Shear Strength and Damage Characteristics of Undisturbed Loess Exposed to Dry–Wet Cycles

Owing to its wind-deposited origin, loess exhibits pronounced water sensitivity and collapsibility. Under extreme environmental conditions, including fluctuations in groundwater levels and rainfall infiltration and evaporation, loess accumulates irreversible damage. Evaluating the effects of dry–wet cycles on loess strength and microstructure is crucial. This study aims to elucidate the degradation mechanism of loess strength by integrating macro and microscale investigations. Undisturbed loess underwent triaxial shear tests under varying dry–wet cycle conditions (dry–wet cycles times N, lower limit water content w1) and stress states (confining pressure σ3). High-power microscopy and scanning electron microscopy were employed to analyze microstructural evolution and reproduce the dry–wet cycle process. The results indicate that dry–wet cycles significantly compromise loess strength, particularly after the first cycle. The dry–wet cycle weakens the connection between soil particles, and the particle contact becomes point–point contact, leading to skeleton transport. Increased confining pressure σ3 and reduced dry–wet cycle amplitude (lower limit water content w1) mitigate mechanical strength attenuation. The failure deviatoric stress and cohesion are negatively correlated with dry–wet cycles times N, while the internal friction angle shows little sensitivity to dry–wet cycle times N. Based on the Duncan–Chang model, the stress index n is the only parameter dependent on lower limit water content w1. Additionally, a damage prediction model utilizing the initial modulus was developed to incorporate both dry–wet cycle conditions and stress states, effectively forecasting variations in the initial modulus. These findings provide a scientific foundation for mitigating geological disasters and enhancing the safety and durability of projects in loess regions under extreme weather conditions.