Who digs the metro tunnels underground?

Building a metro doesn’t mean ripping the whole road open. Below it, a giant called a tunnel boring machine is at work: a steel earthworm over a hundred metres long, digging soil at one end while weaving the tunnel behind. This film shows the most common branch of the family — the earth pressure balance shield (often used in soft ground; geology, water pressure and ground conditioning jointly guide selection). Crews monitor continuously to keep the impact on the surface small — not “nothing at all”.

Its face is a huge rotating cutterhead studded with blades harder than rock. As it spins, the earth ahead is shaved away layer by layer, like peeling an apple, neat and tidy.

With no legs, how does it move? The trick is digging and pushing at the same time: behind the machine stand dozens of thrust cylinders braced firmly against the finished tunnel rings. One shove — and the reaction pushes the whole machine forward. Cutting and advancing overlap; to build a new ring, the cylinders retract in groups for a brief pause, then off it goes again.

Where does the shaved soil go? Down a screw conveyor that twists it out slowly, like a spiral slide. The machine also keeps the face support matched to the surrounding earth — earth pressure balance: too little support and soil may flow in or the ground settle; too much and the surface may heave. Monitoring and adjustment keep the impact in check — not a born “never collapses”. The pressure here is a linear teaching model with a declared band; beyond it the model stops explaining.

Behind the machine, robotic arms lift precast concrete segments into place: six or seven curved “building blocks” close into a ring, ring after ring, and the tunnel takes shape on the spot. The gap between rings and earth is filled with cement grout at the tail shield as it advances — grouting fills the void and supports the surrounding ground. Separate gaskets seal the segment joints. Filling, sealing, inspection and maintenance work together to limit groundwater ingress.

The machine advances a few metres to a dozen a day (depending on ground and equipment); the work is comparatively quiet and weather-proof, though not silent or impact-free. A metro tunnel kilometres long grows out of the city’s underground, metre by metre.

Detailed controls

1. Name the parts. Find the cutterhead, jacks, screw conveyor and segments in the first chapters; watch the screw twist soil out in chapter four.
2. Play the pressure scales. Drag “advance speed” and “extraction speed”: too little support warns of settling, too much of heave; pull them far enough apart and the reading declares itself outside the teaching model’s band; bring them close and it settles on “support right”.
3. Watch a ring close. Scrub the timeline and see six segments close like a clock face.
4. Stop the grout. Pause the grouting and the brown filling vanishes, showing an unfilled tail void. Actual leakage also depends on segment-joint seals and other conditions.

Guess first: how does a legless machine move forward? Choose, then check — not wheels, not a worm: jacks brace against the finished tunnel and push the machine along.

Brunel’s original shield (1818, inspired by shipworms) was a square iron frame: dig at the front, brick up the lining behind — like a shield holding back the earth while workers laboured safely inside. A modern earth pressure balance machine packs the whole process into one steel shell: dig, convey, assemble, grout — all from an operator’s cabin.

The pressure model reduces face support to the difference between advance and extraction, scaled to a kPa order of magnitude (real values range from tens to hundreds of kPa with depth and soil). A six-segment ring is a common layout (three standard, two adjacent, one key), ring width about 1.5 m; eight metres a day is typical for soft urban ground, with good drives exceeding fifteen.

Earth pressure balance shields often serve soft ground and, with suitable conditioning, some sandy or gravelly soils too. Slurry shields support the face with slurry, while hard-rock machines use cutters suited to rock. Water pressure, permeability, depth and construction conditions also guide selection; one soil name does not uniquely determine a machine. See ITA’s EPB explanation and the segment-gasket explanation. The immersed tube of sea tunnels is a cousin that builds first and sinks instead of drilling; the metro rides on what it digs.