Editing Tunnel boring machine (section)

=== Shield types ===
Depending on the stability of the local geology, the newly formed walls of the tunnel often need to be supported immediately after being dug to avoid collapse, before any permanent support or lining has been constructed.  Many TBMs are equipped with one or more cylindrical '''shields''' following behind the cutter head to support the walls until permanent tunnel support is constructed further along the machine.  The stability of the walls also influences the method by which the TBM anchors itself in place so that it can apply force to the cutting head. This in turn determines whether the machine can bore and advance simultaneously, or whether these are done in alternating modes.

==== Open/Gripper ====
Gripper TBMs are used in rock tunnels. They forgo the use of a shield and instead push directly against the unreinforced sides of the tunnel.{{Sfn|Potter|2023}}

Machines such as a Wirth machine can be moved only while ungripped. Other machines can move continuously. At the end of a Wirth boring cycle, legs drop to the ground, the grippers are retracted, and the machine advances. The grippers then reengage and the rear legs lift for the next cycle.

==== Single shield ====

A single-shield TBM has a single cylindrical shield after the cutting head.  A permanent concrete lining is constructed immediately after the shield, and the TBM pushes off the lining to apply force to the cutter head.  Because this pushing cannot be done while a next ring of lining is being constructed, the single-shield TBM operates in alternating cutting and lining modes.

==== Double shield ====

Double Shield (or telescopic shield) TBMs have a leading shield that advances with the cutting head and a trailing shield that acts as a gripper.  The two shields can move axially relative to each other (i.e., telescopically) over a limited distance.  The gripper shield anchors the TBM so that pressure can be applied to the cutter head while simultaneously the concrete lining is being constructed.