Experimental study on micro-scale hydro-mechanical characteristics of unsaturated granular materials based on in-situ triaxial CT scanning tests

The advancement of the knowledge of behaviour of unsaturated soils has 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 soils, a miniature unsaturated triaxial test apparatus suits for μ-CT scanning system is developed. The suction-controlled in-situ triaxial compression CT scanning tests are performed to obtained the micron-level 3D images, and the image processing and RVE analysis are used to investigate the influences of microscopic effects on the global hydraulic-mechanical properties of unsaturated granular soils. The results show that: (1) During the triaxial shear drainage process of unsaturated granular soils, the connected large-volume pore-water clusters are gradually drained out or decomposed into the discontinuous small-volume pore-water clusters in tiny pores, and the strength of unsaturated granular soil samples is vitally affected by the small-volume liquid clusters (liquid bridges) within the shear band, which can 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%), there exist large amount of meniscus air-liquid surfaces, and the increased air-liquid interface area enhances the global strength of unsaturated samples. The solid-liquid interface area is mainly related to the degree of saturation, and its fluctuation has few effects 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, and the liquid bridges at the inter-particle contact can slow down the contact loss of the unsaturated samples and provide the overall strength during the stress growth stage.