discrete element method (dem)

In this study, the numerical Discrete Element Method (DEM) is used for simulating drained and undrained behavior of 2D polygonal granular materials in order to find critical state line. For undrained behavior simulation, two methods including constant volume and cylinder methods have been used. In the constant volume method, it is assumed that the volume of the soil remains constant during the loading due to the incommensurability of water. In the cylinder method, however, a pipe is considered among adjacent pores that provides the water transformation between them. Undrained simulation was performed for sandy samples at the confining pressure of 200 kPa by both methods. Simulations show that the results obtained by cylinder method have good conformity with those of the constant volume method. As the water becomes less compressible, i.e., stiffer, the results of both methods become closer. Also, the effect of confining pressure on the drained and undrained behavior by constant volume and cylinder methods was investigated. The results of the simulations showed that by increasing the confining pressure, the deviatoric stress and the contraction tendency increase in drained and undrained simulations. To achieve critical state, the simulations were performed until large strains where the deviatoric stress and void ratio become constant. Then the critical state line locus and also its parameters are determined. The results show that the critical state line locus does not depend on the stress path and the simulation method for undrained condition has very little impact on the critical state line locus.

It was believed that adding fines grains to a base coarse-grained soil would result in shear strength increase due to filling up the voids among coarse content and reducing global void ratio. However, this belief was against following experiments. New variables referred to as the equivalent intergranular and interfine void ratios, have been proposed to resolve this problem. In this paper, by using Discrete Element Method, several soil samples were created by mixing two coarse- and fined-grained soils. Simulations were performed in two-dimension in which, the particles are circular. After the soil sample preparation, it was consolidated under isotropic confining pressure followed by biaxial compression loading. The loading condition was continued until the samples reached the critical state. Based on the simulated results, the required parameters to estimate the portion of coarse- and fine-grained parts in the global soil behavior, were obtained. Comparison of the results of the this study with those of experiments shows good agreements. The threshold fines content, which indicates the government of either of coarse or fine-graind parts was also derived. The two-dimensioanl simulations indicate that for each sample with a specified fine content, the corresponding critical state line can be obtained. If the definitions of equivalent intergranular and interfine void ratios are used, unique critical state lines are reachable.

In the present study, discrete element method is used to simulate undrained two-dimensional behavior of soil samples containing polygonal particles. Two methods of constant volume and cylinder were used for numerical simulation of undrained behavior and the results from these two methods were compared. In the cylinder method, it is assumed that there are channels among adjacent pores and the possibility of water exchange among the centers of the pores is provided. In this method, the diameter of the cylinder represents the permeability of the soil. In the constant volume method, it is assumed that the sample volume stays constant during loading. The simulations were done by using both methods. The samples were initially subjected to confining pressure of 200 kPa and after consolidation, they were loaded under deviatoric stress. The results showed that the output obtained by the cylinder method are in good agreement with the constant volume method, and by increasing the stiffness of the water, the results of both methods are closer to each other. Then, the undrained behavior was investigated by using cylinder method in terms of shear strength and pore water pressure in a sand sample at the confining pressures of 200, 400, 800 and 1600 kPa. The trend obtained from the simulation results is in good agreement with laboratory results such that with bigger confining pressure, the shear strength and the positive pore pressure are bigger too, but by increasing confining pressure, the amount of pore pressure decreases in the specimen. The pore pressure distribution contour at the strains of 10% and 30% was presented by means of cylinder method and the effect of cylinder diameter changes on the pore pressure distribution contour was investigated. By investigating the pore pressure distribution contour at the strains of 10% and 30%, it was shown that pore pressure in the sample center has the lowest value. In addition, it was seen that by increasing the cylinder diameter, the variation of pore pressure among the voids becomes smaller, which indicates the uniformity of the pore pressure in permeable soils. By reducing the diameter of the cylinder, the variation of pore pressure becomes more through the sample.