Investigation of hydromechanical behaviour of unsaturated hydrophobic soil from micro to macro

Hydrophobic soils, which are commonly found in natural environments and often present after wildfires, necessitate a thorough understanding of their hydromechanical properties for effective hazard mitigation and engineering applications. The generation of convex menisci due to soil hydrophobicity is well documented, and yet its implications for the hydromechanical behaviour of hydrophobic soils are not fully understood. This study investigates the mechanical behaviour of unsaturated hydrophobic grains employing a newly developed custom direct shear apparatus, augmented by microfocus X-ray computed tomography (µCT). The µCT technique enables the visualisation of microstructures, and quantification of water menisci evolution at varying saturation levels. High-resolution µCT images successfully capture the convex shape of water menisci around unsaturated hydrophobic grains, facilitating the estimation of corresponding pore water pressure. This study is the first to capture the interaction between microscopic features (i.e. curvature and air–water interface area) and macroscopic responses (i.e. pore water pressure, dilatancy and shear strength). Findings reveal that the pore water pressure in unsaturated hydrophobic grains is positive and stress-dependent. This positive pore water pressure reduces dilatancy by around 60%, and peak shear strength by 24% in unsaturated hydrophobic grains compared to dry grains. Ignoring the wettability can therefore lead to overestimation of shear strength for unsaturated hydrophobic soils.