Abstract: Coarse-grained soil is widely used for the fill construction of rockfill dams and exhibits macroscopic mechanical properties affected by particle movement and mesoscopic deformation. In this study, Computed Tomography (CT) triaxial tests are conducted on coarse-grained soil of weathered phyllite, combined with digital image processing techniques, to quantitatively analyze its particle movement, local deformation and failure processes under controlled loading conditions (i.e. loading rates), focusing on the macro- and micro-structural characteristics. The results indicate that Unsharp Masking (USM) sharpening and material segmentation, when used to enhance CT images, will effectively highlight the boundary characteristics of soil particles. Significant deformation differences can be observed in various regions within a specimen. Near the two ends of the specimen, deformation takes a major form of relatively uniform axial compression, while near a failure surface, it manifests as compression and lateral spreading. In shear zones, the average CT number increases first and then decreases as an evident trend, indicating strain distribution is reflected by particle movement and local deformation. For a specimen with higher confine pressure, its shear bands show a coupling of tangential dislocation and normal dilation, developing progressively along the shear surface. At an axial strain around 16%, local regions accelerate to failure. Generally, shear band formation involves the stages of compaction, initiation, expansion, and penetration.

Key words: coarse-grained soil, triaxial test, CT test, image processing, shear band, failure