Abstract: In order to study the micro shear mechanical properties of clay during the shear process, a micro mechanism analysis of the shear mechanical properties of this silty clay was conducted using laboratory test results and discrete element simulation methods. Based on the results of experiments, a PFC^2D model that conforms to the shear test characteristics of clay was established to compare and analyze some micro parameters of clay and sand during the shear process. Then micro stress, micro deformation, and micro displacement of clay at different shear stages were analyzed, and the macroscopic shear mechanical properties of the sample were analyzed from a micro perspective. The results show that the shear stress in clay is mainly provided by the force chain between particles. The particles in clay are smaller and the force chain is finer, resulting in a lower bearing capacity. The maximum bearing capacity of the force chain in clay is only about 0.77 kN, while standard sand can reach 1.75 kN. Under the condition of overconsolidation, clay exhibits significant strain softening during shearing, which is related to the directional arrangement of particles and the reduction in the number of strong chains between particles. During the shearing process, the middle coordination number of the clay decreased by 1.70, which is due to the displacement of the clay particles, and an arch like structure between the particles were formed to increase the porosity, which results in an increase in the volumetric strain of the clay. During the shearing process of clay, the contour lines of particle displacement form an approximately diamond shaped area in the middle region of the PFC^2D model, and the shear band of clay is a narrow strip with relatively consistent displacement when sheared.

Key words: clay, sand；discrete elements, volumetric strain, porosity, shear band, PFC2D model