Distribution and evolution of pore structure in 2D granular materials under biaxial compression

The numerical models for ideal granular with different densities are established using the discrete element method to examine the distribution and evolution of pore structure in 2D granular materials under biaxial compression. The voronoi-Delaunay tessellations are adopted to divide the space into Voronoi regions and the irregularly shaped pore geometry is quantified with a best-fitting ellipse with the aid of the boundary-based method. The micro-mechanism of deformation of granular materials is analyzed according to the evolution of representative single pore. It is found that the pore-size distribution of numerical specimens exhibits a bimodal nature with two peaks occurring at certain pore radius. The pores in samples with different densities show different evolution rules with the increasing strain. The proportion of fine pore increases and that of the macro pore decreases in the loose samples. The evolution of pore structure in dense samples shows exactly the opposite evolution trend. The orientation of the pores is associated with loading direction and the initial density has little effect on it. The pores with long semi-axis along the horizontal direction collapse first, while the pores with long axis direction along the axial loading survive. Thus a more stable particle structure is created. The evolution of single pore structure indicates that strong force chain acts on macro pores. With the deformation of samples, macro pores are divided into a number of fine pores due to particle motion in loose samples and the fore chain becomes more uniform throughout the samples. In dense samples, the small-size pore structures decrease and fuse together to form large ones.