Abstract: The soils in the hydro-fluctuation zone on the Three Gorges Reservoir are exposed to dynamic environments of long-term wet-dry cycles and suction variations, leading to its changes in their unsaturated shear strength and failure modes. This study investigates the effects of wet-dry cycles on the unsaturated shear strength characteristics of red soil in the hydro-fluctuation zone of a landslide area on the Three Gorges Reservoir, using nuclear magnetic resonance (NMR) and triaxial shear tests. The study also reveals the variation in the critical saturation at which the failure mode transitions from strain-softening to strain-hardening. Results indicate that matric suction significantly contributes to shear strength at low saturation levels, exhibiting obvious strain-softening and shear failure characteristics. Under high saturation conditions, the soil exhibits strain-hardening and swelling failure characteristics. Taking into account pore-size distribution and suction, we propose a critical-saturation model for the transition from strain-softening to strain-hardening. The model’s applicability and reliability are verified through a comparison of predicted results and experimental data. The critical saturation degree increases with the number of wet-dry cycles, suggesting that the repeated wet and dry cycles cause particle rearrangement, destruction of the original aggregates, and the evolution of the pore structure from dense to loose. The increase in pore size requires a higher saturation state to form continuous water distribution, causing the transition from shear failure to swelling failure.

Key words: shear strength characteristics, critical saturation degree, wet-dry cycles, pore size distribution, Three Gorges Reservoir area