Water Retention Curve-Based Design Method for the Artificial Ground Freezing: The Isarco River Underpass Tunnels within the Brenner Base Tunnel Project

The paper presents an interpretation of the temperature monitored during the application of artificial ground freezing in the Isarco River Underpass tunnel, within the Brenner Base Tunnel project. The observations gathered in the in situ monitoring provided an opportunity to (1) examine the response of a heterogeneous glacial and river deposit composed of gravelly loose sand and boulders subject to seepage, upon freezing; and (2) set up a coupled (thermohydraulic) theoretical formulation, which accounts for the water retention curve and relative permeability function, able to properly simulate the artificial ground freezing process on a heterogeneous coarse subsoil under seepage. The simulation was able to provide a satisfactory representation of the development of the in situ monitoring data and it revealed the dependence between the freezing process, the retention properties of the subsoil, and the seepage velocity. A sensitivity analysis was also carried out to examine the potential effect of water retention parameters on the frozen wall formation in terms of thickness and closing time. The critical examination of in situ observations and the results of the numerical analyses allowed the definition of a new criteria to define the freezing temperature of the soil based on the water retention curve features and the residual degree of saturation, with the aim to optimize the design of the artificial ground freezing process by maintaining the safety level against waterproofing of the excavation area.