Tunnel boring machines

A tunnel boring machine excavates a circular tunnel with a rotating cutterhead and, in most modern urban tunnels, builds the permanent lining as it goes. Its shield, a steel cylinder, supports the ground around the machine; hydraulic jacks push against the last lining ring to drive the machine forward; inside the shield, an erector places precast concrete segments to form the next ring.

The type of machine is chosen from the ground. Open gripper machines suit strong rock that stands unsupported. In soft ground and below the water table, closed-face machines support the excavated face. Earth pressure balance machines hold the spoil in a pressurised chamber and remove it with a screw conveyor at a controlled rate; they suit clays and silts. Slurry machines support the face with pressurised bentonite slurry and pump the spoil out; they suit sands and gravels with high water pressure. Mixed-mode machines can switch between modes.

Face pressure must be controlled carefully: too low and the ground moves into the face, causing settlement at the surface; too high and slurry or ground may blow out. The gap between the segments and the excavated ground is filled with grout injected through the tail of the shield to limit settlement. Surface settlement is monitored over buildings and services.

Metro tunnels in Indian cities, for example, are commonly driven by earth pressure balance machines. Progress is set by the cycle of boring one ring length and building the ring, plus maintenance and cutter changes.