Tunnel Boring Machines

A tunnel boring machine, or TBM, is a large mechanised system that excavates a tunnel with a full-face rotating cutterhead, removes the spoil and, in most cases, installs the permanent lining as it advances. It turns tunnelling into a continuous, factory-like process: the machine cuts, pushes itself forward, builds a ring of lining, then cuts again, often for many kilometres without stopping for anything other than maintenance and cutter changes. TBMs are used for metro and railway tunnels, road tunnels, water and sewer mains, hydropower schemes and utility tunnels under cities and rivers.

Compared with drill and blast or roadheader excavation, a TBM needs a large investment and a long lead time, and it is designed for one diameter and a known range of ground conditions. In return it offers high advance rates, a smooth circular profile, less disturbance to the surrounding ground and much better control of settlement at the surface. For long tunnels and for work under buildings, rivers or the sea, it is often the only practical option.

Brief History of Tunnel Boring Machines

The idea of a protective shield goes back to Marc Isambard Brunel, who patented a tunnelling shield in the 1810s and used one to build the Thames Tunnel in London, begun in 1825. Later in the nineteenth century engineers such as James Henry Greathead refined the circular shield and combined it with cast iron segments and compressed air, the basic ingredients of soft ground tunnelling for the next century. Several attempts at mechanical rock boring machines were also made in the nineteenth century, including machines tested for early Channel Tunnel schemes, but none became established.

The modern TBM is usually traced to 1952, when James Robbins built a machine for the Oahe Dam project in South Dakota, USA, to bore through weak shale. In the following years disc cutters, which crush rock under very high thrust rather than scraping it, became the standard tool for hard rock, and the main beam and double shield designs followed. In soft ground the key advances came from Japan and Germany from the 1970s onwards: slurry shields that support the face with pressurised bentonite suspension, and earth pressure balance (EPB) machines that use the excavated soil itself as a support medium. These developments made it possible to tunnel safely through water-bearing ground beneath dense urban areas, and today TBMs are the normal choice for most large metro and rail tunnels.

How a Tunnel Boring Machine Works

A TBM is a long assembly, often more than 100 metres including its trailing gear, divided into the shield or main body at the front and the backup system behind.

Cutterhead. A rotating steel wheel at the front, fitted with disc cutters for rock, scrapers and knives for soft ground, or a mixture of both. It is driven by electric or hydraulic motors through a large main bearing.
Shield. A steel cylinder slightly larger than the machine body that supports the ground around the machine and protects the crew and equipment while the lining is built.
Thrust and gripping. In shielded machines, hydraulic thrust cylinders push against the last ring of lining to move the machine forward. Open gripper machines instead press large shoes against the tunnel wall and push off them.
Face support. Slurry and EPB machines keep a controlled pressure at the face so the ground does not collapse or flow into the chamber, which limits settlement at the surface.
Muck removal. Spoil is carried away by belt conveyors, by a screw conveyor feeding a belt in EPB machines, by slurry pipes to a separation plant on the surface, or by rail wagons.
Segment erector. A rotating arm that places precast concrete segments to form each ring of the permanent lining, which is then grouted behind to fill the gap to the ground.
Backup system. Gantries carrying transformers, hydraulic power packs, grout plant, ventilation, segment handling, the control cabin and welfare facilities.
Guidance. A laser-based guidance system tracks the position of the machine against the design alignment, and the operator steers by varying the thrust on different cylinders.

Types of Tunnel Boring Machines

Main Beam (Open Gripper) TBMs

Machines for competent hard rock that stands up without immediate support. Grippers hold the machine in place while the cutterhead is pushed forward. Rock support such as rock bolts, mesh and sprayed concrete is installed behind the cutterhead as needed.

Single Shield TBMs

Machines with a full shield that push off the lining segments. They are used in rock that needs immediate support or in mixed conditions, where gripping the tunnel wall is not reliable.

Double Shield TBMs

Machines with a front shield and a gripper shield that can work in two modes: gripping the wall and boring while segments are installed at the same time, or pushing off the segments like a single shield in poor rock. They are used on long rock tunnels where high advance rates matter.

Earth Pressure Balance (EPB) Machines

Soft ground machines that fill the excavation chamber with conditioned soil and control the pressure by adjusting the rate at which a screw conveyor removes it. They are widely used for metro tunnels in clays, silts and mixed soils.

Slurry Shields and Mixshields

Machines that support the face with a pressurised bentonite suspension, which also carries the spoil to the surface through pipes. They suit permeable, water-bearing ground such as sands and gravels, and river or sea crossings. Mixshield is a name used by Herrenknecht for a slurry machine that can be adapted to different ground.

