Abstract: In the view of the effect of average particle size on the shear behavior of sand, a meso-level research was carried out by indoor tests and discrete element simulation methods. Based on the results of three direct shear tests with different average particle size, the PFC (particle flow code) particle flow model reflecting the characteristics of sand shear test was established to study in detail the variation characteristics and laws of soil sample changes, force chain network, porosity, coordination number,and other microscopic structural parameters of sandy soil with different particle sizes during the shear process;And the effect of particle size on the macroscopic shear behavior was analyzed from the microscopic perspective. The research results demonstrate that there are significant differences in the sand shear meso-parameters with different average particle sizes, and the differences are mainly reflected in the shear zone. The effect of average particle size of sand on its shear mechanical properties is mainly reflected in its shear strength and dilatancy deformation. The larger the average particle size is, the higher the shear strength is,and the more obvious the dilatancy effect is. There is a certain difference between the soil particle movement law and the morphological change characteristics of the shear zone during the shearing process:the larger the average particle size is, the larger the proportion of up span particles in the shear band is, and the larger the thickness of the shear band is.

Key words: sand, average particle size, direct shear test, discrete element method, shear band