Editing Tunnel boring machine (section)

==== Slurry shield ====
Slurry shield machines can be used in soft ground with high water pressure or where granular ground conditions (sands and gravels) do not allow a plug to form in the screw. The cutter head is filled with pressurised slurry, typically made of [[bentonite]] clay that applies hydrostatic pressure to the face. The slurry mixes with the muck before it is pumped to a slurry separation plant, usually outside the tunnel.

Slurry separation plants use multi-stage filtration systems that separate spoil from slurry to allow reuse. The degree to which slurry can be 'cleaned' depends on the relative particle sizes of the muck. Slurry TBMs are not suitable for silts and clays as the particle sizes of the spoil are less than that of the bentonite. In this case, water is removed from the slurry leaving a clay cake, which may be polluted.

A [[Caisson (engineering)|caisson]] system is sometimes placed at the cutting head to allow workers to operate the machine,<ref name="Descend">{{cite web |author=Walters, D |title=Sydney Airport Link Rail Tunnel Project, Des Walters: Under Pressure Underground |url=http://www.descend.com.au/commercial/projects/2003413191.htm |url-status=dead |archive-url=https://web.archive.org/web/20030924093233/http://www.descend.com.au/commercial/projects/2003413191.htm |archive-date=2003-09-24 |access-date=2008-10-08 |publisher=Descend Underwater Training Centre.}}</ref><ref name="spums">{{cite journal |author1=Bennett, MH |author2=Lehm, J |author3=Barr, P |title=Medical support for the Sydney Airport Link Tunnel project |url=http://archive.rubicon-foundation.org/7673 |url-status=usurped |journal=[[South Pacific Underwater Medicine Society]] |volume=32 |issue=2 |archive-url=https://web.archive.org/web/20081211090605/http://archive.rubicon-foundation.org/7673 |archive-date=December 11, 2008 |access-date=2008-10-08}}</ref> although air pressure may reach elevated levels in the caisson, requiring workers to be medically cleared as "fit to dive" and able to operate pressure locks.<ref name="Descend" /><ref name="spums" />