Artificial ground freezing

Artificial ground freezing (AGF) is a method for temporarily increasing the strength and decreasing the permeability of ground. As AGF works in all ground types, it is particularly useful in highly variable water-bearing ground when sinking deep shafts and in difficult tunnelling situations. The ground is frozen by passing a cold fluid through pipes buried in the ground. The system creates overlapping cylinders to form a freeze-wall to stabilise the void and exclude groundwater. The moisture content of the soil must normally be above 10% and the rate of flow of groundwater less than about 2 m/day if brine is the coolant or 20 m/day for nitrogen. Recent developments involve initial freezing with nitrogen and then maintaining the freeze with brine. The shape of the frozen ground is limited only by the ability to orientate the freeze tubes. Several weeks are required for the frozen ground to be ready for use if using brine, but nitrogen is much quicker. The system design needs specialist knowledge of the complex properties of frozen ground. The structural design of vertical freeze-wall cylinders can be based on empirical formulae. For other applications, conventional analyses can be used to determine the required strength and dimensions of the frozen ground. Numerical thermal modelling is required to predict freeze-wall growth. Monitoring of the work throughout the entire process is essential.