Artificial Ground Freezing (AGF) is a specialized technique commonly used to control groundwater flow and settlement during underground construction. Generally, a gallery of well loops are sunk into the soil and circulated with brine fluid to freeze and subsequently support the soil structure.
While the soil-thermal dynamics can be modeled to characterize and forecast changes in the soil during the various phases of construction, boundary conditions are not always well known. Thus, it is critical to physically verify the integrity of the freeze wall through discrete monitoring of in-situ soil temperatures.
Variables associated with the operational construction and heterogeneity of the soil pose a potential safety hazard during active ground freezing, as crews and equipment can be susceptible to unprecedented groundwater inflows and soil instability during excavation and operations. Additionally, AGF requires deployment of ground-freezing infrastructure and equipment, qualified personnel, and significant energy resources in order to effectively freeze the ground and keep it frozen.
Actively monitoring temperatures is crucial in order to reduce costs and unnecessary risks associated with unfrozen soils during operational construction phases:
- Verify design-phase modeling efforts.
- Drive and validate operational thermal models with real temperature data to improve model performance and reduce uncertainty.
- Validate the degree and extent of frozen soils, both temporally and spatially.
- Well-placed temperature sensing systems can be used to confirm which soil segments are sufficiently frozen.
Check out these links below, two examples of temperature monitoring used in artificial ground freezing applications:
Artificial Ground Freezing for Rehabilitation of Tunneling Shield in Subsea Environment: AGF methods were employed in the rehabilitation of a subsea tunnel. Special design and research was conducted that considered the influence of the salty water on the freezing effect. Temperature data was used to verify design goals.
Artificial Ground Freezing in Clayey Soils: This doctoral thesis is concerned with the thawing process related to the settlement in fine-grained soils and describes the artificial freezing and thawing process, identifying differences and similarities with natural freezing. Temperature data is used to characterize the freeze/thaw processes.
Coming soon, more information on temperature monitoring technologies for geotechnical applications. See 'Mind the Bandgap' for a previous post on Temperature instrumentation.