The m4 line or Blue Line is an ongoing project to improve Milan’s transport system and interconnection with the underground and the suburban rail network. After excavating underneath Milan’s suburbs, TBMs are now working in the city centre, passing under highly populated areas and existing buildings. Excavation of the first drive of the running tunnels began from Linate station on 4 April 2014. On 11 November 2014, the TBM on the next drive reached the Forlanini FS station, completing the installation of approximately 2,370 rings of precast segments. At the end of November, TBMs started working on the East side of the M4 route, while the opposite side will be excavated in 2017. In 2018 TBMs will excavate the old town centre, carefully crossing some archaeological sites.
The M4 metro line was designed by Rocksoil, which provided data planning and analysis of the underground and civil works. The total value is EUR 1.8bn (USD 1.9bn) and will be mainly funded by the “MetroBlu” consortium, which is composed of Astaldi and Salini-Impregilo. The project involves the design and construction of two single-track tunnels, one in each direction, with 21 stations, 30 associated structures and a depot/workshop.
The 15km-long M4 will cross Milan from west to east passing south of the old town up to Linate Airport. The new line will be a fully automated light rail system, driverless, and with automatic platform doors and a CBTC (Communication Based Train Control) signalling system. The trains will be 50m long, considerably shorter than rolling stock in circulation today. The 50m-long stations will also be shorter than the 110m stations on lines M1, M2 and M3. The compact dimensions of the structures, especially in the stations, means that construction work on the line can be carried out more easily and with less impact.
MECHANISED TUNNELLING
Six EPBMs will be used to excavate the M4 project. Two are from NFM with a diameter of 9.15m and four are from Herrenknecht with diameters of 6.36m and 6.5m. Two Herrenknecht TBMs are currently working on the East side. The final lining of the tunnel will be made of precast segments. The ring for the 6.3m-diameter tunnels comprises six segments in a 5 + 1 key configuration with a thickness of 280mm. The 9.15m tunnels consist of a seven segment 6 + 1 keystone ring with a thickness of 350mm. “For the TBMs with a 6.36m diameter, a 3.85m3 of grouting is required to get a complete filling of concrete between the excavation and lining extrados,” says Stefano Gazzola, Rocksoil’s M4 project manager. “During excavation, data from the TBMs is available and compared to expected values. Grouting is being used to protect buildings and improve soil stiffness, particularly in areas with low overburden or structures of historical significance. So far, ground injections are expected in up to four locations and they have been designed to be made from the surface.”
The segments are equipped with EPDM gaskets and are assembled with longitudinal mechanical dowels. The maximum advance thrust is 42,500KN for the 6.36m diameter TBMs with 32 jacks and 82,000 KN for the 9.15m diameter TBMs with 38 jacks.
“The 6.36m TBM will be used for the sections from Manufatto Ronchetto, placed in the San Cristoforo area, to the Parco Solari station, and from Linate Airport to the Tricolore station,” says Davide Fraccaroli, M4 TBM manager. “The 9.15m TBM will be used for the section from the Parco Solari to the Tricolore station.”
Gatti explains, “The 9.15m machine will be used in the historic centre in order to enable the installation of the station platforms directly inside the inner contour of the tunnel in segments. It allows us to significantly reduce the impact on existing structures compared to the use of conventional tunnelling methods.”
To place deep stations in the historical part of the alignment, a central shaft will be excavated at a depth of up to approximately 35m and transverse (outward) dimensions limited to approximately 10m. This central shaft will be used as a permanent access. Outside this, running tunnels are built using the 9.15m TBM, which provides sufficient width to accommodate station platforms within the bore. The connecting bypass between the station shaft and the platforms will be excavated after the soil has been grouted from the ground level with the continual use of cement and silicate mixes.
The open excavation method will be used for stations that are generally at depths of about 15m, while the station in the old town centre will reach depths of approximately 26 and 30m because of a higher overburden. Reinforced concrete diaphragm walls will support the open bottom-up method. In some areas a top-down method will be also used to cast immediately the top roof and reduce impact on urban areas.
