The Non-Monotonic Shape of the Loading-Collapse Curve for Partially Saturated Soils

This study experimentally investigates the shape of the Loading-Collapse (LC) curve of unsaturated soils, focusing on the evolution of the yield stress with suction across the full range of degree of saturation. The experimental campaign included soils with distinct geotechnical characteristics - a quartz sand, Speswhite kaolin, and a scaly clay - subjected to controlled-suction loading. The results indicate that the yield stress does not increase monotonically with suction, as commonly assumed when the soils are in partially saturation conditions; instead, the yield curve reaches a maximum point, followed by a continuous decrease in yield stress as it approaches the residual saturation state. The experimental data were analyzed using both net stress and effective stress approaches, confirming that the observed shape of the LC curve is not influenced by the choice of the stress variables. Additionally, a specific controlled-suction oedometer test demonstrated that this new shape of the LC curve enables the activation of plastic strains due to drying processes conducted at vertical stress levels between the maximum yield stress and the yield stress at the driest state explored.