Effect of confining stress and lateral boundary conditions on the drained instability response of sand: a DEM based assessment across the length scales

Instabilities in geomaterials significantly influence its strength mobilization and often act as a precursor to failure in geotechnical structures. Such instabilities in a granular assembly can vary drastically from localized to a diffused mode owing to the changes in density state, i.e. from a dense to a loose one. The present study elucidates the effect of confining stress and lateral boundary conditions on the mechanisms associated with these instability modes through particle-scale observations and relates it to the macro-scale shearing response of the specimen with a particular focus on the density state. In this regard, 2D DEM simulations of drained biaxial test have been carried out for the dense and loose sand specimens at different initial confining stress, and under both flexible and rigid lateral boundary conditions. Analysis of force chain network indicates that the formation of higher number of elongated strong force chains increases the susceptibility of local buckling of strong force chain in dense specimen, resulting emergence of shear bands. Conversely, buckling of a large number of strong force chains of smaller lengths, distributed across the loose specimen, causes diffused instability as can be perceived from the scattered large particle rotation and relative displacement field. Assessment on fabric anisotropy reveals that, with increasing confinement, the weak force chain network provides enhanced lateral support to the strong force chain network and retards the force chain buckling process, resulting in a reduction in the shear band thickness and delayed peaks in the macro-scale strength response. Shear band thickness and inclinations are noticed to be higher for rigid lateral boundaries, which due to constraint kinematics leads to a constant shear band inclination regardless of the level of confinement.

Graphical abstract