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.