Ground freezing is one of the most reliable techniques for excavating unstable soil prone to collapse. Recently employed in the construction of tunnels for Milan’s M4 metro line, this method actually dates back much further.
It was first employed around 1862 in Welsh mines and patented in 1863 by the German engineer F. H. Poetsch. Later, it was widely adopted in the United States starting in 1888. In Italy, the method was first employed in 1937 during the recovery of the Ara Pacis Augustae in Rome.
Ground Freezing Technique: how does it work
Once the ground is frozen, it becomes more stable and easier to work with, allowing excavation to proceed safely. One of the main advantages of this method is its low environmental impact, as no chemicals are released into the soil. So, how does the ground freezing technique work?
Freezing probes are inserted into the soil to stabilize it. These small tubes are placed at the required depth and connected to a refrigeration system. Typically, a refrigerant such as ammonia or a freon substitute is used, along with a saline solution of water and calcium chloride, cooled to temperatures between -15°C and -35°C. Alternatively, the probes can be connected to tanks containing liquid nitrogen, reaching temperatures as low as -196°C. This latter method was used for all the connecting tunnels between the stations and the galleries of Milan’s M4 metro line.
