Tunnel boring machine

A tunnel boring machine excavates a circular tunnel through soil or rock using a rotating cutterhead. Many machines also support the ground and install a lining as excavation advances.

Cutterhead: The rotating front assembly of a tunnel boring machine that breaks or cuts the ground. Its tools and shape determine how the machine excavates a particular ground type.
Gotthard Base Tunnel: A 57-kilometer railway tunnel through the Swiss Alps, opened in 2016. Its construction used large TBMs alongside conventional excavation methods.
Drill and blast: A tunneling method that fractures rock with explosives after drilling blast holes. Unlike a TBM’s continuous cutting, it excavates in discrete drilling and blasting cycles.
Tunnel boring machine performance: The rate and reliability with which a TBM excavates and supports a tunnel under given conditions. Advance rate and downtime strongly influence a project’s schedule and cost.
Tunnel boring machine shield: A protective cylindrical structure that surrounds a TBM’s excavation and work areas. The shield supports unstable ground while the machine and lining advance.
Channel Tunnel: A rail tunnel beneath the English Channel connecting the United Kingdom and France. TBMs excavated its twin rail tunnels and service tunnel beneath the seabed.
Sequential excavation method: A tunneling approach that excavates in stages and uses the ground itself as part of the support system. Its staged adaptation contrasts with a TBM’s fixed diameter and continuous advance.
Ground settlement: The downward movement of the ground surface caused by changes in soil or underground support. Poor face-pressure or excavation control can damage buildings above a TBM tunnel.
Tunnel lining: A structural layer installed along a tunnel’s interior to stabilize it and resist ground and water loads. Many TBMs erect lining segments immediately behind the shield.
Crossrail: A major railway project in London that created the Elizabeth line and new underground sections. Its urban tunnels illustrate TBM construction beneath a dense city.

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