What is a tunnel segment?

A tunnel segment is a precast, curved element of reinforced concrete that, together with other segments, forms a ring: the lining of a bored tunnel. A tunnel boring machine (TBM) excavates the ground and, behind its shield, builds the lining ring by ring. The German term “Tübbing” is also widely used for these units.

How a ring is built

• A ring consists of several standard segments and one smaller key segment, placed last to lock the ring, much like the keystone of an arch.
• The TBM has an erector, a lifting arm that picks up each segment and presses it into position.
• Rings are usually built with staggered joints, so the longitudinal joints of successive rings do not line up.
• Bolts or dowels hold segments and rings together during construction.
• The annular gap between the ring and the ground is filled with grout as the machine advances. This supports the ring all round and limits settlement at the surface.

Sealing

The joints between segments contain elastomer gaskets, often made of EPDM. When the TBM’s thrust rams press the segments together, the gaskets are compressed and seal the tunnel against water. That matters especially when a tunnel runs below the groundwater level, as is common in the soft soils of the Netherlands.

Application

• Rail, metro and road tunnels built with a tunnel boring machine.
• Utility tunnels for cables, water or sewage with larger diameters.

Points to watch

• Tolerances: segments are cast in precision moulds. An out-of-tolerance unit can lead to leaks or cracking when it is pressed into place.
• Edge damage: edges and corners are vulnerable during transport and erection, and damaged gasket zones can leak.
• Fire safety: road and rail tunnels may need protection against concrete spalling in a fire.

Related terms

• Reinforced concrete
• Grout
• Grout injection
• EPDM
• Groundwater level
• Pulled tunnel

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