Macro-meso mechanical behavior and degradation mechanisms of silty clay subgrade subjected to coupled traffic loading and dry-wet cycles

Existing studies on subgrade soil degradation predominantly isolate traffic loading or dry-wet cycles, neglecting their synergistic impacts and leaving microscale mechanisms underlying macroscale weakening poorly quantified. This study addresses these gaps through a multiscale investigation of silty clay subgrades under coupled traffic loading and cyclic hydration-dehydration using direct shear tests, dynamic triaxial tests, and advanced imaging techniques (computed tomography [CT], scanning electron microscopy [SEM], and mercury intrusion porosimetry [MIP]). Key findings reveal biphasic strength degradation: cohesion (c) decreases by 13.02–21.68 % after five cycles, stabilizing after the second cycle, while the internal friction angle (φ) maintains 89.49 % of the initial capacity. Dynamic resilience modulus (M_R) decreases by 33 % during the initial cycles, with cumulative strains reaching 12.16 % under low confinement (20 kPa) and high moisture (22 %). CT quantification identifies macropores (>100 μm) as dominant (99.95 % porosity), driven by micropores coalescence, linearly correlating with axial strain (0.27 % per 1 % porosity increase at 20 kPa. A refined hyperbolic model, distinct from standard formulations by incorporating moisture-dependent decay parameters (a, b) that capture cyclic hydration effects, accurately predicts strain stabilization (R² > 0.96), validated against experimental data. These results establish explicit linkages between pore network dynamics and mechanical degradation, offering a paradigm shift from isolated to coupled conditioning frameworks and providing practical implications for designing climate-resilient subgrades.