Freezing ground is a technique in general use for making excavations, shafts and galleries beneath the water table. It may also be used to "save" a structure after fill being washed out, etc.
It has the advantage of rendering the soil not only watertight, but stronger as well.
There are essentially two freezing techniques: one using brine, the other liquid nitrogen.
Open method and closed method
The refrigerant fluid is liquid nitrogen. It is transported to the site by special tanker trucks where it is kept at a temperature of - 196░C under a pressure of about 0.5 MPa.
This pressure ensures the flow of nitrogen through the probes. After the last probe, a valve allows the now gaseous nitrogen to escape to atmosphere, at around - 60░C. Venting nitrogen gas to atmosphere is not a hazard even in towns.
A primary refrigerating circuit liquefies the fluid by means of compressors and condensers. By evaporating again, this cools the refrigerant liquid which circulates in the freezing tubes in a closed circuit.
The primary fluid may be ammonia or freon. The refrigerant liquid is generally a brine with an operating temperature of between - 25░ and - 30░C.
This consists of combining the two preceding methods, using the same freezing tubes. Thus the rapid cooling of the nitrogen is combined with the economical continued operation of the brine. Often it is necessary to pretreat the ground by grouting. A light treatment with the object of closing voids or major flows and strengthening the soil a little is required. The advantage is twofold: economy of refrigerants (less losses), and increased safety.