Incremental stress and microstructural response of granular soils under undrained axisymmetric deformation

The influence of the micromechanics on the response of stress and fabric on strain is an essential part of a constitutive model for granular matter, because it contains the information of how different deformation paths (history) affect the actual mechanical state of the system. Here, we study a granular aggregate that is first isotropically compressed and then subjected to large axisymmetric strain extension at constant volume. At various axial strain amplitudes, we determine the stress and fabric incremental response envelopes that result from the application of strain-probes with identical total magnitude but different contribution of isotropic relative to deviatoric strain (which span an orthogonal two-dimensional subset in "strain-space", in which the "direction-angle" defines the relative magnitude of the two contributions). For very small strain increments the fabric response is null in shear direction, i.e. the structure does not change and the elastic stress-response is probed. Larger strain are associated with rearrangements of contacts probing the combined plastic stress-and fabric-response. With increasing shear-induced anisotropy, the centers, inclinations and the thickness of the response envelopes in the stress-space change, involving the maximum and minimum stiffness.