Macroscale and microscale triaxial compressive behaviors of loose saturated sand under excess pore water pressure generation

Investigating the triaxial compressive behavior of loose saturated sand under excess pore water pressure generation, based on both macroscale and microscale observations, contributes to a better understanding of the instability mechanisms associated with the non-localized failure mode. In this study, the original intention was to conduct undrained triaxial tests on dense and loose saturated sands. However, due to the unavoidable limitations associated with X-ray CT imaging, the tests were consequently conducted under a partially drained condition. Nevertheless, the use of X-ray CT imaging, combined with image analyses, revealed clear differences in both the macroscopic responses and microscale structural evolution between dense and loose saturated sands that exhibit localized and non-localized failure modes. The loose sand examined in the present study showed a gradual increase in deviator stress until the end of shearing. During this process, the local void ratio gently decreased and the number of particle contacts gently increased due to compression. The particle-contact orientation did not change significantly from the initial state to the end of shearing. Shear strain was found to be relatively uniformly distributed over a wide region in the loose sand specimens. It is likely that these deformation characteristics of loose sand correspond to positive second-order work, indicating the stable state of the material. This behavior was clearly different from that of the dense sand specimens, for which significant dilation, a decrease in the number of particle contacts, and a change in the particle-contact orientation that occurred, were associated with a localized failure mode characterized by negative second-order work.