How Tunnel Boring Machines Build Subways Beneath Cities

Tunnel boring machine (TBM) technology is transforming how modern subways and tunnels are constructed beneath dense urban environments. Using massive underground machines, cities can excavate deep below the surface without having to open up entire streets, as the TBM moves through soil, excavates material, shields the work area against collapse, and forms the tunnel walls all at once.

What Is a Tunnel Boring Machine and Why Is It Called a Shield?

A tunnel boring machine is a specialized mechanized system for excavating tunnels through clay, sand, water-saturated soils, and hard rock. The TBM's design is tailored to the geology of each project: a machine for urban soft soils differs greatly from one used to bore through solid rock.

The unique feature of a TBM is its ability to perform multiple operations simultaneously: it crushes soil at the front, removes spoil from the work area, advances forward, and installs permanent lining elements as it goes. This means the process is much more than simply digging a hole underground.

The term "shield" comes from the massive cylindrical shell that houses the equipment and crew, temporarily separating the tunnel's interior from surrounding ground. Until the permanent lining is installed, the shield body itself prevents collapse and protects the construction site.

TBMs vary in diameter depending on the tunnel's intended use-subways require machines sized for future rail tunnels, while highway projects use even larger TBMs. Smaller versions are used for sewer lines, water pipes, and utility conduits.

The visible "shield" is only the front section of a much longer system, which may include conveyor belts, transformers, pumps, ventilation, and equipment for delivering concrete segments. The full TBM often stretches far behind the cutting head, making it a moving factory rather than a simple drilling machine.

Key Components of a Tunnel Boring Machine

Rotating Cutterhead and Cutting Tools

The front of the TBM features a giant rotating cutterhead outfitted with cutting elements that break up the soil or rock. The cutterhead's design depends on ground conditions: blades and scrapers are used for soft soils, while disc cutters tackle hard rock. The cutterhead rotates slowly but with massive torque, ensuring a steady excavation rate across the face of the tunnel.

Openings in the cutterhead allow spoil to enter the machine for removal, keeping the operation continuous and efficient.

Working Chamber and Face Stabilization Systems

One of the TBM's most important jobs is to prevent ground collapse at the face. This is especially critical for urban tunneling, where buildings, roads, and utilities lie above. Between the cutterhead and the main TBM body is a working chamber, where pressure is carefully controlled to balance the pressure of the surrounding soil and groundwater.

In EPB (Earth Pressure Balance) TBMs, the excavated soil itself is used to stabilize the face, with its amount and pressure precisely regulated. For waterlogged or loose soils, slurry TBMs use a special suspension to support the face, and all parameters are monitored by sensors to maintain safety.

Conveyors, Screw Transporters, and Spoil Removal

Excavated material must be removed continuously. In EPB machines, large screw conveyors move soil from the pressurized chamber to the rear of the TBM. From there, the spoil travels by conveyor belt or wagons to a shaft for removal to the surface. Slurry TBMs mix soil with fluid and pump the mixture to surface separation plants, where solids are removed and fluids recycled.

The rate of spoil removal must match the excavation rate to maintain pressure balance and safe operations.

Hydraulic Jacks and Tunnel Lining Installation

TBMs don't use wheels to move; instead, they advance by pushing off the already installed tunnel lining using powerful hydraulic jacks arranged around the shield. When a new section is needed, the TBM stops, some jacks retract, and a ring of precast concrete segments is installed by a robotic erector. Once the new ring is in place, the jacks gain a fresh surface to push against, and the cycle repeats.

How the TBM Works: Step by Step

1. Excavation: The cutterhead rotates, breaking up soil and rock. Operators monitor and adjust torque, chamber pressure, and spoil removal to match changing ground conditions.
2. Advance: Hydraulic jacks press against the last installed ring, pushing the TBM forward by the width of one concrete segment ring.
3. Lining Installation: Automated manipulators place precast concrete segments to form a new ring, with seals between segments protecting against water ingress.
4. Grouting: Any gap between the lining and the surrounding ground is filled with grout to stabilize the ring and prevent voids.
5. Cycle Repeats: The process repeats hundreds or thousands of times, forming the tunnel as the TBM advances.