One of the project’s main challenges is working in highly-populated areas and in the historical centre of the city. “We carefully assess any possible disturbance related to existing structures such as sewerage systems, water pipes, phone and gas cables, and so on. We also pay attention to the potential impact of works on existing roads and road traffic, says Gatti. “In order to find the best traffic solutions, the local Council may require diversions or alternative routes for buses or cars. Of course, each stage needs to follow such requests, which come from a specific site area.
Archaeology
The presence of archaeological sites has presented challenges as the majority of M4 stations are located in the historical city centre, which includes Roman ruins and buildings.
“For example, Vetra Station will be located near to the Saint Lawrence Church with the famous Roman columns,” says Gatti. To identify the grade of archaeological risk, archaeological surveys were carried out along the M4 route between 2008 and 2009, paying attention to the Cerchia dei Navigli (‘Ring of Canals’) area, the historical centre of the city. The cultural heritage department approved the project on condition that archaeologists can constantly monitor excavation works as the TBMs advance stroke by stroke. Excavations at the depth of 4m will be carefully performed in order to look for archaeological findings.
An analysis of disturbance to buildings caused by excavations has been carried out in two main steps. The first used subsidence theory and mathematical formulas to analyse surface settlements as a result of tunnel excavation. Thanks to this calculation, as Gazzola explains, it’s also possible to include the effects of deep station excavation. “Through this process we can estimate a building’s settlement, and strain, and calculate a damage class,” says Gazzola. “Considering a building’s state of repair, support material and year of construction, there are reference tables to reach a primary acknowledgment of the effects on each building.”
The second step was to evaluate if it was necessary to carry out numerical analysis by using various FEM and FDM software. That meant the opportunity to get a more accurate settlement calculation, which also checks the presence of the building and its stiffness. 3D FEM models for buildings could be also implemented in addition to a possible estimation of the “damage class”.
“After carrying out these two different analyses, we are able to evaluate if grouting is necessary to prevent building settlements that have been considered to be critical,” Gatti adds. “Consolidation works mainly consist of improving soil stiffness with concrete injections at various pressures. With deep station excavation, retaining structures, such as micro pile walls or reinforced concrete diaphragm walls, are used to reduce building settlement.”
“A fundamental aspect of the tunnel construction in urban areas is the control of subsidence induced by excavation through a monitoring system,” Gatti says.
“Monitoring during excavation is undertaken using settlement gauges set up approximately every 100 or 150m along the route at ground level.
“From the accumulated data we can compare theoretical and real results and calculate real volume loss caused by tunnel excavation. During the first 3.3km of excavation from Linate Airport to Forlanini Railway Station settlements have been on acceptable levels and close to expected values, with a ground loss of 0.5-0.7 per cent of tunnel excavation. It has to be noted that the first part of the track is located mainly in a suburban area without many buildings. Ground level settlements have been considered acceptable even where, in some cases, they were over predicted values.”
For buildings’ settlements operation the threshold is between 25 and 30mm, Gazzola says, but each case is carefully analysed as necessary. When real settlements exceed the calculation values a detailed explanation is usually provided, including analysis of the TBM’s data, specifically the pressure values and the chamberexcavation. The monitoring programme also consists of strain gauges positioned inside concrete segments in order to control internal forces acting in the final lining. One ring is equipped with instrumentation every kilometre, increasing for particular areas of interest.
For buildings near station excavation, a specific monitoring programme has been provided considering also interaction with tunnel excavations where necessary. On buildings, settlement gauges and instrumentation for inclination measurement are provided. In order to get information on foundation settlements, displacements at deeper levels can also be investigated with specific inclinometer instruments.
If ground movements increase in a way not predicted by calculations, measurements are intensified (two for each day) in order to clarify the situation.
“To control varying settlements as indicated by monitoring data, the excavation’s chamber pressures and both pressure and quantity of conditioning grouting are revised,” Fraccaroli says. “During the design of TBM working pressures, acting inside the excavation chamber, we considered any possible interferences with the existing underground infrastructures such as the Milan Passante railway, which crosses the city of Milan from the North to West side,” Gatti says. “In order to avoid heave of the existing tunnel due to low cover separating the two, a lower working pressure will be used and a monitoring system on the existing railway will be carried out with settlements and slant measurements. The railway manager has also provided the limit values compliant with the operation of trains.”
