Editing Tunnelling shield (section)

==Modern tunnel boring machines==
[[File:Tunnnel boring machine case.JPG|thumb|Tunnel boring machine case left in the tunnel and used as part of the support structure]]
{{main|Tunnel boring machine}}

A tunnel boring machine (TBM) consists of a shield (a large metal cylinder) and trailing support mechanisms.

A rotating cutting wheel is located at the front end of the shield. Behind the cutting wheel there is a chamber where the excavated soil is either mixed with [[slurry]] (so-called slurry TBM) or left as-is (earth pressure balance or EPB shield), depending on the type of the TBM. The choice of TBM type depends on the soil conditions. Systems are also present for removal of the soil (or the soil mixed with slurry).

Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which are used to push the TBM forward. Once a certain distance has been excavated (roughly {{convert|1.5|–|2|meters|feet|sigfig=1}}), a new tunnel ring is built using the erector. The erector is a rotating system which picks up [[precast concrete]] segments and places them in the desired position.

Several support mechanisms can be found behind the shield, inside the finished part of the tunnel, which are part of the TBM: dirt removal, slurry pipelines if applicable, control rooms, rails for transport of the precast segments, etc.