The work and the technique: Progetto Terzo Valico dei Giovi Genoa Junction, Italy
Progetto Terzo Valico Junction is a huge work, in terms of both numbers and means of transport, met the challenges brought about by an underground route by using cutting-edge technology and the work of premium engineers and personnel, who were ready to meet the difficulties brought about by the fact that the terrain had different geological characteristics.
In fact, every new excavation requires skills and tools specific to carry out works in the best possible way. Studying the area where works are carried out is fundamental and constant: this way it is possible to make the right decision, kilometre by kilometre, both with regard to the excavation method to use, and the conferment procedure or the reclamation of the excavated material.
The excavation techniques for the Terzo Valico had to consider the variable geological conditions of the areas interested by the route. Two excavation methods were used in the project: the traditional method (the so-called hammer demolition), and the mechanized method, with Tunnel Boring Machines (TBMs).
In the southern part of work’s route, considering the terrain’s articulated geological composition and its poor geo-mechanical features, the traditional excavation is the best method to use for the results it provides. The work carried out by the personnel working in the tunnels is fundamental too: their experience and their developed knowledge of being able to interpret signals from the rock, allows progressively adapting the excavation practices.
If geological conditions allow it, it is possible to take advantage of the excavation technique done with TBMs, which allows having an improved construction speed.
When it comes to the Terzo Valico, the excavation fronts are concomitantly simultaneous, not with regard solely to two route terminal points.
In fact, also, to reduce construction timings, 4 intermediate tunnels were also built that intersect in depth with the main route, allowing excavation works to be carried out from further intermediate points.
The traditional excavation method
60 km of the total 90 km to be excavated within tunnels are carried out with the so-called traditional method: the advancement that is directly experienced by personnel foresees 4 phases carried out on different shifts, without ever stopping.
The first, the so called "consolidation", is carried out only for particularly poor-quality rocks naturally inclined to instability. Hole are drilled into the mass. These holes are then filled with fiberglass nails and also filled with numerous cement injections so as to confer greater stability to the rocks.
The second phase consists in demolishing the rock surface, using demolishing methods with a jackhammer or explosives, advancing a daily 3-4 metres.
During the third phase, pre-covering laying activities are carried out to secure the excavation works, and support the tunnel: centerings are installed, metal profiles supporting the excavation included with the concrete jetting. Finally, during the last phase, the final tunnel lining is carried out, after having laid a waterproof layer, touching the pre-lining made of non-woven fabric and PVC; the final tunnel lining is made with reinforced concrete and will make up the internal ring of the tunnel that will be seen by passengers.
To excavate a tunnel using the traditional method, many teams are required, each specialized in a particular work phase with shifts every 8 hours, allowing works to advance 24/7.
Mechanized excavation
The second excavation technique is instead more modern. It is done with a Tunnel Boring Machine (TBM), that allow excavating and lining the tunnel also placing prefabricate concrete segments that make up the final tunnel lining.
To excavate the Terzo Valico, 5 TBMs were used. With a diameter of 9.73 m, they are 75 to 115 metres long and run at an average daily advancement speed that ranges from 8 to 14 metres per day.
The cutting head is the main part of the TBM. A series of teeth on its surface cut and break the rock, rotating approximately twice every minute.
The excavated material is collected behind the head: all the rocky debris is conveyed into a chamber and through an auger, which is basically a "worm screw", is moved, on a belt system, outside the tunnel.
A back-up part is on the rear of the TBM, where the equipment is located, and the control cabin and a vacuum system are used to install the precast concrete segments that go to make up the tunnel lining.
Inside the TBM, works are also carried out 24/7, with various work teams (each with over 10 people), on 8 hour-shifts.