Work begins on west section of Milan Metro Line 4
Work begins on west section of Milan Metro Line 4

Milan metro sees TBM progress on Line 4 15 Nov 2018

Patrick Reynolds for TunnelTalk

Extensive tunnelling is underway by the Salini Impregilo/Astaldi JV to complete the second stage of works on Metro Line 4 in Milan, Italy. The first phase excavations on the east section were completed earlier this year. The JV has launched the first of twin 6.36m diameter Herrenknecht EPBMs on the west section of Line 4.

Line 4 is a 15km long, fully-automated, driverless light rail system with short trains to run underground, east-to-west, across Milan. Main construction work has been underway since 2013 with cut and cover excavations for most of the 21 stations along the route running from Linate Airport in the east to San Cristoforo in the west, and with limited surface works and different, and deeper, station layouts in the historic central stretch.

Also known as the Blue Line, the new route’s stations are much shorter and more compact than earlier metro lines in Milan. A key factor in the change comes from advances in technology to use shorter, four-carriage trains for Line 4.

Map of Milan Metro with detail of Line 4
Map of Milan Metro with detail of Line 4


Marco di Paola, construction manager with Line 4 concessionaire, M4 S.p.A, said: “The new subway Line 4, as well as the entire territory of Milan, is located in a flat area, occupied and modelled in the Pleistocene Epoch by different glacial phases, during which debris was deposited from the Alpine mountain range.”

He adds that, lithologically, the excavation zone for Line 4 is dominated by ‘Fluvio glaciale Wurm,’ consisting of sandy-gravelly terraced soils, sometimes weakly silty zones.


TBM tunnelling on Line 4 has been split into three sections – east, central and west – to suit the station construction needs and the deeper alignment in the central section. The stations are between 400m-800m apart along the route.

All running tunnel excavation is by Herrenknecht EPBMs, boring at depths of 12m or 20m, depending on the section of the project. The groundwater level along the alignment of Line 4 is about 8m-10m below the surface, said di Paola.

Senior managers explain Line 4 scope

Four TBMs of just over 6m diameter are excavating the twin tube running tunnels in the east and west sections, and two machines of 9.15m diameter will bore the deeper tunnels of the central section, the tunnels being larger to house station platforms. Spoil removal is by conveyor. The excavation works are supported from two sites – Manufatto Ronchetto and Manufatto Sereni, said di Paola.

The first TBMs began their drives in the east section in early 2014 and achieved the major milestone of completing the last of their drives earlier this year, achieving progress rates of 10m-15m per day. The work pattern is 24/7, commented di Paola. The second major milestone took place in the third quarter with the start of TBM boring on the west section.

East section

All sections of twin tube running tunnels linking underground stations on the approximately 6km long east section of Line 4 have been excavated by refurbished 6.36m diameter EPBMs, previously used in the Middle East. The machines worked in two phases, in 2014-15 on the Linate Airport to Forlanini FS section, and then 2017-18 on the Forlanini FS to Tricolore section, said di Paola. The TBMs were walked through the intervening station boxes.

Twin tube tunnelling between the stations of the approximately 4km long west section recently got underway using two refurbished 6.36m diameter EPBMs that were previously used on Line 5 of the metro. The machines are built with a 5.50m i.d. lining with 1.4m wide rings formed of six segments, each 280mm thick, said di Paola.

Starting from a working shaft at Ronchetto, near San Cristoforo station, both TBMs are to proceed eastwards at an average advance of 16m/day. By early November, the first TBM had passed San Cristoforo and reached Segneri station. They will continue to advance towards the Parco Solari station, at the edge of the central section, where they will terminate. Ahead of them, at Parco Solari, the two larger 9.15m diameter TBMs will be launched, also to drive eastwards, to bore the deep tubes in the central section towards Tricolore.

Successive breakthroughs to complete east section

Central section

Located in the historic centre of the city, the approximately 5km long central section will have the deepest stations and running tunnels on Line 4. The two new 9.15m diameter shields will bore eastwards, from Parco Solari to Tricolore. Among the principal challenges on the project are the practicalities of working in the city centre and also accommodating archaeological excavations.

The design solution is to use larger TBMs to create 8.15m i.d. tunnels able to accommodate station platforms and therefore only require slender, central box excavation and minimal surface works. The TBMs will build 1.5m wide rings with seven segments, each 350mm thick.

To limit open cut works on the surface, platforms for central section stations are in larger diameter running tunnels
To limit open cut works on the surface, platforms for central section stations are in larger diameter running tunnels


Salini Impregilo and Astaldi are working on a 50:50 JV to complete the civil works and tunnelling, operating as a subcontractor, named Metro Blu, to the MM4 consortium. The engineering, procurement and construction (EPC) contract to build Line 4 was awarded to MM4 consortium. Led by Salini Impregilo and Astaldi, MM4 includes Ansaldo and Sirti, both of which have their own subcontracts from the EPC contractor.

The EPC contract was awarded by a special purpose vehicle, M4 S.p.A, which holds the concession to build and operate Line 4. The concessionaire is 67% majority-controlled by the City of Milan Government, and includes private sector shareholders Salini Impregilo at 9.6%, Astaldi at 9.6%, the Hitachi Rail sister companies Ansaldo STS and Hitachi Rail Italy (formerly AnsaldoBreda), and Sirti and ATM holding the remaining shares and contracted to provide operations and maintenance services when the short metro trains start running, scheduled for 2023. Once opened, the new line is forecast to carry 86 million passengers per year.

The 30-year concession for M4 S.p.A was formalised after financing was secured at the end of 2014, shortly after the first section of tunnel boring was completed from Linate to Forlanini FS station, as the first phase milestone on the early works. Prior to that, from 2013, the companies involved worked together in a JV contract on the early phase of construction of Line 4.

Larger EPBMs will bore central section of Line 4
Larger EPBMs will bore central section of Line 4

Milan Metro Line 5

Astaldi also helped to build Line 5 of the metro, the first fully automated medium-capacity metro line in the city. Until last year it held a large, majority equity stake in M5 S.p.A, the concessionaire that built and operates the line.

Project planning for Line 5 took place over 2003-7 and construction began soon after. TBMs on the line were supplied by Seli, Lovat and Herrenknecht. In mid-2017, with Line 5 built and in operation, Astaldi sold most of its holding in M5 S.p.A to Italy’s state rail company Ferrovie dello Stato Italiane S.p.A (FSItaliane).

FSItaliane bought the largest stake in M5 S.p.A in its strategy to improve transport integration for urban areas in Italy. Its move is expected to support extending Line 5 by 13km in projects to Monza and Settimo Milanese, said Astaldi. The other shareholders in M5 S.p.A are Ansaldo STS, Hitachi Rail Italy, ATM and Alstom.

Astaldi sold its large stake in Line 5 as part of its own wider, strategic programme of divesting stakes in many concessions where it had been involved also as a contractor, including its holding in the Chacayes hydro facility in Chile which called for significant tunnelling works.


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