Strength Degradation Mechanisms of Structural Loess Subject to Dry–Wet and Freeze–Thaw Cycles

Structural loess is highly susceptible to structural deterioration and strength degradation under alternating dry–wet (DW) and freeze–thaw (FT) cycles, posing a significant threat to the long-term stability of loess slopes. To clarify the mechanisms and coupling effects of different environmental actions, this study conducts a series of triaxial compression tests and nuclear magnetic resonance (NMR) measurements on structural loess samples under DW, FT, and dry–wet and freeze–thaw (DWFT) cyclic conditions. The evolution of shear strength parameters and pore structure with increasing number of cycles is systematically analyzed along with the macroscopic strength–internal pore correlation mechanism. The results show that all three cyclic actions significantly reduce the shear strength of loess, with DW having a stronger effect than FT. Notably, under DWFT, a negative synergistic effect is observed, in which the strength degradation is less than the linear sum of DW and FT effects. Nonlinear degradation models for cohesion and internal friction angle are established based on regression analysis, and a strength prediction model incorporating the synergistic effect is developed using response surface methodology. NMR results further reveal the micromechanisms underlying the negative synergy, including the pre-release of pore expansion pathways, pore redistribution with localized healing, and structural fatigue–induced damage saturation. These findings provide a theoretical basis for modeling loess strength evolution under complex environmental disturbances and contribute to improved slope stability assessments in seasonally affected loess regions.