The speed of construction depends on tunnel diameter, soil type, TBM design, and worksite logistics-not just the cutterhead's rotation speed. Segment transport, spoil removal, maintenance, and quality control all impact productivity.

How TBMs Build Metro Tunnels Under Cities

Subway construction beneath dense urban areas starts long before a TBM begins its journey. Engineers analyze geology, groundwater, building foundations, and existing infrastructure to determine tunnel depth, route, and machine type. A launch shaft is prepared, where the TBM is assembled underground before it sets off toward the next station or retrieval shaft.

Mechanized tunneling enables long tunnel stretches to be built under active streets and buildings without disrupting life above. This technology has allowed cities to expand their underground infrastructure for subways, highways, and utilities.

Learn more about the future of underground cities in our detailed article: Explore how megacities are moving underground.

How a TBM Navigates Below Buildings and Roads

The biggest challenge of urban tunneling is maintaining the stability of the ground around the tunnel. Even minor ground movement at depth can cause surface settlement, so TBM parameters are constantly adjusted.

Pressure in the working chamber is set to balance soil and water pressure ahead of the cutterhead. The volume of removed spoil is carefully matched to the excavated tunnel space to avoid voids that could cause ground deformation. After the TBM passes, grout is injected behind the lining to transfer ground loads to the tunnel structure and minimize settlement. Sensitive areas, such as beneath old buildings or infrastructure, are closely monitored both inside and outside the tunnel.

How a TBM Stays on Course

Operators cannot see where the TBM is heading underground. The tunnel route is predetermined, and the machine's position is tracked by geodetic systems and reference points inside the tunnel. Sensors continually measure how much the actual path deviates from the design, and the hydraulic jacks are controlled to gently steer the machine as needed. Due to the TBM's length, curves in the tunnel are designed with generous radii to allow gradual turns.

Monitoring the Surface Above the TBM

Throughout tunneling, engineers monitor buildings, roads, and other structures above the TBM using survey markers and settlement sensors, often capable of detecting movements of just millimeters. If settlement exceeds safe limits, TBM parameters-face pressure, advance rate, spoil removal, or grouting-are adjusted accordingly. The tunnel itself is also monitored for alignment and integrity, with modern TBMs collecting data on machinery status and environmental conditions in real time.

As a result, tunneling under city life is much less dramatic than one might imagine: above ground, the city continues as normal, while far below, the TBM steadily advances with engineers vigilantly monitoring every parameter.

What Happens to the Ground and Tunnel After the TBM Passes?

The TBM leaves behind a fully formed tunnel lined with concrete segments. The excavated soil, often thousands of cubic meters per section, is removed to the surface and either reused or disposed of as required. In slurry TBMs, spoil is separated from slurry at the surface, with fluids cleaned and recycled.

The space between the tunnel lining and surrounding ground is filled with grout to lock the ring in place and distribute ground pressure evenly. Seals between the precast segments prevent groundwater ingress, especially below the water table.

What Remains After the TBM Completes Its Job?

When tunneling is finished, the TBM arrives at a retrieval shaft. Sometimes the entire machine is dismantled and reused on other projects; in other cases, parts may be left underground if removal is impractical. The tunnel, now a continuous reinforced concrete tube, is ready for the next stage of construction: installing tracks, cables, utilities, and safety systems before the subway line opens for service.

Conclusion

The tunnel boring machine is more than just a giant drill-it is a fully integrated underground construction system. While the cutterhead excavates, the TBM simultaneously maintains face stability, removes spoil, advances, and assembles the tunnel lining. This technology enables subways and tunnels to be built beneath cities with minimal surface disruption, thanks to precise control over pressure, spoil removal, machine alignment, and surface monitoring.

After the TBM passes, a reinforced tunnel shell remains, ready for further outfitting. The core mission of the TBM is to safely create the underground space and structural support that will become the city's next subway tunnel.