Traditional tunnel excavation using ground freezing technology
The construction of Line C boasts some major technological innovations that make it a modern and functional underground transit line: one of these is certainly ground freezing technology, a perfectly eco-friendly process.
Ground freezing is used to guarantee the stabilisation and waterproofing of the terrain by lowering the temperature of the water present in the soil. It is a non-impacting process; in other words, it is a procedure that preserves the environment by not releasing any product into the terrain.
Artificial ground freezing is a technique for waterproofing and/or stabilising the ground at the same time, and for the purpose of excavation loose earth beneath groundwater level. The method consists of freezing the water within a volume of soil by circulating low-temperature liquid, “nitrogen or brine,” which extracts the heat and dissipates it to the outside.
The freezing modifies the ground’s thermal properties – its resistance/permeability – thus allowing workers to continue with the lining and consolidation operations.
Once the “ice wall” has formed, a provisional lining may be installed, composed of steel ribs and shotcrete, before going on to the final sealing and the casting of the final lining.
In urban environments, ground freezing technology is the best solution, because it guarantees the greatest level of safety possible both for those doing the work, and for the pre-existing constructions on the surface.
In the case of Line C, this technique was used to construct the line tunnels underpassing the existing Line A San Giovanni station, with a length of about 40 m for each direction of road, done entirely by traditional blind-hole excavation.
The project’s complexity required using a two-phase mixed freezing system.
Phase one employed liquid nitrogen freezing, in which the cooling liquid, at -196 °C, circulated in an open circuit and, after passing inside the freeze pipes, was dispersed as gas into the atmosphere. This made it possible to reduce performance times and to obtain a safer closure of the ice wall.
The second phase went from the “open-cycle” liquid nitrogen system to the “closed-cycle” system: the refrigerant, consisting of brine in this case, after its passage into the freeze pipes, warms through heat exchange with the ground, and then returns to the brine refrigeration plant, which cools it and sends it back into circulation.
We were thus able to monitor the development of the ice wall over time, thereby limiting deformations. Everything was monitored closely: the percentage of oxygen underground, existing structures, and ground temperatures, at all times protecting those working in the work site, as well as citizens. The use of nitrogen is fundamental, since 78% of the atmosphere is nitrogen; it is non-toxic, odourless, colourless, and non-inflammable. The white vapours originating from the subsoil, visible at the San Giovanni work site and dispersed into the air, thus presented no health risk. The experience of ground freezing applied in the Metro C working method will be an important point of reference for other future applications of this technology in similar contexts.