Freezing

The construction of the platform tunnels for the stations on the Naples Line 1 (lower section), in tuffaceous rock and under a hydraulic head of more than 3 bar, posed the twin requirements of waterproofing the tuffaceous mass by sealing the inevitably present cracks and providing the static resistance necessary in the presence of loose material inclusions (pozzolana). A ‘crown’ of frozen soil more than a meter thick was therefore created over a length of nearly 50 m (184 ft) around the perimeter of the excavation.

The freezing process was achieved by the circulation of a liquid refrigerant, brine, or alternatively liquid nitrogen, in special pipes.

The freezing technique was also used in Milan for the construction of the Line 3. This technique overcame a difficult geological situation due to the presence of a thick layer of clay below the water table, which had not previously allowed static consolidation of the soil by sealing.