It was long awaited, and now it’s reality: Webuild has completed the construction of the M4 line in Milanwhich connects the city from west to east in just 30 minutes and the center with Linate Airport in just 12 minutes. We were lucky enough to see behind the scenes of its construction, and we want to tell you about it technical challenges and in particular understand how was it possible to dig under the city, without damaging the buildings on the surface.
Consider that to prevent them from being ruined during the excavations, some famous monuments were even dismantled, made safe and then relocated once the excavation work was completed.

Some (surprising) data on the new blue line
As of July 2023, a section of the M4 was already active, from Linate to San Babila, which connects Linate airport with the city center in just 12 minutes.
It has now been completedentire linewhich crosses the city from west (San Cristoforo terminus) a east (Terminus Linate).
The new metro line therefore guarantees a fast and direct connection, also thanks to the train frequency: during rush hour one will pass every 90 seconds, for a total of 40 trains in operation (plus 7 in reserve). The optimized times are possible thanks to the guide driverlessi.e. autonomous driving (without a driver), which reduces downtime to a minimum, making traffic as efficient as possible.
The machinery used to dig the tunnels of the M4 Line
The M4 line passes right in the centre, under historic buildings, churches and works of art of inestimable value: for this very reason it was dig deeper than that standardi.e. approximately 20-25 meters (30 meters at Dateo station) below the road surface.
They were used to build the tunnels 6 mechanical molesalso known as Tunnel Boring Machines (TBMs), which are capable of digging and simultaneously lining the tunnel for approximately 20 meters per day safely. Just think that to make these prodigious machines that you can see in the video work (the construction of which lasts around 10 months) they work 24 hours a day, thanks to teams made up of around 80 professionals. If you are curious to see the head of one of the TBMs that dug this line, at the National Museum of Science and Technology in Milan you will find one on display, called Stefania. Yes, all TBMs have female names!

How the monuments on the surface were preserved during excavations
But when you dig underground in a city, above, on the surface, there is no risk of damage i palaces, historic buildings, monuments?
Well no! This is because the excavations were carried out by generating a very vibration level Bass And in total safety precisely thanks to the technological excellence of these machines and the feasibility studies carried out during the design phase. Furthermore, the Consortium of builders has structured a technical collaboration with the Superintendency, and for each site affected by the passage of the TBMs geometric and structural data were collectedso as to identify any critical issues in advance and foresee all the necessary consolidation actions for the monuments.
Just think that in some cases it was even decided to reduce the risks to zero move some monuments (Colonna del Verziere with the Statue of the Redeemer in Largo Augusto, the Bust of Cesare Correnti in Piazza della Resistenza Partigiana and the medieval wall in via Francesco Sforza), to restore them and return them to their place once the work is completed.

But that’s not all! Another very interesting aspect is the method used to dig some sections, which is the liquid nitrogen method. If you dig soil that is not hard and consistent (but loose and saturated with water) the risk is that the excavation face will come down, collapsing; but if instead before digging ground water freezes contained in the spaces between the granules of loose soil through the passage of liquid nitrogen at -196 degrees Celsius in pipes inserted into the ground, which becomes more consistent and compact. This way, you can dig better and more safely. In practice the method of artificial freezing of soils it is a temporary waterproofing and consolidation technique necessary to carry out excavations in the ground, avoiding possible collapse of the walls during excavations.
This excavation method, however, is not new: it has been first used in second half of the 19th century. In Italy it was put into practice for the first time in 1937 in Rome and then spread almost everywhere starting from the 1950s.
The M4 line turns greenAnd
During the works there was also a lot of attention to environmental aspects. For example, to avoid the use of trucks in the city center to take away excavation material, a complex system of underground conveyor beltswho from the center brought the material further to the periphery. By doing so they were avoided approx 75 thousand truck trips in full historic center and therefore there was less impact on city traffic. Furthermore, it is estimated that this subway line it will transport 86 million people every yearwhich translated means around 180 thousand fewer car trips, and therefore approx 75 thousand tons of CO2 less emitted into the atmosphere every year.
And finally, here’s the greenest point: the project includes the planting of 1900 new trees along the line, 246,000 m2 of redeveloped urban areas and approximately 66,000 m2 of green areas and those who live in the city center know well how important the presence of greenery is.