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Construction techniques


As regards the civil works and in particular the methods of tunnel construction, these are now carried out from the viewpoint of minimizing the environmental impact of the excavations. For this reason it is increasingly rare to dig tunnels using the open-cut Milan Method (better known today as cut-and-cover). This method, devised by MM in the second half of the 1950s for the construction of the Line 1 and subsequently adopted worldwide, is now only used in less congested urban areas, almost always on the outskirts of the City.

The worsening problem of urban traffic and the consequent need to limit the impact of new work on highways has led to the need to adopt less invasive techniques, such as blind hole excavation.

Depending on the length of the tunnel, the existence of buildings around the excavation site and varying levels of soil cohesion, blind hole excavations are performed in different ways. The technique of soil consolidation can be employed, with cement mixtures or jet grouting in advance, or mechanized excavation with balanced earth pressure TBMs (Tunnel Boring Machines). For minor construction jobs and certain underpasses, the jacking technique is used.

In special cases, innovative systems have been developed, such as the so-called cellular arch which was used to build the Venezia station on the Milan Underground Urban Rail Link: with this method it is possible to carry out very wide span excavation with reduced overhead cover and the presence of vehicle and tram traffic. The method consists of constructing the entire roof dome before excavating the tunnel.

In the presence of special geological conditions and to prevent landslips, Milan Metro has experimented with the technique of freezing the ground to a temperature of -196° C using liquid nitrogen. These experiments were made both during the construction of the Naples metro system, where the presence in the subsoil of soft rocks such as tuff and pozzolana was detected, and during the construction of Line 3 in Milan due to the presence of clay close to the subsoil water table.