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The line tunnels underpassing the existing San Giovanni Line A station, about 40 m in length per track, were made entirely by traditional blind bore excavation, under groundwater level, through the stabilisation and waterproofing of the soil with freezing technology.

The artificial ground freezing method is a technique of simultaneous waterproofing and/or stabilisation, under groundwater level, of loose earth or fractured rocks. It consists of freezing the water within a volume of soil by circulating a low-temperature liquid that extracts heat and dissipates it to the outside.

The earth is cooled by circulating, within the volume to be stabilised, a low- temperature liquid that extracts heat and dissipates it to the outside. For the ground to be frozen effectively and evenly, the freeze pipes are positioned to respect the design’s geometry and interaxes for their entire length.

The freezing technique is the most appropriate choice particularly when working in urban environments, to guarantee the greatest level of safety possible both for the labour employed, and for the pre-existing constructions on the surface.

The terrains affected by the stabilisation and the subsequent excavation of the line tunnels are within pre-volcanic river/lake deposits, consisting mainly of sandy and clayey silts, silty clays, and clayey silts above a powerful and continuous layer of sands and gravels resting in turn upon a substratum of Pliocene clays.


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