TBM – Tunnel Boring Machine

It is a large full section mechanical cutter, commonly called Mole or Mechanical Mole, used for excavating tunnels.

The construction of the tunnel is completely mechanized, in fact it allows at the same time the excavation of the entire section of the tunnel, the recovery of the excavated materials and the laying of the segments, which will form the lining of the tunnel itself, giving it a self-supporting structure.

More about TBM

Structure and operation

The full section mechanical cutter consists of a metal cylinder called shield, at the end of which there is a rotating head that normally rotates between 1 and 10 revolutions per minute and on which the actual excavation tools are mounted (rotating tips or cutting discs), which can be of different sizes and different construction materials. The speed of rotation, the speed of advancement and the manufacturing of the tools depend on the dimensions of the TBM and on the compactness, hardness and density of the material to be broken up and removed. Behind the rotating head there is a chamber where all the excavated material, through a screw, is collected and deposited on a conveyor belt to be taken outside the tunnel.

The advancement of the TBM is guaranteed by powerful hydraulic jacks which, aiming at the last segments installed, push the cutter towards the excavation face. All the services pass inside the shield, including energy and pipelines for the mixing sludge.

Furthermore, the TBM is assembled by all the systems that are needed for its operation:

the control chamber, the mechanized system for positioning the prefabricated segments and the hyperbaric chamber.

Useful tips

Choice and advantages of the TBM

The costs of these industrial machines are very high, so the choice of the TBM model (dimensions and tools, etc.) is important for optimizing costs and obtaining performance that allows good productivity; fundamental importance is given by the advance design studies and geological prospecting.

TBMs have revolutionized the world of construction of large infrastructures, replacing the old excavation techniques that remain used for excavating small tunnels or where TBMs cannot be used.

The speed of realization and the quality of the industrial product are the strengths of this system, the timing of the work and the costs can be calculated exactly.


In recent years, the mechanized excavation technique is experiencing an enormous development, this technique is divided into four major applications:

  • TBM (Tunnel Boring Machine), for the excavation and construction of tunnels with a diameter greater than 3000 mm.
  • MICROTUNNELING, for the excavation and construction of tunnels with a diameter of less than 3000 mm.
  • HDD (horizontal directional drilling) for horizontal drilling, used for positioning steel or polyethylene pipes.
    In excavations in general, but especially in urban environments or in the presence of aquifers, the main problem is represented by subsidence of the surface above the excavation and it becomes essential not to modify the tensional state of the above ground and any subsidence phenomena, which would cause a significant lowering of the surface. There are TBM models, called EPB (Earth Pressure Balance) and BS (Bentonite Slurry) specially designed to address these issues, where the balance of the excavated material is maintained by injecting pressure into the excavation face. These TBMs are equipped with a Hyperbaric Chamber which allows the workers to reach the pressure front.

© Copyright - Hyperbaric Work Services - Centro Iperbarico Romano - Hyperbaric Medicine - Tunneling