Ground freezing is a construction technique used in circumstances where soil needs to be stabilized so it will not collapse next to excavations, or to prevent contaminants spilled into soil from being leached away. Ground freezing has been used for at least one hundred years.
Pipes are run through the soil to be frozen, and then refrigerants are run through the pipes, freezing the soil. Frozen soil can be as hard as concrete.
A project in Boston known as the Big Dig used ground freezing during some of its tunneling, to allow its wide tunnels to be built under or through soil that supported existing infrastructure that would have been difficult or expensive to support using more traditional excavation methods.
In Northern Canada and Arctic Alaska, passive pipe systems are used that do not require any external power to keep the ground frozen. These systems use in-ground evaporators and above-ground radiators filled with liquid refrigerant. When ambient temperatures fall below ground temperatures, the liquid vapor starts condensing in the radiator, reducing the pressure in the system causing the liquid in the evaporator to boil and evaporate. This process results in heat transfer from the ground to the air and keeps the ground in a permanent frozen state.
The basic premise behind ground freezing is that soil—made up of bits of minerals and organic matter, water, and air—becomes stronger and less penetrable when its water freezes and expands.
In order to maintain support to the tunnel face, excavation and jacking normally carried out alternately in small increments, typically in the range of 2 to 4 feet. In most cases, the soft ground must be treated by means of ground improvement techniques such as ground freezing, jet grouting, etc. as discussed in Chapter 7 Soft Ground Tunneling to enhance its stand up time.
During the last two decades soil freezing with liquid nitrogen (LIN) has developed from an exotic gas application with lots of uncertainties into a standard procedure for treating unstable soil and leakages.
Passive cooling by means of pressured heat exchange pipes was developed in Alaska by the U.S. Army Corps of Engineers in 1965 to preserve foundations in ‘warm’ permafrost.
The basis of thermosyphon thechnology is a heat transfer device (thermosyphon) which extracts the heat from the soil during winter and passes it to the environment.