Experimental Study on Mesoscopic Shear Behavior of Calcareous Sand Material with Digital Imaging Approach

The study of the mesostructure of soil under loading is the basis for understanding its macromechanical properties and for establishing its constitutive model. In this study, a series of shear tests was performed on dry calcareous sand under constant normal stress by a modified direct shear apparatus. Digital images of the sample at different shear stages are obtained. The mesostructural parameters of the sample are then extracted and analyzed using an image analysis technique. The results show that the shear-band is located at the junction of the upper and lower shear boxes with a thickness of 0.79–1.59 mm. During shearing, the position of the maximum shear strain incremently shifted to the junctions between the two shear boxes. The azimuths of the particles prior to the test distribute symmetrically on both sides of 90°. After the test, the azimuths of the particles are mainly obtuse angles (150–180°) and the long axis of the particles generally points in the opposite direction from the shear-band. The sand particles undergo four stages: random arrangement during initial sample preparation, compaction under normal stress, particle rotation during shearing, and ordered alignment after shearing. The test results help to reveal the movement mechanism of calcareous sand at the mesoscopic level during the direct shear process.