An experimental study of micro-scale hydro-mechanical characteristic of unsaturated granular materials based on in-situ triaxial CT scanning test

The advancement of the knowledge of unsaturated soil behaviour developed rapidly over the past decades, but the understandings remain largely at the macroscopic or mesoscopic element scale, and precise microscopic experimental studies on unsaturated soils still need to be improved. Aiming at the experimental requirements of non-destructively observing the micro-scale hydraulic-mechanical behavior of unsaturated soil, a miniature unsaturated triaxial test apparatus suits for μ-CT scanning system is developed. The suction-controlled in-situ triaxial compression CT scanning tests were performed to obtained the micron-level 3D images, and using image processing and RVE analysis to investigate the influence of microscopic effects on the global hydraulic-mechanical properties of unsaturated granular soils. The results show that: (1) During the unsaturated granular soil triaxial shear drainage process, the connected large-volume pore-water clusters gradually drained out or decomposed into the discontinuous small-volume pore-water clusters in tiny pores, the strength of unsaturated granular soil specimens is vitally affected by the small-volume liquid clusters (liquid bridges) within the shear band, which could provide the inter-particle cementation effect to enhance the global strength. (2) At about 30% of saturation, the air-liquid interface surface area reaches the peak value, in the scale of 25%~35% of saturation (axial strain 2%~5%), existing large amount of meniscus air-liquid surface, and the increased air-liquid interface area enhance the global strength of unsaturated sample. The solid-liquid interface area is mainly related to the degree of saturation, and its fluctuation has little effect on the overall strength enhancement. (3) The changing of the fabric structure can be characterized by the evolution behaviour of inter-particle contact coordination number at the sub-particle scale, the liquid bridges at the inter-particle contact can slow down the contact loss of the unsaturated sample and provide the overall strength during the stress growth stage.