Editing Tunnel (section)

=== Shafts ===
[[File:Mersey Railway Tunnel - ventilation and drainage machinery.png|right|thumb|1886 illustration showing the ventilation and drainage system of the Mersey railway tunnel]]
A temporary access [[Shaft (civil engineering)|shaft]] is sometimes necessary during the excavation of a tunnel. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, TBMs are lowered to the bottom and excavation can start. Shafts are the main entrance in and out of the tunnel until the project is completed. If a tunnel is going to be long, multiple shafts at various locations may be bored so that entrance to the tunnel is closer to the unexcavated area.<ref name="Engineers. 1978">United States Army Corps of Engineers. (1978). Tunnels and shafts in rock. Washington, DC: Department of the Army.</ref>

Once construction is complete, construction access shafts are often used as [[ventilation shaft]]s, and may also be used as emergency exits.