Experimental analysis of the thermo-hydro-mechanical (THM) coupling in freezing vertical shafts of unsaturated sandy soil

The evolutionary mechanism of frozen soil involves complex dynamic coupling among temperature, moisture and the stress field. However, existing research has struggled to adequately describe the interplay between these factors. To address this, we independently designed and developed a multifunctional loading system appropriate for geotechnical engineering experimentation and a corresponding loading technology. Using a model of vertical shaft freezing, we studied the spatiotemporal evolution of thermo-hydro-mechanical (THM) multi-field coupling. The research findings indicate that, compared to the freezing interface, the main section experiences not only more intense variations in the temperature field but also heightened activity in terms of in-situ freezing and moisture migration. Both the radial and circumferential characteristic faces exhibit wave-like variations in moisture gradient evolution. The circumferential face features a critical gradient at 3.8 m⁻¹, whereas the moisture gradient curve of the radial face undergoes temporal elongation at the peak, resulting in no discernible extremities. Each characteristic position of the frost heave force growth undergoes three distinct phases: incubation, rapid increase and stabilisation. During the same phases, the response times for the growth of frost heave forces on the radial characteristic face are roughly equivalent. However, when moving outward along the equivalent freezing radius, the response time on the circumferential face becomes progressively delayed.