Editing Tunnel boring machine (section)

==== {{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>