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 theevolution of the yield stress with suction across the full range of degree of saturation. The experimental campaign included soilswith distinct geotechnical characteristics——a quartz sand, Speswhite kaolin, and a scaly clay——subjected to controlled-suctionloading. The results indicate that the yield stress does not increase monotonically with suction, as commonly assumed whenthe soils are in partially saturation conditions; instead, the yield curve reaches a maximum point, followed by a continuousdecrease in yield stress as it approaches the residual saturation state. The experimental data were analyzed using both netstress and effective stress approaches, confirming that the observed shape of the LC curve is not influenced by the choiceof the stress variables. Additionally, a specific controlled-suction oedometer test demonstrated that this new shape of the LCcurve enables the activation of plastic strains due to drying processes conducted at vertical stress levels between the maximumyield stress and the yield stress at the driest state explored.