Ground freezing has proven to be a feasible and adaptable technique for underground construction applications.

The additional cost (in most cases) is justified by the wide range of soil and ground conditions where it can be used effectively. In fact, in situations where no groundwater is acceptable or when restrictions limit the use of other ground improvement techniques, it becomes the cost-effective option. Compared to other standard support methods, ground freezing is unique because it can create very robust support without causing any vibration or ground loss.

For the freezing methodology to be effective, sufficient water must be present within the soil, however freezing will not change the properties of dry soils significantly. It requires in situ porewater to be converted to ice by extraction of the latent heat. The ice then acts as a cement to bind the soil grains together, thereby increasing the strength and lowering the permeability of the soil mass.

Preliminary steps to design a ground freezing system involve looking into the thermal conductivity of the soil that needs to be supported and the chemistry of the existing groundwater.

Usually, simple empirical methods with sufficient factors of safety could be used to determine the energy and time of operation. However, projects which are more complex in nature can make use of computer models and analytical software. The instrumentation involved (thermocouple strings in most cases) usually allows more accurate monitoring of the operation.

A typical freezing system comprises a refrigeration plant that chills a brine solution. This is then pumped through the inner section of an annular freezepipe and subsequently returns through the outer annulus which is in contact with the ground that needs to be supported. The brine that has extracted heat from the ground is then returned to the refrigeration plant where it is chilled to allow the cycle to continue.

In most projects, arrays of freeze pipes are installed to cover the entire zone that needs to be supported. Initially, circulation takes more energy to freeze the zone around each pipe and to overlap with other pipes. However, after that stage, the refrigeration needs only to maintain the already frozen area. Creating a freeze zone stronger than necessary can result in lower advance rates, larger ground movements and additional costs.

One of the most commonly-used solutions in the industry is calcium chloride brine. Other brines are available with lower freezing points but they are more costly. For example, liquid nitrogen can reduce the initial freezing process time from weeks (or even months) to days, but the cost is high. Additionally, special safety measures and precautions must be taken when it is used.

Underground elements suited to ground freezing include cross passages, TBM break-in/break-out zones, fire/exit passages and shafts below the water table. Excavation of frozen ground can be carried out by drilling and blasting, or by conventional mining equipment such as excavators and mechanical hammers. Roadheaders also appear to be particularly effective in the conditions created by freezing.

Below there follow a few tips on ground freezing:

  • The existence of organic material (common in silts and clays) or saltwater will typically have the effect of lowering the groundwater freezing point and therefore result in greater freezing complexity.
  • Moving groundwater will be a challenge since it can bring heat into the work zone faster than the cooling system can remove it.
  • In some ground conditions, the expansion of water by freezing will first cause heaving of the ground surface, and later settlement upon thawing so the ground could be considered as disturbed afterwards.
  • Short tunnels (e.g. cross passages) may be frozen by installing the freezing elements horizontally around the perimeter of the excavation.
  • Any utilities or structures in the vicinity that might be affected by freezing must be looked at and protected as needed.
  • In most tunnel ground-freezing applications, it is only necessary to stabilise the soil so it supports its own weight and becomes impermeable; strength is usually secondary. Frozen soil strengths typically lie in the 3-10MPa range.