Pore-structure evolution and net bulk shrinkage of undisturbed soft clay during closed-system freeze–thaw: The role of internal hydrothermal redistribution

Controlling and predicting frost heave and subsequent thaw settlement are critical challenges in applying artificial ground freezing (AGF) to soft-soil underground engineering. When the soil between adjacent freeze pipes is hydraulically isolated, it behaves as a closed system with limited external water supply. This study investigates the hydrothermal behavior and microstructural evolution of undisturbed soft clay subjected to one-dimensional closed-system freeze-thaw. Freeze-thaw experiments were carried out at different cold-end temperatures, and the resulting pore-structure changes were characterized using mercury intrusion porosimetry (MIP) and fractal analysis. The evolution of the temperature field, freezing-front progression, and vertical displacement was monitored and related to changes in pore structure. The results show that slower freezing, associated with a smaller imposed temperature gradient, allows more internal water to migrate toward the freezing front and results in larger frost-heave strain. At the same time, the resulting closed-system moisture redistribution produces a clear depth-dependent contrast in post-thaw microstructural response. MIP reveals systematic pore-size redistribution and changes in mercury-accessible pore volume and pore-volume distribution heterogeneity after freeze-thaw. Final thaw settlement exceeds maximum frost heave, indicating net bulk shrinkage over the freeze-thaw cycle. Updated mass-balance analysis indicates that this bulk shrinkage was dominated by freeze-thaw-induced structural rearrangement under nominally closed conditions, rather than by appreciable external water loss during thawing. This interpretation is consistent with the post-thaw increase in mercury-accessible pore space, because MIP reflects pore accessibility and connectivity within the soil skeleton rather than specimen-scale volume alone. These findings clarify how thermal boundary conditions regulate internal hydrothermal redistribution and pore-fabric evolution under closed-system freezing and provide an experimental basis for interpreting freeze-thaw deformation in water-limited AGF conditions.