What is a TBM and what does a tunnel boring machine do?

A TBM (Tunnel Boring Machine) is a large mechanised system that excavates ground in a circular cross-section while combining digging, support and spoil removal into a single integrated process. Its fundamental difference from classic drill-and-blast is that it builds the tunnel in one continuous operation: as the ground is cut at the front, the tunnel wall behind it is lined with permanent precast concrete rings. In other words, a tunnel boring machine is at once an excavator and a moving assembly factory.

A TBM is far more than the cutter head at the front. Behind the head runs a back-up gantry train that can stretch for tens of metres, housing hydraulic power packs, grouting pumps, conveyor belts, ventilation ducts, electrical cabinets and the operator cabin. Machines are built in sizes ranging from three-metre service tunnels to road and metro tunnels exceeding fifteen metres in diameter.

The machine is selected to match the ground. Soft, water-bearing soils call for an EPB (Earth Pressure Balance) machine; sands and gravels under high water pressure suit a slurry machine; competent rock favours a gripper or double-shield hard-rock TBM. Choosing the right machine is the single most critical engineering decision shaping the project's speed, cost and safety.

How the cutter head works and how the machine advances

The heart of a TBM is its cutter head. In hard rock, steel disc cutters mounted on the rotating head are pressed against the face and fracture the rock as the head turns; this is not blasting but a form of controlled crushing and chipping. In soft ground the discs give way to scraping blades and teeth. The cut material enters the machine through openings in the head and is carried out by a conveyor or a slurry circuit.

Unlike classic tunnelling, the machine does not advance by a motor driving it forward but by the principle of reaction force. Powerful hydraulic thrust jacks in the body push against the concrete segment ring already installed behind, and the reaction they gain there drives the head into the ground. In short, the tunnel boring machine moves by bracing against the very ring it built one step earlier. This elegant loop is the essence of how a TBM works.

When a thrust stroke (typically 1.5 to 2 metres) is complete, the jacks retract, a new segment ring is installed, and the cycle begins again. The operator continuously monitors thrust force, head rotation (torque and RPM), penetration rate and, especially in soft ground, the pressure in the excavation chamber. Balancing these parameters governs both the advance rate and the risk of surface settlement.

Segment installation: how the tunnel lining is built

The permanent lining of a TBM tunnel is made of factory-cast precast concrete segments. Usually five to eight pieces combine to form a complete ring; each segment is a reinforced concrete element cast to precise geometry, its outer arc matching the tunnel diameter. These pieces are installed inside the machine's tail shield, before any ground is left exposed, so the surrounding soil is never unsupported.

Segment installation is carried out by a robotic arm on the machine called the erector, fitted with vacuum or mechanical grips. The erector lifts each segment delivered from the back-up train with millimetric precision, rotates it to the correct angle and bolts it to the previous ring. Gaskets between the segments make the completed ring watertight, which is vital for metro and pipeline tunnels below the groundwater table.

Once the ring is closed, the thrust jacks brace against this new ring to push the machine forward, and the cycle continues. Correct ring building matters enormously: a wrong sequence, missing bolts or damaged gaskets lead to water ingress, ring deformation and long-term maintenance costs. This is why the discipline of the erection crew is as decisive as the technical capacity of the machine itself.

Backfill grouting and waterproofing

Between the outer diameter of the segment ring and the bore the machine has cut, there remains a void as thick as the tail shield, known as the annular gap. Left as is, the surrounding ground would settle onto the ring, triggering both surface subsidence and ring deformation. For this reason the gap is filled with backfill grouting the moment each ring is installed.

Grout is injected through channels in the tail shield or through ports in the segments, pumped under pressure as cement-based mortar simultaneously while the machine advances. The aim is to fill the void completely, seat the ring firmly against the ground and balance the surrounding stresses. Correct pressure and mortar consistency are decisive: insufficient grouting causes surface settlement, while excessive pressure causes ground heave or segment damage.

Waterproofing is a layered defence. First the compressible gaskets between segments stop the water; then the backfill grout closes seepage paths; where needed, secondary contact grouting is applied behind the ring. Because these layers work together, even metro tunnels tens of metres below the water table can stay dry for decades.

Advantages of the TBM: speed, safety and surface protection

The most talked-about strength of the TBM method is speed. Where classic drill-and-blast may advance about one metre per day in good ground, under suitable conditions a TBM can reach ten metres per day and beyond; on favourable projects, several hundred metres a month is routine. In schedule-tight projects such as urban metro lines, this gap translates directly into cost and delivery-time advantages.

The second major advantage is safety. Because excavation happens under the protection of the shield, the risk of collapse is largely eliminated and workers are never exposed to unsupported ground. With no blasting, vibration, dust and gas risks are minimal. As the process is largely mechanised and automated, the share of accidents tied to human error is markedly lower than with classic methods.

The third is surface protection. Thanks to pressure balance in the excavation chamber (EPB or slurry), a TBM can pass beneath dense urban fabric without damaging the buildings, roads and utilities above. Settlement is held to millimetric levels. By contrast, the high upfront cost means a TBM is not always economical for short tunnels or highly variable ground; choosing the method is a feasibility decision.

Common mistakes and engineering risks

The most common mistake is inadequate ground investigation. A TBM is selected for the ground class it is planned for; unexpected fault zones, large boulders, excessive water inflow or pockets of pressurised gas can stall or even trap the machine. On projects with thin borehole data, a jammed machine and months-long rescue operations are among the most expensive scenarios. A sound geotechnical model is the foundation of a successful TBM project.

The second critical area is cutter tool management. Disc cutters wear and must be replaced periodically; this often requires a hyperbaric intervention into a pressurised chamber, which is both hazardous and time-consuming. Continuing to advance with worn discs permanently damages the cutter head and slows progress. Planned management of the maintenance schedule directly affects efficiency.

Other frequent mistakes include incorrect grouting pressure (surface settlement or heave), neglecting tail-seal maintenance (backfill grout escaping into the machine), flawed ring-selection logic (drift from the tunnel axis) and poor monitoring of spoil removal. In EPB machines, letting the volume of material in the chamber go unchecked can cause sudden surface collapse. All of these risks can be managed with an experienced crew and rigorous data monitoring.

KMB's approach to TBM tunnelling

Mechanised tunnelling is a matter of crew and methodology more than of the machine itself. KMB Metro Altyapı draws on more than seventy-five years of accumulated know-how born from the partnership of Troy from Türkiye (since 1996) and Kyivmetrobud from Ukraine (since 1949). Holding experience in both NATM and TBM methods brings the ability to match the right method to the right project, because for some works a TBM is wiser, and for others a classic method is.

The company's field experience on metro and pipeline tunnels is the real-world counterpart of the principles described here: from the Kyiv metro stations to the Dwarka Metro line in New Delhi, from the Novoriski oil-pipeline tunnel in Russia to the Voronezh railway tunnel, works delivered across different grounds and cultures have demanded site-specific solutions in excavation, segment installation and grouting alike. An ISO 9001 quality management system ensures these processes are carried out in a repeatable and auditable way.

In practice the difference usually hides in the detail: a sound ground model, planned cutter maintenance, disciplined ring building and real-time grouting control. On a tunnel project, decisions from method selection to machine sizing shape the cost and schedule of the work for years. TBM tunnelling, therefore, is not about buying a machine but about building the right engineering culture.

Frequently Asked Questions

Below are short, clear answers to the most frequently asked questions about TBM tunnel construction.