||The micromechanical behavior of granular samples in direct shear tests using 3D DEM|
Direct shear test is known as one of the most generally-performed laboratory tests in geotechnical engineering area. For granular material, the friction angle can be determined by obtaining the peak or residual shear strengths under different normal stresses from this test. However, our understanding of the microstructure and micromechanical behaviors of granular material under specified stress conditions in the direct shear tests is very limited because of the design of the direct shear box itself. In this study, a simulation of the direct shear test in the 3-dimensional discrete element method (3D-DEM) model is conducted to evaluate its micromechanical behaviors during direct shear simulation.
Huang et al. (2015)  have conducted a series of studies for direct shear simulation through DEM under the two dimensional (2D) condition. It was shown that the stress paths in dense granular material are very different from those in loose granular material. The stress path variations in the dense material are quite uniform and less complicated compare to the loose material. By contrast, the stress states in the loose material during direct shear tests are really complex and erratic. The simulations of the direct shear test in 2-dimensional DEM have evidently pointed out the potential to apply DEM for providing an in-depth understanding of the micromechanical behaviors of granular materials. Considering that there are some restrictions in the 2D-DEM model because real granular material should be spheres instead of circular plates. In this study, 3D DEM model of the direct shear test was employed to evaluate and compare to what observed in 2D DEM model by Huang el al. 2015.
A comparison of the micromechanical behaviors between 2D and 3D DEM models was also performed to understand the effect of dimension in the variations of micromechanical behaviors in such tests. The analysis result shows the substantial advantages of the 3D DEM because behavior of real soil particles is simulated directly. For example, there is a significant increase in friction angle of particle assembly (from 260 in 2D model to 450 in 3D model). The main reason was attributed to the interaction of particles in model, especially in the out-of-plane direction. The contact forces in 2D simulation can only exist in the shear direction, not in the out-of-plane direction. However, in the 3D simulation, the contact forces could exist in the third dimension, which contributes to much more shearing resistance during direct shear simulation. The additional shear forces might come from the resistance forces at other planes (parallel to the 2D shear plane). The numerical model showed an obvious evidence of non-uniformity of stress under various normal stresses. Most of contact forces were distributed between the lower-left and upper-right wall of the direct shear box because the shearing was performed with the lower part of the box moving to the right during the shear time. The distribution of contact forces at the shear plane was more uniform than other places.
Key words: direct shear test, dense granular material, 3D DEM, contact forces, stress paths.
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