Drying soil-water characteristic curve of compacted kaolin prepared at optimum water content and saturated under free swelling

Abstract

Water retention and deformation evolve simultaneously during drying, making their interaction an essential aspect of the hydro-mechanical behaviour of compacted unsaturated soils, particularly when deformation is not mechanically constrained. This study investigates the drying soil-water characteristic curve (SWCC) of compacted kaolin samples initially prepared at optimum water content and subsequently saturated under free vertical swelling conditions. After saturation, the samples were air dried across a suction range of 25 kPa to 300 MPa using the combined axis-translation and vapor equilibrium techniques. In parallel, independent shrinkage measurements were performed to characterize the soil shrinkage curve (SSC). By integrating the two experimental techniques, the soil water retention behaviour can be characterised using either water content or degree of saturation. The results indicate a continuous transition in water retention behaviour, characterized by three distinct regimes associated with changes in desaturation mechanisms. The corresponding volumetric response reveals typical shrinkage stages, reflecting the progressive rearrangement of the soil structure as moisture is removed. By incorporating the measured mass-volume relationship, the water retention results were re-plotted using degree of saturation as the state variable. The experimental data were successfully represented using established constitutive models for the SWCC and SSC, enabling consistent interpretation of the coupled evolution of water retention and volume change throughout the drying process. The findings demonstrate the importance of accounting for volume change in water retention analyses and advance the understanding of the coupled hydro-mechanical response of compacted clay, supporting more reliable characterization of unsaturated soils and their application in geotechnical and environmental engineering.