Tunnels are constructed using various methods, with the choice depending on factors such as ground conditions (soft clay vs. hard rock), water presence, tunnel length, diameter, depth, and the final use.
The three basic types of tunnel construction are:
- Bored Tunnels: Constructed in situ (in place) without removing the ground above. This is typically done using a Tunnel Boring Machine (TBM), which excavates the ground and simultaneously installs the precast concrete tunnel segments to form the circular or horseshoe-shaped lining.
- Cut-and-Cover Tunnels: Constructed in a shallow trench that is excavated, built, and then covered over. This is common for shallow subway lines or underpasses.
- Immersed Tube Tunnels: Used to cross a body of water. Precast concrete or steel tunnel sections are fabricated on land, towed to the site, sunk into a dredged trench on the waterbed, and then joined together.
