Exploring the critical state properties and major principal stress rotation of sand in direct shear test using the distinct element method

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This paper provides insights into the critical state properties and major principal stress rotation of sands in direct shear tests using the distinct element method (DEM), in which a three dimensional contact model considering rolling and twisting resistances proposed by the authors was implemented. Firstly, the DEM was used to simulate a series of direct shear tests. Then the macroscopic characteristics of the DEM samples and the critical state properties within the shear band were analyzed, including the major principal stress rotation and the stress level dependency of peak strength. Finally the effect of the grain angularity was investigated. The rolling and twisting resistances were separately considered to check their individual effects. The results demonstrate the strong capability of DEM with the 3D contact model to capture the mechanical behavior of sands in direct shear tests. After a global shear strain of 15%, the shear zone reached the critical state while the whole sample did not. The major principal stress rotation angle evolved in a similar trend to that of the shear stress, and the stress level dependency of the peak strength of dense sand can be attributed to the fact that the vertical stress affects the rotation of the major principal stress. Higher shear strength, more obvious dilatancy and upward move of critical state line on the $$e\hbox {-lg}p$$ plane were obtained when larger values of shape parameter (leading to greater rolling and twisting resistances) were used. And twisting resistance is much less important relatively in the presence of rolling resistance.