AbstractThe undrained response of sand under complex loading conditions involving principal stress rotation (PSR) is of particular interest in practical engineering. Although this subject has been studied extensively through laboratory experiments, more in-depth investigations of the microscopic mechanism underlying the macroscale observations under PSR have not been reported adequately. The discrete element method (DEM) plays an important role in the investigation of the elementary behavior of sand subjected to various complex loading conditions. It could enable us to comprehend the evolution of particle-scale quantities. Therefore, an advanced discrete element approach that can apply an arbitrary undrained loading path is implemented in this study. Based on this approach, numerical algorithms that implement undrained rotational shear are elucidated, and undrained pure PSR tests are conducted on anisotropic specimens with varying stress ratios, densities, and intermediate principal stress ratios. The evolution of the fabric anisotropy of specimens under PSR is quantified by a contact normal fabric tensor. The macroscopic mechanical results are found to be consistent with the experimental results. The interplay between fabric evolution with stress and strain increments is examined. The findings provide effective microscopic insights into the anisotropic responses of granular materials under rotational shear.