Ground Freezing
GROUND FREEZING
Ground consolidation by means of freezing
Freezing as a method of soil immersed into water is a technique known for several decades in the field of geotechnical engineering. Ground freezing can be achieved by the direct (liquid nitrogen) or indirect method (brine). For both systems thermometric data points, placed inside thermometers distributed within the volume to be frozen, allow an indirect control on the formation of the frozen structure.
In the direct method, nitrogen (close to the atmospheric pressure is liquid at a temperature of about -196 ° C) circulates in closed metal pipes causing a thermal shock in the groundwater surrounding the tube itself. Using liquid nitrogen it is possible to freeze the pore water present in a cylinder of soil of about 1 meter diameter within 3-4 days. The liquid nitrogen is distilled from the air and is transported and stored on site in special refrigerated tankers. Once used, the nitrogen is dispersed into the air again as gas.
In the so-called indirect method, brine (a solution of calcium chloride in water) is cooled by means of an electric refrigeration (chilling) unit at temperatures of -35 ° -40 ° C and is circulated in metal tubes placed in the soil (freezing pipes) returning after to the chilling unit to be cooled. In this case it will take about 3-4 weeks to freeze the water present in a cylinder of soil of about 1 meter in diameter. Also in this case the circulating system must be closed, it is essential to avoid any leakage of brine into the ground.