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Artificial Ground Freezing: Applications, Techniques, and Key Considerations

*Ice body formation behaviour of different geological formations. Source: Snapshot of ICE video (lecture on cross passage construction including the use of artificial ground freezing)*

Artificial Ground Freezing (AGF) is a temporary ground improvement technique widely applied in tunneling, shaft sinking, and underground construction below the water table. The method involves lowering the temperature of water-bearing soil until pore water converts to ice, creating a strong and impermeable soil mass. Frozen ground acts as both a structural support and a barrier against groundwater inflow, providing a stable environment for excavation.

Installation of freeze-pipes for artificial ground freezing on a tunnel project. Source: Soletanche Bachy

The process is achieved through the installation of freeze-pipes or lances arranged around the targeted zone. A coolant is circulated through the pipes to extract heat from the soil, forming cylindrical frozen bodies that eventually merge into a continuous frozen wall. Two main approaches are used: the closed-circuit brine method, where brine cooled to around –30°C is recirculated, and the open-circuit liquid nitrogen method, where nitrogen at –196°C rapidly freezes the soil. In practice, the two systems may also be combined, with liquid nitrogen used for rapid initial freezing followed by brine circulation to maintain stability over longer durations.

Applications and Advantages

AGF is particularly valuable in conditions where conventional methods such as dewatering or grouting are ineffective. It has been applied successfully for tunnels beneath rivers, cross-passages between twin tunnels, and shafts in aquifer zones. Beyond infrastructure projects, AGF has been used for containment of contaminated sites, preservation of permafrost, and temporary stabilization of roadways.

One of its key benefits is environmental compatibility. Since the refrigerant circulates in a closed system, no chemicals are introduced into the soil, and natural hydrology is not permanently altered. The frozen barrier provides a complete seal against water ingress, reducing the need for pumping and treatment. Additionally, the technique is adaptable to a wide variety of ground conditions, provided sufficient moisture is present to freeze.

Illustrative chart for AGF. Source: HUININK

Design and Monitoring Considerations

The performance of AGF depends on accurate thermal and geotechnical design. Factors influencing design include groundwater seepage velocity, soil saturation, grain size, and required frozen body strength. High seepage rates can hinder freezing, requiring mitigation measures such as pre-grouting, closer spacing of freeze-pipes, or switching to liquid nitrogen.

Monitoring is essential throughout the process. Soil and coolant temperatures must be continuously recorded to confirm the development of the frozen wall, while piezometers monitor water pressures inside the frozen mass. Displacement measurements are also critical, as frost heave and thaw settlement can pose risks to adjacent structures. Modern AGF projects employ automated monitoring systems to collect and share real-time data, ensuring safe construction practices.

Although AGF is often more expensive than alternatives, its reliability and ability to achieve impermeability make it indispensable in challenging projects. In certain conditions, it is the only viable technical solution, emphasizing the importance of experienced geotechnical design and specialized contractors in its application.

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Sources: soletanche-bachy.com, cdmsmith.com, tunnelsandtunnelling.com