Unstable creeping in geomaterials

Time plays a dominant role in many areas of geomechanics, such as in the assessment of landslide hazard and territorial vulnerability. In this context, the reliable description of the temporal evolution of both soil mass movements and their dependence on loading rate effects is deemed crucial. From a theoretical standpoint, the time-dependent behaviour of geomaterials can be described by means of viscoplastic constitutive theories, also beneficial to foster the objectivity of numerical results from strain localisation simulations. However, not all the aspects of viscoplastic modelling have been so far clearly acknowledged in the literature. For this reason, in this paper the authors focus on creeping strain localization phenomena under simple shear conditions, discussing, from a theoretical point of view, the mechanical condition to be satisfied for tertiary creep to occur.