Characterizing stress variability within granular samples upon liquefaction

Abstract

A tensor-based approach is adopted to investigate the evolution of stress variability within granular media based on DEM simulations of both cyclic and static liquefactions. The bulk stress dispersion is evaluated using the effective variance of the entire stress tensor field and the local stress fluctuation is quantified by the Euclidean distance between the stress tensor of individual particles and the global stress tensor. The orientation discrepancies between the local stress tensor and global fabric tensor, and between the local and global stress tensors are examined by their joint invariants. We observe that the bulk stress dispersion decreases but the discrepancy of local stress fluctuation increases upon liquefaction. The initially deviatoric stress-dominated stress variability gradually becomes hydrostatic stress-dominated, and returns to deviatoric stress-dominated again after liquefaction. The liquefaction process is characterized by the degradation of coaxiality between the local stress tensor and the global fabric tensor and that between the local and global stress tensors. There exists a range of threshold values for such coaxiality, below which liquefaction occurs. Particles with coaxiality higher than the threshold value and particles with coaxality lower than the threshold value exhibit different roles in sustaining external loads. However, the discrepancy becomes neutral upon liquefaction.