Abstract: The macro- and micro-numerical simulations of biaxial shear tests for ideal granular samples with different initial densities were carried out using discrete element method. Loop structures, represented by Voronoi polygons, were taken as the basic meso-mechanical unit of granular material by mesh-subdivision. The evolution of numbers, geometrical morphology and mechanical characteristic for different loops were simulated. The meso-mechanical characteristic at critical state for granular media was especially analyzed. Numerical results show that the evolution of higher-order loops and lower-order loops are different. The evolution of same types of loops in granular samples with different initial densities are different before critical state reaching. The number proportions, geometrical morphology, contact forces of side particles and inner sliding rates of same kinds of loops in granular samples with different initial densities have reached critical state respectively. From mesoscopic point of view, critical state of granular media is the consequence of mutual transformation for higher-order loops and lower-order loops, and is the combined average and external manifestation of all loops. Meso-scale loop structures under a state of dynamic equilibrium at critical state reflected by the constant shear stress and development of shear deformation under constant volume on macro-scale. Numerical results also show the evolution of numbers, geometrical morphology, mechanical characteristic and contact stabilities of meso-scale loop structures, are closely related to the development of strength, dilatancy and critical state for granular material. So it can be taken as the basic unit of meso-mechanism for granular media.

Key words: granular material, meso-scale structure, critical state, discrete element method