4D microstructural impacts on strength degradation in reservoir landslide slip zone soil subjucted to hydraulic cycling

Reservoir water level fluctuations expose adjacent bank slopes to cyclic hydraulic changes， i.e.， wetting-drying cycles （WDCs）， progressively degrading the shear strength of landslide slip zones and threatening long-term reservoir landslide stability. While wetting-drying cycles （WDCs） are known to alter soil microstructure， a quantitative characterization linking four-dimensional （3D space + time） pore evolution to strength degradation mechanisms remains limited. This study investigates the Huangtupo landslide slip zone soil through an integrated approach combining time-lapse X-ray computed tomography （CT） and triaxial shear testing. A four-dimensional （4D） analytical framework was employed to quantitatively track the evolution of pore morphology， porosity distribution， and fractal dimensions across successive WDCs， identifying the key microstructural drivers of mechanical weakening. The results show a systematic microstructural coarsening process， featuring rapid pore nucleation and coalescence into an interconnected network， culminating in a quasi-stable fabric. Quantitative analysis identified 3D porosity (φ_3d) and fractal dimension (F_3d) as the most sensitive 4D microstructural indicators， exhibiting strong correlations with the number of WDCs. Concurrent triaxial tests demonstrated a corresponding exponential degradation of both cohesion and internal friction angle. A novel microstructure-based constitutive model was developed， directly linking the accumulated 4D structural damage to the macroscopic strength decay. The findings establish a direct causal pathway where hydraulic cycling drives microstructural alteration， which in turn governs mechanical weakening， providing profound insights into the evolution of reservoir landslides and a quantitative basis for stability assessment.