To dig the tunnels, the TBM (Tunnel Boring Machine) was used, consisting of a cutter head of 6.70 meters in diameter and a cover 10 meters long, in addition to a backup of about 90 meters which contains the installations (driving cabin, wiring installations, conveyor belt).
The cutter excavates the soil, which is mixed with various foams and kept under pressure to ensure the stability of the excavation face; subsequently it is channeled towards the rear of the tunnel and removed via a conveyor belt.
When progress has been made, the workers proceed directly with the implementation of the final lining of the tunnel: a robotic arm applies the precast segments of concrete on the walls of the excavation. Each segment is arc-shaped and is connected with the others by means of special screw anchors, so as to form a ring - which constitutes the tunnel lining. After the passage of the TBM, then, the tunnel is substantially finished and ready to be fitted with the tracks and the technological systems.
The tunnels, located at 25-30 meters deep, are connected with the surface only in correspondance with stations and ventilation shafts.
The TBM by the numbers
- 90 meters in length
- 6.70 meters in diameter at the shield cutterhead
- 8-12 meters is the average daily speed of advancement, with peaks of 30 meters
- 25-30 meters the depth of the tunnel excavation, below the archaeological layer (6-12 meters)
- 4 teams of 12 people (engineers and workers) for each TBM
- 11 employees working on the external piazza