The work and the technique: Milan's M4 Metro Line, Italy
Milan's M4 line joins the two terminuses of Linate and San Cristoforo, passing through the city center with two 15 km single-track tunnels, its 21 stations and interchanges: Linate airport, the M1 red line at San Babila, the M2 green line at Sant'Ambrogio, the M3 yellow line at Sforza Policlinico and the suburban railways at the Forlanini FS, Dateo, and San Cristoforo stations.
The tunnels were excavated by six EPB TBMs working simultaneously.
Construction of the stations began with the diaphragms, i.e., the underground perimeter retaining walls. Then the excavation work was carried out and the reinforced concrete structures were built.
Work involved a series of significant engineering challenges that required great caution: the route, which often passed close to the foundations of buildings and monuments, the depth that was reached, which had to consider the features of the terrain and Milan's aquifer, the city center's heavy traffic, which made the logistics of the construction sites and the removal of the excavated earth difficult. The use of conveyor belts inside the tunnel to carry away the earth from the excavation face and take it to the discharge points at Ronchetto and Forlanini made it possible to minimize the use of trucks, and therefore the impact on surface road traffic, taking 75,000 truck journeys off Milan’s roads.
To safeguard the city's historical and artistic heritage, the consortium led by Webuild adopted a cutting-edge technical control method, which combines preventive analyses and continuous monitoring of buildings above ground. During the work, all geometric and structural data of the buildings affected by the excavations were collected and analyzed in detail by specialized technicians, so that any necessary preventive safety measures could be put in place, for example for Villa Necchi, Casa Silvestri and Palazzo Serbelloni.
As an extra precaution, the M4 tunnels were built at a greater depth than the industry standard, from the terminus tunnels at 15 meters down to 40 meters for the Dateo and Santa Sofia stations. At this depth, Milan's subsoil is mainly loose. In order to excavate the passages connecting the stations and the tunnels, avoiding the risk of landslides and ensuring total safety, Webuild used a cutting-edge technique: artificial freezing of the soil, a temporary waterproofing and/or consolidation technique employing liquid nitrogen, as well as the so-called “brine” method.
Through tubes positioned inside the excavation perimeter of the connecting passages, liquid nitrogen at -196°C was pumped into the soil; through a heat exchange with the soil, in just 15 days this created a stratum of ice of more than 1 meter in the soil along the excavation perimeter. In the longest connecting passage, at the Vettabia structure, the soil was frozen with the "brine" method to a temperature of approximately -32/-35°C, with longer freezing times of about 35-40 days.
To ensure the safety of Milan's artistic heritage as the TBMs passed through the tunnels, some monuments were temporarily moved. They included the Verziere Column with the Statue of the Redeemer in Largo Augusto, the Bust of Cesare Correnti in Piazza della Resistenza Partigiana, the medieval wall in via Francesco Sforza and the Monument to the Madonna della Rassegnazione.
During the excavations, numerous finds of great historical and scientific value for archaeologists and the Superintendency were discovered. They included burial grounds, filled with items from the Roman era, near the Basilica of Sant'Ambrogio, one of Milan's oldest churches.