Phase change-driven structural degradation of ice-bearing glaciofluvial deposits from initial thawing to freeze–thaw cycling

Global warming and engineering activities have reduced the stability of widespread ice-bearing glaciofluvial deposits in China’s southeastern Xizang Plateau, increasing the risk of landslides and debris flows and threatening tunnel safety. The freeze–thaw response of such deposits remains insufficiently understood because their initial ice content, manifested as pre-existing ice particles, induces strong phase change effects and controls the evolution of the soil–ice composite load-bearing skeleton. This study investigates the coupled macroscopic deformation and mesostructural pore network evolution of such deposits under a unified loading sequence of initial thawing followed by repeated freeze–thaw cycling. A one-dimensional temperature boundary was applied to drive water migration, and temperature, water content, and top displacement were continuously monitored. X-ray computed tomography was employed to assess the porosity and pore size distribution, shape evolution, and throat tortuosity at selected stages, supported by three-dimensional pore network reconstruction. Results indicated that initial thawing triggered pronounced thaw settlement and downward water migration, with thaw settlement increasing with increasing initial ice content. Freezing induced a partial rebound during subsequent cycles but could not offset the preceding thaw settlement, leading to cumulative net thaw settlement. For a specimen with 10% initial ice content, top displacement became nearly stable after 7–8 cycles and reached approximately − 6.84 mm after 10 cycles. Mesostructural analysis indicated that thawing markedly increased porosity and shifted the pore size distribution toward larger pores, and the first cycle induced local compaction and redistribution. The porosity increase slowed with continued cycling and approached a plateau after ~ 6 cycles, whereas pore connectivity and pathway complexity continued to increase, reflected by higher pore shape diversity and spatial heterogeneity, as well as greater high-percentile pore throat tortuosity. Results supported a two-stage mechanism involving phase change-driven disturbance and skeleton reorganization during initial thawing, followed by progressive pore network loosening and reconfiguration under cyclic water redistribution. These findings provide new insight into the full freeze–thaw evolution path of ice-bearing glaciofluvial deposits and are of direct relevance for evaluating geotechnical risks in tunnel entrance ground and similar cold region engineering settings.