Variable Density and Multi-Mode Machines

Machines designed to switch between modes, for example EPB and open mode or EPB and slurry, where the ground along a single drive changes significantly.

Special Shapes

Rectangular, double-circular and other non-circular machines have been built for pedestrian subways, road underpasses and shallow tunnels where a circular section wastes space.

Size Classes

Class	Typical excavation diameter	Typical use
Small	about 2 to 4 m	Sewers, water mains, utility and cable tunnels
Medium	about 5 to 8 m	Single-track metro and rail tunnels, water transfer
Large	about 9 to 13 m	Double-track rail tunnels, road tunnels
Very large	about 14 m and more	Multi-lane road tunnels, combined road and rail tunnels

Figures are indicative. Machines below about 2 to 3 m are usually microtunnelling machines rather than conventional TBMs.

Main Manufacturers

Herrenknecht, the German manufacturer active in all diameters and ground types.
Robbins, the American company that built the first modern TBM, known for hard rock machines.
China Railway Construction Heavy Industry (CRCHI) and China Railway Engineering Equipment Group (CREG), which supply many machines in China and increasingly abroad.
Japanese manufacturers, with a long tradition in soft ground shields, EPB machines and special shapes.

Machines are often designed for a specific project and many are refurbished and reused on later drives.

Main Uses of Tunnel Boring Machines

Metro, light rail and mainline railway tunnels.
Road tunnels, including large diameter urban motorway tunnels.
Water supply, stormwater storage and sewer tunnels.
Hydropower headrace and tailrace tunnels.
Utility, cable and district heating tunnels.
Long access and conveyor tunnels in mining.

Planning a TBM Tunnel

Ground investigation first. The choice between hard rock, EPB, slurry or multi-mode machines depends on geology and groundwater. Gaps in the site investigation are a common cause of delays.
Length matters. A TBM carries high fixed costs for design, manufacture, transport and assembly. It is usually most economical on long drives, while short tunnels may suit drill and blast or roadheaders.
Launch and reception shafts. The machine needs space for assembly, a launch structure and an area at the end to be recovered or buried.
Logistics. Segment supply, spoil disposal, slurry treatment, power supply and ventilation must keep up with the machine, or the logistics will limit the advance rate.
Settlement control. In urban work, face pressure, grouting and monitoring of buildings and utilities above the tunnel are essential.
Cutter and tool wear. Abrasive ground increases the number of interventions to change cutters, which can be slow and hazardous under pressure.

Safety

The main hazards in TBM tunnelling include working in confined space, fire, ventilation failure, heavy lifting of segments, moving rail traffic in the tunnel and contact with rotating parts. Interventions into the excavation chamber to inspect or change tools are among the riskiest tasks, especially when they must be carried out in compressed air, which requires trained workers and strict decompression procedures. Refuge chambers, gas monitoring, fire suppression, emergency lighting and clear communication with the surface are standard on modern machines. Tool changing systems that allow cutters to be replaced from inside the cutterhead arms, without entering the pressurised chamber, reduce this exposure.

Trends and Technology

TBMs are getting larger and more versatile, with more machines designed to switch between operating modes along a single drive. Real-time data from sensors on the cutterhead, thrust system and backup is analysed to adjust machine parameters and predict maintenance. Automated segment erectors, cutter wear monitoring and robotic tool changing are being developed to reduce manual work at the face. Contractors are also paying more attention to energy use, low carbon segment concrete and the reuse of excavated material.

Key Terms

Cutterhead: the rotating front wheel that carries the cutting tools.
Disc cutter: a hardened steel disc that crushes rock under high thrust.
EPB: earth pressure balance, a method of supporting the face with conditioned excavated soil.
Segment: a precast concrete element forming part of a lining ring.
Backup: the trailing gantries that carry the machine’s services and logistics.
Intervention: entry into the excavation chamber to inspect or change tools.

Frequently Asked Questions

How fast does a tunnel boring machine advance? It depends on ground, diameter and logistics. Rates of a few metres to a few tens of metres per day are common, and faster rates are possible in favourable rock with good logistics.

What happens to a TBM at the end of the tunnel? It is usually dismantled and removed through a reception shaft, then refurbished for another project. Where removal is impractical, parts may be left in the ground.

Why do TBMs have names? It is a tradition in many countries to give each machine a name, often a woman’s name, before it starts boring.

Can one TBM handle both rock and soil? Some machines are designed for mixed ground and can switch modes, but every TBM is optimised for a particular range of conditions.