Milan Metro M4, a Major Project Reaches Its Destination

With the inauguration of the section from San Cristoforo FS to San Babila on October 12, the entire route of Line M4 is now operational: 21 stations, with several interchanges, covering approximately 15 kilometers from the far western outskirts of Milan to Linate Airport in the east, passing through the city center and within the Navigli circle. Travel time is 30 minutes. While reducing the transfer from the airport to the historic center (just eight stops), it takes only 12 minutes. Up to 24,000 people per hour (in each direction), with an estimated total of 86 million passengers per year. The driverless trains pass every 90 seconds during peak hours. As the city’s fifth metro line, it adds another level to Milan’s metropolitan network within the European Union, now ranking seventh in terms of total kilometers with its 104 kilometers in total. A result that the contractor, Consorzio MM4 (led by the Webuild Group), achieved after overcoming several challenges. «We are very pleased to have finally reached this milestone. The M4 represents Milan’s gateway to Europe. From the ancient heart of the city to the airport with European capital-worthy timing, offering a better experience for tourists (and Milanese)» comments Guido Mannella, Operations Director Metro Italy Webuild.

One of the most challenging aspects of the Milan Metro M4 project was the passage through the historic city center by TBMs, which dug the tunnels for the new metro line. The tunnel boring machines passed almost right next to historic and artistic landmarks like the Forecourt of the Basilica of Sant’Ambrogio, the Columns of San Lorenzo, and the Merlata Tower between Via San Vittore and Via Carducci. These treasures had to be carefully preserved. The consortium of builders, in collaboration with the Superintendence of Archaeology, Fine Arts, and Landscape for the Metropolitan City of Milan, applied a control procedure that combined preventive analysis and continuous monitoring of the buildings above to identify potential risks and carry out necessary preventive reinforcement of buildings and monuments. A total of 631 buildings were monitored.

To minimize risks along the path of the tunnel boring machines, in some cases, it was even decided to move monuments, which were later placed back in their exact locations. Another precaution was digging at a greater depth than industry standards, reaching as far as 30 meters below street level at the Dateo station.

Finally, to avoid the risk of collapses during excavation, the “liquid nitrogen excavation method” was adopted, another significant challenge. This technique is applied in cases where the ground is unstable and at risk of collapse during excavation. All the connecting tunnels between the stations and the M4 tunnels were constructed using this technique: nitrogen at a temperature of -196°C is circulated in the ground to create an ice block. Liquid nitrogen was pumped seven days a week, at night for about 8 hours straight; for the remaining 16 hours, the walls remained frozen, ensuring extreme stability, allowing for safe digging even very close to building foundations. Under Milan’s surface, 42 connecting passages between stations and tunnels were excavated using the ground freezing method.

«The M4 Line was entirely constructed using underground ‘blind hole’ excavation by TBMs, except for excavations for the construction of some stations, utilities, and structures which were ‘open-air’, dug according to the Milan Method (better known today as ‘cut and cover’)» adds Francesco Stopponi, CEO of MetroBlu Scarl (a Webuild subsidiary), part of the line’s construction consortium. «Building this line was like a journey through time because as we advanced, significant archaeological discoveries of great historical value emerged, starting with ancient medieval walls at the De Amicis station site, which were restored and relocated in the station atrium.»

Redesigning urban spaces was another major challenge of the Milan Metro M4 project, aimed at redeveloping Milan’s surface from east to west along the entire route of the new metro line. The interventions involved numerous neighborhoods, covering a total of 246,000 square meters of urban areas, including around 66,000 square meters of green areas. A total of 1,900 trees were planted. A single green space was created along Viale Argonne, from Piazza Susa to Via Lomellina, featuring bike lanes, playgrounds, and pedestrian paths. Largo Augusto and Via del Verziere were pedestrianized. There are bike lanes and tree-lined paths along Corso Europa, and a new bike-pedestrian path in San Cristoforo that connects the two areas of the district by passing over the Naviglio.

With the inauguration of the entire M4 route, Milan is becoming increasingly sustainable, modern, and connected, but above all, more people-friendly.