Mechanical Properties and Microstructural Evolution of Dispersive Soils Under Freeze–Thaw Cycles

Dispersive soils are widely distributed in the seasonally frozen regions of northeastern China, where hydrothermal dynamics driven by seasonal freeze–thaw (FT) cycles dominate the hydrological evolution and mechanical deterioration of soil masses, posing a serious threat to the long-term stability of hydraulic engineering in cold regions. However, the hydro–thermo–mechanical (HTM) coupled degradation mechanisms of dispersive clay from the South Nenjiang Main Canal remain poorly understood, particularly the linkage between FT-induced microstructural evolution and macroscopic mechanical behavior. In this study, low-plasticity dispersive clay specimens were subjected to 0–12 FT cycles. Unconsolidated undrained (UU) triaxial tests were conducted to evaluate mechanical behavior, while scanning electron microscopy (SEM) combined with the Pore and Crack Analysis System (PCAS) was used to quantify microstructural evolution. Results indicated that increasing FT cycles transformed the stress–strain response from mild strain-softening to strain-hardening, with the failure mode evolving toward bulging-type ductile failure. Cohesion exhibited a pronounced exponential decay, with the most significant degradation occurring within the first three FT cycles and stabilizing after approximately six FT cycles, whereas the internal friction angle showed only minor variation. At the microscale, porosity and total pore area increased continuously through micropore coalescence and macropore development, with a slight decrease in fractal dimension indicating reduced pore boundary complexity and smoothed pore interfaces due to frost heave-induced pore merging. The FT-induced hydrothermal disturbance promoted pore-water phase transition and redistribution, resulting in progressive pore enlargement and loss of structural integrity. Because the specimens were tested in sealed, closed-system conditions with a nearly constant total water content, this degradation chain is attributable specifically to in situ ice–water phase transitions and internal pore-water redistribution, i.e., water-phase-change-driven processes, rather than to external water supply. It is demonstrated that interparticle bond breakage and pore expansion–coalescence driven by ice–water phase transitions dominate strength degradation, promoting a transition from structure-dominated to friction-dominated strength behavior. A normalized cohesion reduction factor and a cohesion degradation index are further proposed to quantify the progressive loss of structural integrity and to provide a design-oriented tool for cold-region geotechnical practice. These findings provide a basis for stability assessment and hazard mitigation of dispersive soils in cold-region engineering.