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Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art

The central section of new metro line M4 in Milan cross an high-density urban area with a lot of issues cause by historical and archaeological building, one of the most complex case is the St. Ambrogio station. The initial solution for the connection between existing metro line M2 and the new one M4 in St. Ambrogio station was to build a 100 m long rectangular open-air tunnel placed above and practically on the same axis with the existing tunnel of M2 built in the 80s with a traditional excavation method, with a gap between the two facilities being quite strait, of about 1.50 m. Such a solution created problems for the further development of the overall project in terms of the traffic/construction site organization, because the connection had to be placed exactly along an important traffic corridor making part of internal ring road of the city of Milan. An alternative engineering and architectural solution has been developed in collaboration with the archeological supervision authorities. The integration and utilization, in the tunnel connecting M2 and M4, of the belowground part of the complex of Pusterla of St. Ambrogio, built in XII century a.C. and under protection of the archeological supervision authorities, helped to avoid a complete surface traffic block, to resolve a number of critical issues from the geotechnical and structural points of view, and to revaluate the archeological site that was incorporated into the connection between the M2 and M4 metro stations. 1 GENERAL INFORMATION OF NEW METRO LINE M4 The construction of Line 4 of the city of Milan has been set up and managed in a Public Private Partnership (PPP); for this purpose an ad hoc company has been set up, two-thirds owned by the Municipality of Milan and a third by the private partners in charge of designing, constructing and managing the new line 4 of the Milan underground (M4 Spa society). The duration of the concession is approximately 31 years (370 months) from the date of signature of the Convention (December 22, 2014), of which 88 months for the construction of the entire work. The Private Members, selected through a specific call for tenders, are formed the CMM4 consortium consisting in a temporary grouping of the companies Salini-Impregilo, Astaldi, Ansaldo STS, AnsaldoBreda (later Hitachi Rail), ATM and Sirti. The same is given the burden of design the Executive Project of the work, as well as the realization of the same (EPC contract). Within the above framework, MM company, in addition to drawing up the project in the preliminary phase, has had a controlling role in the executive design developments, the Technical Assistance to the Client, the Safety Coordination and Work Supervisor. The new metropolitan infrastructure create a fast public connection along the east/south-west route through the historic city center, developing for 15km from San Cristoforo to Linate, with 21 stations, 30 line shafts and 1 depot-workshop. It is an automatic metro without driver on board (driverless) with automatic dock doors and CBTC signaling system (Communication Based Train Control); the fleet consist of 47 vehicles.