Variation in principal curvature of air-water interface inside and outside shear band of dense unsaturated sand

Triaxial compression tests were conducted on dense unsaturated sand specimens under constant suction and constant water content conditions. X-ray micro-computed tomography (CT) scans were performed at different deformation stages to visualize the development of the shear band with high resolution sufficient to identify individual soil particles and pore water morphology. Once segmentation was applied to the CT volumes, small sub-volumes corresponding to the inside and outside of the shear band were extracted. The principal curvature of the air-water interface, which correlates with matric suction in each sub-volume, was measured using an image analysis technique developed in this study. Finally, relationships between the degree of saturation and the principal curvature of the air-water interface inside and outside the shear band, identified as water retention curves, were obtained. The principal curvature inside the shear band was slightly higher than that outside, even under the constant suction test condition. It was also found that the local matric suction inside and outside the shear band, calculated using the principal curvature (1/r1) and the surface tension of water in accordance with the Young-Laplace equation, was slightly lower than the global matric suction of the triaxial specimen applied by the negative water column technique.