A three‐dimensional (3D) anisotropic plasticity model for sand is formulated in this study to provide a constitutive description for both radial and principal stress axes rotation (PSAR) loading‐induced behavior under various conditions with a single set of model parameters. The model has zero elastic range, with plastic loading and flow direction dependent on both current stress and stress rate direction. Fabric tensor is introduced along with its evolution to achieve anisotropic plastic modulus, dilatancy, and flow rule formulations. Increase in plastic modulus under continuous PSAR achieves eventual convergence of strain accumulation. A unique decomposition of dilatancy controls the overall contraction and periodic dilatancy oscillation under PSAR. The performance of the model is first thoroughly evaluated based on drained/undrained, monotonic shear/PSAR tests on Toyoura sand, showing its effectiveness in reproducing the behavior of real sand. Discrete element method numerical test results are then adopted for comprehensive calibration of the model parameters, and then for validation of the model under 3D PSAR in any arbitrary direction. These comparisons highlight the model's capability in simulating the behavior of granular soil under 3D stress paths.

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The shape of particles can have a great impact on the mechanical behavior of granular material, which is not only limited to general shape measures such as sphericity and roundness, but also applies for small scale measures such as surface asperities. In this study, triaxial tests on 3D printed particles and DEM tests show that small surface asperities on spherical particles can contribute significantly to the peak strength and dilatancy of granular material, which cannot be reflected by simply altering the traditional contact parameters in DEM. Using polydispersed particles with artificially designed surface asperities in the form of bumps for laboratory and DEM triaxial tests, both macro and micro scale behavior of granular material are analyzed to reveal the influence of these small surface asperities. The result shows that surface asperities can change the coordination number, anisotropy, and normal contact force on the micro scale, increasing the interlocking between particles and restricting particle rotation, which leads to the increase of peak strength and dilatancy on the macro scale. Although the commonly adopted approach of introducing rolling resistance to reflect shape influence can help to enhance the simulation of stress‐strain relationship, especially for particles with relatively small artificial asperities, the micro scale influence of rolling resistance and surface asperities are found to be different as the size of the asperities increase.