Abstract: Sand is a kind of typical granular material. Its mechanical and deformation characteristics are macro phenomenons of its meso-particles under loading. In view of the sand with graininess and fragmentation, the meso-structure level research on sand is necessary. In this research, the soil particles are simplified as ellipsoids. The direction angles of the soil particles obey normal distribution. The friction characteristics and failure mechanism of sand material are studied at meso-particles level based on meso-statistics. The friction characteristic of sand is composed of the resistance caused by the rise of the sand particles around the plane which is normal to the shear plane. The angle between the particle movement direction and the shear plane is considered as the friction angle of a single particle. The friction angle and direction angle meet the same normal distribution law. The shear failure process of sand composes of particle rearrangement and internal stress redistribution. During this process, some particles friction angles gradually become weak and the number of particles sustaining load increases. Moreover, the distribution of sand internal stress gradually become uniform. With the increases of the vertical load, the internal friction angle of the sand decreases. The relationship between the internal friction angle and the applied load can be therefore quantitative calculated.

Key words: meso-statistical, normal distribution, shear capacity factor, friction angle decrease