Editing Tunnel boring machine (section)

=== Tunnel-face support methods ===

In hard rock with minimal ground water, the area around the cutter head of a TBM can be unpressurized, as the exposed rock face can support itself.  In weaker soil, or when there is significant ground water, pressure must be applied to the face of the tunnel to prevent collapse and/or the infiltration of ground water into the machine.

==== {{anchor|Earth Pressure Balance Machine}}Earth Pressure Balance ====
[[File:Zürich - Oerlikon - Tunnelbohrmaschine IMG 0002.JPG|thumb|Tunnel boring machine at the site of Weinberg tunnell Altstetten-Zürich-Oerlikon near [[Zürich Oerlikon railway station]]]]
[[File:2014-06-30_7356x4904_chicago_tbm.jpg|thumb|right|Urban installation for an {{convert|84|in|m|order=flip|adj=on|sp=us}} sewer in [[Chicago|Chicago, IL, USA]]]]
[[File:Yucca Mountain TBM at South Portal.jpg|thumb|left|The support structures at the rear of a TBM. This machine was used to excavate the main tunnel of the [[Yucca Mountain nuclear waste repository]] in Nevada.]]

Earth pressure balance (EPB) machines are used in soft ground with less than {{cvt|7|bar|psi}} of pressure. It uses [[Muck (mining)|muck]] to maintain pressure at the tunnel face. The muck (or [[Spoil (archaeology)|spoil]]) is admitted into the TBM via a [[screw conveyor]].  By adjusting the rate of extraction of muck and the advance rate of the TBM, the pressure at the face of the TBM can be controlled without the use of [[slurry]]. Additives such as [[bentonite]], polymers and foam can be injected ahead of the face to stabilize the ground. Such additives can separately be injected in the cutter head and extraction screw to ensure that the muck is sufficiently cohesive to maintain pressure and restrict water flow.

Like some other TBM types, EPB's use thrust cylinders to advance by pushing against concrete segments. The cutter head uses a combination of [[tungsten carbide]] cutting bits, carbide disc cutters, drag picks and/or hard rock disc cutters.

EPB has allowed soft, wet, or unstable ground to be tunneled with a speed and safety not previously possible. The [[Channel Tunnel]], the [[Thames Water Ring Main]], sections of the [[London Underground]], and most new [[rapid transit|metro]] tunnels completed in the last 20 years worldwide were excavated using this method.  EPB has historically competed with the slurry shield method (see below), where the slurry is used to stabilize the tunnel face and transport spoil to the surface.  EPB TBMs are mostly used in finer ground (such as clay) while slurry TBMs are mostly used for coarser ground (such as gravel).<ref>{{cite web|first1=Si|last1=Shen|access-date=2023-10-29|title=TBM selection - EPB and SlurrySi Shen|url=https://www.si-eng.org/post/tbm-selection|date=25 October 2021|website=The Si-Eng}}</ref>

==== 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" />

==== Open face soft ground ====
Open face soft ground TBMs rely on the excavated ground to briefly stand without support. They are suitable for use in ground with a strength of up to about {{cvt|10|MPa|psi}} with low water inflows. They can bore tunnels with cross-section in excess of {{cvt|10|m|ft|sigfig=1}}. A backactor arm or cutter head bore to within {{cvt|150|mm|in|sigfig=1}} of the edge of the shield. After a boring cycle, the shield is jacked forward to begin a new cycle. Ground support is provided by precast concrete, or occasionally [[Ductile iron|spheroidal graphite iron]] (SGI) segments that are bolted or supported until a support ring has been added. The final segment, called the key, is wedge-shaped, and expands the ring until it is tight against the ground.