, floated out and lowered into a prepared trench. The elements are then connected to form a continuous tunnel.
For an immersed-tunnel connection, water is pumped from the sealed space between adjacent end bulkheads. The resulting pressure difference helps press the new element against the previous one and compress the joint seal. The temporary bulkheads are removed after the connection is secured.
From structure to service
Joining the tunnel elements creates a continuous enclosure, but it does not finish the transport system. Femern describes a further installation stage covering tracks, ventilation, cameras, communications and signage. It is useful to separate these jobs: structural construction provides the enclosure; the later systems equip it for its intended use.
The connection sequence also explains why removing the temporary bulkheads comes later. The element must remain sealed while it is positioned and joined. Read Femern’s construction sequence ↗
Bored tunnel construction
Water control begins at the excavation face. In water-bearing ground, a suitable pressurized tunnel-boring machine supports the ground ahead and opposes groundwater pressure as excavation advances. This temporary protection has a different job from the permanent tunnel lining. ITA mechanized tunnelling guidance (PDF)
In a segmentally lined tunnel, precast concrete pieces form the enclosure. Sealing gaskets around the segments are compressed during assembly to close the joints. Their performance depends on water pressure and how accurately the segments fit together. A bored tunnel therefore relies on both the lining and its connections, rather than the surrounding ground alone. ITA segmental lining guidance, chapter 10 (PDF)
The choice depends on the crossing. Ground conditions, construction risk, cost and environmental effects all matter. For the Fehmarnbelt link, the project owner compared both methods and selected immersion partly because the seabed conditions made a bored tunnel more expensive and risky. That is a site-specific decision, not evidence that one method is always better. Read Femern’s planning comparison
Questions, answered
Yes. Bored tunnels are excavated through the ground below the water. Immersed elements remain sealed while they are lowered and connected, so the surrounding body of water does not need to be drained. Sources and further reading
Both approaches exist. Bored tunnels pass through ground below the seabed. Immersed tunnels are assembled from sealed elements lowered into a prepared trench.
Temporary end bulkheads keep the sections sealed. Water is pumped from the space between adjacent ends; the pressure difference helps compress the joint seal. The bulkheads are removed after the connection is secured.
No. The Channel Tunnel was bored through ground beneath the seabed. It is different from the immersed-tube method shown elsewhere in this episode.
Go to the source
Official references for the concepts explained on this page.
How the Channel Tunnel runs beneath the seabed.
Producing, immersing and connecting tunnel elements.
An overview of the immersed-tunnel construction method.
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Keep exploring · Episode 02
Published by MAKESTEAD · Page updated October 4, 2026 · Our sources and editorial approach
