A large proportion of tunnel projects are excavated by a Tunnel Boring Machine or TBM. These machines provide a system of equipment for excavating and supporting the ground at the face of the tunnel, removing and transporting the exavated ground to the surface, and installing any necessary ground support to keep the tunnel in a stable condition. At one time, the term TBM was used for rock tunnels and the term Shield Machine for soft ground tunnels but the term TBM is broadly used today. There is a huge variety of different types of specific equipment to deal with different ground conditions and the ground support installed may only be intended for temporary support prior to final tunnel lining or may be installed as a final lining in one pass. A common form of tunnel lining is a segmental lining in which lining segments are erected within the tail of the TBM to form supporting rings. A TBM pushes itself forward through the ground by gripping the surrounding ground (in rock) or by thrusting against the previously completed tunnel lining.