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What is a tunnel-boring machine?

A tunnel-boring machine (TBM) is a machine that can dig full-face tunnels underground; this means it is done in a single mechanical operation by drilling. They usually weigh over 6,000 tons and span more than 150 meters in length, and they work in a variety of terrains, from sedimentary and volcanic rock to sandy or clay soils. 

Using a tunnel-boring machine requires certain characteristics of both the terrain and the tunnel to be dug. The latter must have large radii of curvature since the machine cannot make sharp turns. This is why the machine is usually quite expensive. Those measuring 4 to 6 meters in diameter can cost up to €11 million, while those measuring 15 meters in diameter can reach €25 million. 

A tunnel-boring machine can also be used to place shoring systems, i.e., temporary containment mechanisms to support excavations and structures in construction projects. 

How does a tunnel-boring machine work?

A tunnel-boring machine usually performs the excavation process with its rotating head that makes a hole in the ground by using a motor with a hydraulic or electric system. The tunnel-boring machine with cutting elements that can have different shapes and be made of different materials. Both depend on the tunnel’s characteristics. 

The tunnel-boring machine’s pushing force is a result of the action of perimeter jacks supported on rings which are, in turn, pushed against the tunnel wall by other jacks. This action gives the machine enough strength and grip for the rotary head to continue fulfilling its drilling function. 

Behind the tunnel-boring machine is a platform on rails called “lagging equipment” or “buck-up,” where other pieces of work equipment are located, such as fans, transformers, and a belt to remove the material being excavated. 

After the excavation is done, the tunnel-boring machine stops, and next is installing the rings in the tunnel structure that will keep it steady. For this end, pieces of prefabricated, reinforced concrete called voussoirs are used. 

What types of tunnel-boring machines are there?

Tunnel-boring machines are classified into two main groups:

  1. Tunneling machine: designed to excavate hard or semi-hard rocks without the need for a support system. Its pushing force is transferred to the rotating cutting head through thrust cylinders. This type of tunnel-boring machine doesn’t have steel cylinders behind the cutting wheel, so it doesn’t perform the function of temporary shoring. 
  2. Shields: designed to excavate in terrain that requires installing support systems. These are usually soft rock soils. The shields have an outer metal chassis that temporarily holds the ground and is positioned from the front to facilitate the assembly of the final shoring that ensures the tunnel’s stability.
    The shields can be: open front, closed front, mechanical closure, pressurized closure with contained air, hydro shields, or earth pressure balance shields (EPB). 

There is a third group of double-shield tunnel-boring machines. This type of tunnel-boring machine can work like a tunneling machine or a shield, depending on the quality of the rocky terrain. This alternative is ideal for stretches of terrain that vary between soil and rock. 

What is the difference between a tunnel-boring machine and a wall chaser?

There are different methods for mechanically excavating tunnels; the most outstanding are tunnel-boring machines and wall chasers. While both fulfill the function of excavation, there are some differences between them, such as:

  • In addition to the rotating head of a tunnel-boring machine, wall chasers also have an articulated arm with friction spikes, and they move on belts.
  • The wall chasers are used to tear off pieces of ground of low and medium resistance, while tunnel-boring machines are also used for high-strength terrain. 
  • In contrast to tunnel-boring machines, wall chasers are used for civil engineering and construction projects whose excavation lengths are no more than 2 kilometers, while tunnel-boring machines can execute projects of greater length. 

What requirements should a tunnel-boring machine operator meet?

For handling and using a tunnel-boring machine, operators must meet certain requirements and have specific knowledge, such as:

  • Use of safety equipment at the construction site.
  • Transporting materials to be used on the work. 
  • Use of drilling machines. 
  • Caring for infrastructure for services when putting the machine into operation, preventing any damage. 
  • Ergonomic work. 
  • Using protective gloves, helmets, safety shoes, signaling clothing, and a harness.
  • Knowledge of the relevant procedures to react in emergency situations. 
  • Respect for health and safety protocols in construction.
  • Use of hearing protection, whether that’s earplugs or headphones.

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