Abstract: The micro-scale hydraulic-mechanical mechanism of unsaturated soil is still unclear, and the relevant microscopic experimental researches are insufficient. Aiming at the experimental requirements of observing the microscopic behavior of unsaturated soil, we developed a miniature triaxial test apparatus and a miniature soil-water characteristic curve test device for unsaturated granular soil, which bases on the similar experimental principle of conventional unsaturated soil test device and suits for μ-CT scanning system. Two devices have a miniaturized size, light weight, and high compatibility, which are able to achieve the non-destructive and dynamic high-resolution 3D tomography of unsaturated granular soil samples under unsaturated triaxial shear test and SWCC test respectively. The experimental accuracy and reliability of these two developed mini-devices were verified by comparing with the results of conventional triaxial test and conventional SWCC test devices respectively. We carried out in situ CT scanning tests of triaxial shear and unsaturated seepage, and obtained micrometer resolution 3D images. The multiscale evolution trends of the parameters obtained from the image analysis accurately reflect the macroscopic stress-strain relationships and global water-retention capacity characteristics of the in situ tests.

Key words: unsaturated granular soil, mini-triaxial device, mini-SWCC test device, X-ray micro-CT scan, microscopic behaviors