A CLOSER LOOK: TUNNEL CONSTRUCTION
The construction of the Line C tunnels, dug under the groundwater level, at a depth between 20 and 35 m below road level, calls for the use of TBMs (Tunnel Boring Machines), one of the most sophisticated technologies in the field of mechanised excavation in an urban setting.
To build the section between the Giardinetti station and the San Giovanni station, four TBMs of the EPBS (Earth Pressure Balance Shield) type were used, all the same as one another, manufactured by HERRENKNECHT.
In May 2008, the first two, after being lowered into the shaft introducing the shields of the TBM work site at the Giardinetti station, bored the line tunnels up to Shaft 5.4, while the other two performed the excavation from the Malatesta TBM shaft to Shaft 5.4.
At the same time, the first two TBMs were pulled from Shaft 5.4 and, after maintenance of the machine’s components, were lowered into the Malatesta TBM shaft, from which they excavated the line tunnel to the San Giovanni station.
The excavation of the TBM tunnels, for a total length of approximately 19.5 km, was completed in June 2011.
Upon completion of the passage of the TBM, which advanced about 8-12 metres a day, with peaks of 20 metres, and can work 24 hours a day, seven days a week, the tunnel was essentially finished, complete with its lining and ready to be fitted with tracks and the technological systems essential for controlling and operating railway traffic.
It takes about 10 months to build a TBM. Once the individual components are transported, they are assembled at the excavation site, a job that takes at least two months.
They are then lowered underground via vertical shafts, which are also used to transport the muck up to the surface.
Each TBM is manned by a 25-person crew per shift.
The TBM used to excavate the Line C tunnels has a diameter of 6.70 m, while the internal lining consists of precast segments in reinforced concrete with a thickness of 30 cm, and an internal diameter of 5.80 m.
Each precast ring, 1.40 m in length, consists of 6 main segments + 1 key segment. The rings are of the “universal” type and adapt to 19 possible different positions depending on the radius of curvature to be obtained.
The segments are made at a prefabrication facility using a fixed plant with steam curing.
For the Underground C tunnels built to date, four TBMs owned by Metro C were used.
The TBMs achieved excellent output, for a total advance of approximately 19.5 km in just over a year and a half of activity along sections T6, T5 and T4, with satisfactory results in terms of maintaining the excavation parameters and the values of measured surface subsidence.
For the extraction breakthrough inside the constructions, a steel ring system was used, with a seal allowing the shaft diaphragm wall to be broken down in the presence of groundwater behind.
This technique may be considered an alternative to head consolidation plugs, as it makes it possible to perform the phases of entering (as in the case in point) and exiting the shaft or station through a seals system that guarantee the sealing around the TBM shield against the groundwater present behind the diaphragm walls.
This is Italy’s first instance of employing this technology during the phase of the “arrival” of the EPB TBMs (that is to say with earth pressure at the face) inside a shaft.
The arrival phase is far more complicated than the starting one, mainly for the following reasons:
- High-precision TBM topographic guidance required.
- Pressure balance at the moment of the breakthrough.
The device, built by MSD- HERRENKNECHT, depending on the installation sequence, consists of the following elements:
- Ring with radial pegs
- Ring with seal
- Front closure plug.
The breakthrough operation by steel ring requires preparing a detailed sequence of operative phases starting from the latest excavation advances (nearing the final diaphragm wall to be broken through) and up to the shield’s entry inside the seals, and their pressurisation.
- Assembly of the elements composing the device inside the extraction shaft.
- TBM advance in EPB mode until the cutterhead enters into the consolidated plug.
- Gradual reduction of pressure supporting the face during the final excavations and until breaking down the final consolidated diaphragm wall.
- Breakthrough and advance without pressure.
- Entry of cutterhead inside the seal, and pressurisation.
- Removal of closure plug.
Use of the device offers the following benefits:
- Control of water outflows.
- Reduction of consolidations from the surface.
- Reusability of the structure.
Along the Line C route, the choice was made of full passage through all constructions – stations and/or shafts – except in the cases of the Teano and Malatesta stations. This choice, which consists of making the TBM passage inside the construction when the excavation depth is at least one diameter above the crown, allows continuity to be given to the building of the line tunnels regardless of the progress of the constructions, which offers clear benefits in terms of production and independence of the construction activities. Once the excavation of the construction has reached the tunnel, the tunnel is demolished for the segment within the perimeter diaphragm walls.
Geological and geotechnical aspects
From the geological-geotechnical standpoint, the excavation path carried out thus far has involved essentially pyroclastic terrain belonging to the Lazio volcanoes complex. The roofing of the tunnels above the crown varies from a minimum of about 6 m to a maximum of 25-30 m in correspondence with the Lodi–San Giovanni section, with a hydraulic groundwater load that varies along the route, but is always present for the entire work except for an initial section starting from the Torrenova tunnel entrance.
At particular points on the line, such as the platforms of five stations (Torre Spaccata, Torre Maura, Alessandrino, Parco di Centocelle, and Mirti), the Malatesta and Alessandrino sidings for stabling trains, and the Alessandrino railway connection, since they require a net internal tunnel diameter greater than 5.80 m, the tunnel was enlarged after the passage of the TBM, by traditional excavation, through the prior stabilisation of the ground surrounding the excavation using chemical and cement grouting having the function both of waterproofing the ground and of improving the mechanical and resistance features.
This type of excavation was performed for a total length of about 1.4 km.
Moreover, the line tunnels that underpass the existing Line A San Giovanni station, having a length of about 40 m per track, are done entirely by traditional blind hole excavation, under the groundwater level, through the stabilisation and waterproofing of the ground by freezing technology.
This technology is used in excavations below groundwater level in difficult conditions in the presence of loose earth or fractured rocks.
The freezing technique is the most appropriate choice in cases where working in urban settings with highly permeable earth and major hydraulic heads, so as to guarantee the greatest possible level of safety, both for the labour that is employed, and for the pre-existing constructions on the surface.
This type of intervention has exceptional technical value, due to the presence of the constraints dictated both by the pre-existing structures (in particular Line A’s foundation piles that required spacing the freeze pipes more than three and a half metres apart), and for the presence of the tunnel beneath an operating underground line, without covering.