Investigation of instabilities in granular media and their numerical simulation

Abstract

The inception of instabilities in sand across different "length-scales," viz., continuum, discrete, and laboratory element tests, has been highlighted in this study. With instability onset, the material behavior no longer remains "single element? in the sense of continuum mechanics, and due care is required while calibrating different constitutive relationships for use in various numerical simulations. A laboratory "element" test can instead be viewed as a boundary value problem at the instability onset. The post-instability response of a soil specimen represents the "system-response" with the evolution of inhomogeneities being primarily influenced by the boundary conditions among various other factors. Instability onset in drained and undrained biaxial tests has been explored by adopting the bifurcation framework with the aid of a generalized nonassociative elastoplastic material model. For the locally drained globally undrained scenario within a continuum numerical framework, instability onset is found to be influenced by the refinement of mesh discretization. Signatures of rate-dependent localized behavior of sand specimens are also commented on from numerical simulations of drained biaxial tests. To address the "pathological mesh dependence" of classical continuum modeling, we resort to a micromechanical discrete granular framework that considers the actual particle morphology. The numerical predictions match reasonably well with the flexible boundary plane strain experiments. The inherent grain fabric arrangement is found to be the triggering mechanism behind the localized strain accumulation at multiple zones. The influence of loading boundary conditions and various signatures of local nonuniformities is also explored with the aid of image analysis in plane strain and triaxial tests.