Tunnel Boring Machine

TBMs walk themselves forward using thrust cylinders at the rear of the shield. After a concrete segment ring is bolted into place behind the machine, hydraulic rams (typically 16 to 30 of them, each rated 1,500 to 3,000 kN) extend against that ring. The reaction force pushes the cutterhead into the rock or soil face. A typical full thrust is 50,000 to 80,000 kN — enough to lift several aircraft carriers. After advancing one segment length (1.5 to 2 m), the rams retract, the next ring is built, and the cycle repeats. There are no rails on the floor; instead a backup train rolls behind on temporary tracks laid as the tunnel grows.

Three families, each tuned to ground type. Earth Pressure Balance (EPB) is for soft, cohesive soil — clay, silty sand. The cutterhead chamber stays full of muck under controlled pressure, balancing groundwater and earth pressure at the face. A screw conveyor extracts muck. Slurry shield is for permeable, water-bearing ground like coarse sand or gravel below the water table. The face is supported by pressurized bentonite slurry; muck is pumped out as a fluid mixture and separated at the surface. Hard-rock TBMs (gripper or shielded) bite competent rock with disc cutters; they may not need any face support and can use rock anchors instead of segment rings.

The lining is precast concrete, typically 30 to 50 cm thick, in segments that bolt together to form a complete ring. A 6 m diameter tunnel might use 6 + 1 segments per ring (six trapezoidal pieces plus one wedge-shaped key). After each forward stroke, the thrust rams retract one at a time while a vacuum-suction erector arm lifts each precast segment from a feed train, rotates it into position, and bolts it to its neighbors and the previous ring. The annular gap between segment ring and bored hole is filled with grout pumped through tail-shield ports, locking the lining to the surrounding ground. A skilled crew builds a 7-segment ring in 25 to 40 minutes.

Geological surprises are the largest risk in tunneling. Rare boulders in clay can crack disc cutters; a sudden water inrush can flood the cutterhead chamber; karst voids can drop pressure and cause sinkholes above. Mitigations include probe drills (3 to 5 forward boreholes scanning 30 to 50 m ahead), ground-penetrating radar, and the option to switch operating mode (open versus closed face). Bertha in Seattle famously stopped for 2 years (December 2013 to December 2015) after overheating when something — possibly a steel pipe casing or trapped grout — damaged the main bearing seal. Repair required excavating a 36 m deep rescue shaft to access the cutterhead. Cost overrun: roughly $223 million.

Yes, but only along a gentle curve. Minimum radius is roughly 3 to 5 times the tunnel diameter for shielded TBMs and 10-plus for hard-rock gripper machines. Steering is by selectively extending some thrust rams more than others, which yaws the shield toward the desired direction. Articulated middle sections in the shield allow tighter bends. Sharp turns or vertical crossovers are made at hand-mined chambers between TBM drives. The machine cannot reverse — if it must back out, the only path is dismantle, retreat, rebuild.

Three options. (1) Reception shaft: bore into a vertical shaft pre-excavated at the destination, then disassemble the machine on the surface and crane out the parts. Most common for urban subway projects. (2) Bury at the endpoint: cut the cutterhead off, leave the front shield underground encased in concrete, recover only the back-up gear. The Channel Tunnel used this approach; the British TBMs are buried under the seabed near Calais. (3) Daylight: drive into open air at a portal, hand-dismantle outside. TBMs are largely single-use; resale is rare because each machine is custom-sized to its tunnel diameter, with non-trivial refurbishment required to reuse.