Perturbation Intensity and Mesh Convergence in Coupled Undrained Instability Analysis in Sands under Biaxial Loading

The influence of perturbation magnitude and extent of area on the emergence of localized instability in sand specimens subjected to biaxial loading has been explored within a numerical framework using a generalized 3D material model. In this regard, this article seeks to examine the effect of perturbation type in terms of material imperfection and subsequently attempts to characterize mesh dependency on localization onset in sands. Coupled undrained transient (globally undrained locally drained) analysis of Hostun RF sand has been performed to investigate the onset of instability by inducing material heterogeneity within the numerical model considering small strain formulation. For a given magnitude of perturbation, it is observed that instability onset requires a specific mesh convergence study, and with mesh refinement it practically becomes asymptotic to the theoretically predicted value estimated by bifurcation analysis. Perturbation magnitude significantly influenced onset strain of instability when compared with the effect of extent of the area for a fixed perturbation magnitude. However, all the sand specimens revealed emergence of localized instability modes in both cases.