A Tunnel Boring Machine TBM is a machine which is able to perform a full-face excavation of a tunnel and, simultaneously, it contributes to the ground support of the excavation either  temporarily or definitively.

The TBM’s may be classified as shown in the following figure:

• Moles :

A mole is a type of hard rock tunnel boring machine that has a cutter head capable of excavating the entire face in a single pass. The cutter head is driven by struts that push in a radial direction against the rock of the tunnel gables, advancing sequentially in two phases (ITA).

- Excavation (the struts remain fixed).

- Advance (the struts retract and the assembly advances the excavated distance).

Topo-type tunnel boring machines are used exclusively in specific geological conditions, always in competent hard rock terrain. Gripper TBM video

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• Shields :

The general principle on which a shield is based is that it is a cylindrical piece of steel that is pushed along the axis of the tunnel while the ground is being excavated. The steel piece supports the excavation opening until the lining is installed.

The shields can be differentiated according to the type of excavation element, support and extraction of the material extracted from the face or according to the type of lining of the excavation:

- Open Shield: Natural hold and dry extraction

- Hydroshield

- Earth Pressure Balancing Shield

 

• Double Shields :

The microtunnelling process by means of a TBM, regarless its type, will always start from a launching shaft and will pierced the ground to get an exit shaft or final position where the TBM will be recovered by lifting it by means of cranes from this location.

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The most usual auxiliary rigs pairing the TBMs during the excavation are:

• Control and guidance systems

• Spoils handling and treatment equipment

• Thrust or jacking systems

• Equipment for perforation, lubrication and lining inyection and preparation

• Power supply equipment