Linea C’s underground section is built with two side-by-side circular, single-track tunnels, with a usable internal diameter of 5.80 m, that, under the groundwater level, lies at a depth between 20 and 35 m below road level. The tunnels constructed to date for the Monte Compatri/Pantano–San Giovanni segment have a total length of 19.5 km (9.5 km per direction of traffic).
The tunnels are excavated through the use of Tunnel Boring Machines (TBMs) of the earth pressure balance shield (EPBS) type.
The TBMs used for Line C have a bore diameter of 6.70 m, with lining rings 30 cm thick.
Following the TBM’s passage, the tunnel is completed with its final lining and ready to be equipped with the installation of the tracks and technological systems essential for controlling and operating railway traffic.
The excavation of the Line C tunnels was completed in June 2011 for the Giardinetti-San Giovanni section (the underground portion of the section from the Monte Compatri/Pantano terminus to San Giovanni). The 4 TBMs used achieved a total advance of 19 km in the two tunnels.
On some points of the line (such as some station platforms, the sidings for stabling the trains, and the railway connections), since a net internal tunnel diameter greater than 5.80 m was needed, the tunnel was enlarged after the passage of the TBM, by traditional excavation, through the prior stabilisation of the ground surrounding the excavation, for a total length of approximately 1.4 km.
The line tunnels underpassing the existing Line A San Giovanni station, having a length of about 40 m per track, are instead done entirely by traditional blind hole excavation, under the groundwater level, through the stabilisation and waterproofing of the ground by freezing technology.