Editing Tunnel boring machine (section)

==== 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.