Sereni Shaft
“The Pozzo Sereni is a shaft excavated in open-work, carrying out lateral support works which are reinforced concrete diaphragms. It is located near the Forlaninini Station and it will be used as emergency exit and fire fighters’ access. At the moment it is the launching shaft for the TBMs in the east side. Pulling together these diaphragms, we obtained a support area where it’s possible to excavate,” Gatti says.
“Working with a high aquifer, it’s essential to do concrete injections under the level of the final dig. This waterproof barrier let us excavate without any problems, avoiding water inflow with flowing ground from the excavation.” The TBM’s conveyor belt is vertically oriented to save space on site. Fraccaroli adds: “This 100m-long TBM is not fully assembled because we are working in confined spaces. Thus, the back-ups are temporarily placed in an open-air area and when the TBM advances, they are lowered down the shaft.”
The TBM’s average progress has been 20m per day, working in what is a high-density area. “For example, we experienced this average for the section from Linate airport until the Sereni shaft, achieving some peaks of 35m per day, as well,” says Fraccaroli.
Forlanini FS Station
The tunnel beneath of Forlanini FS railway has been completed, including the platforms and all the civic works. On its route it meets the railway at several points and each stop is 600m apart.
“In these locations we have adopted a system of cladding with precast concrete segments and it has been designed on universal ring logic,” Gatti says. “This allows us to navigate bends at 180m of planimetric radius. These precast concrete segments have longitudinal connections that prevent adjacent rings disconnecting. “It’s important to pay attention to the second tube’s excavation to consider the ground movement caused by the first tube’s excavation. During the second tube’s excavation higher settlement may be detected as generally happens.” Moving from east to west the next stop after Forlanini FS is Argonne.
“Here there is an open-air excavation, where a team specialising in waterproofing is working to place the PVC membrane around the station before casting the internal reinforced concrete lining. However, the membrane is not placed in the space where TBMs break-in the station.
“The retaining wall of this area is reinforced through fibreglass structural elements to be easily cut by the TBM’s cutterhead,” Gatti says.
Dateo Station
“A team specialised in grouting is currently working at the Dateo station at the level of the acquifer (14m down from ground level) to perform the bottom plug and to protect the future excavation from water inflow. The Dateo station requires grouting plug because the excavation will reach the depth of 32m,” Gatti explains. “The grouting is performed by PVC tubes, equipped with manchette valves.”
Gatti adds: “We use a mix of water and concrete in addition to waterproof materials such as silicate. The combination of these materials not only allows a good permeability on the ground, but also a waterproofing to avoid leakage of water into the excavation. After the perforation, we can drop tubes to cement them to the ground with the concrete mix. There are also valves at some points to inject the mixture of materials in some empty intergranular spaces. Once they are filled, the flow of water can’t pass through inside them.
“Designers usually identify the right distance between these PVC tubes to ensure the efficiency of this grouting,” Gatti says. “The fundamental parameters also include pressures and the quantity of mixtures so that waterproofing can be extended on the whole face.”
SuStainability plans
The Blue Line is being carried out in full compliance with sustainability and environmental criteria. Thanks to this new line, it will be possible to significantly reduce traffic congestion and air pollution, improving the quality of life in Milan. The planning of surface sites will be compliant to ISO 14001, a series of environmental management standards, which includes a waste plan and noise control, and each activity to reduce the environmental impact.
According to the Environmental Monitoring Plan it will also be possible to analyse the potential effects of the M4 on the surrounding environment. Controls will include measurements of biological, chemical and physical parameters relating to the construction works. Results based on monitoring will drive the main operations in terms of possible adjustments, updates and running changes. The Environmental Monitoring Plan will also check the hydro-environment, noise and air pollution and the state of vegetation.
“For example, an acoustic restriction to 50dB (measured near the work site) has to be observed in the city centre,” says Gazzola. “Each zone of the city has been mapped with a specific limit on acoustic pollution”.
