The different types of containment
Tunnel Boring Machines

Tunnel excavation alters the initial state of stress in the soil or rock along the tunnel route. Tunnel excavation creates an excavation face that advances as the tunnel is dug, and a perimeter excavation wall.

The excavation face and excavation wall are surfaces on which tunnel excavation cancels out normal stresses if no provision is made. These boundary conditions can bring the soil or rock to a state of failure around the tunnel, and external action is then required to avoid reaching this state of failure.

In the case of a tunnel excavated by a TBM, the shield, which can be likened to a cylinder the diameter of the excavated tunnel, can at least provide a "passive" reaction by opposing the movements...