A non-associated constitutive model with flexible surfaces for modelling the cohesive-frictional structure degradation of soft rocks and fine-grained soils

In this paper, a new hierarchical constitutive law is proposed for reproducing both soft rocks and natural fine-grained soils behaviours. The wide applicability of the model is mostly related to the flexible shape of the yield surface as well as to the plastic-driven softening law accounting for the structure degradation of both cohesive and frictional components of strength. Other important features of the model are the non-linear stress-dependent hyperelastic formulation and the non-associated flow rule, this latter controlled by a plastic potential having the same degree of flexibility of the yield surface in the meridian plane. The constitutive model is implemented in a Finite Element code by means of an explicit modified Euler scheme with automatic sub-stepping and error control. The predictive capability of the model is first discussed with reference to single element tests. The constitutive formulation proves to be able to describe specific features of the mechanical behaviour of structured clayey soils and soft rocks which all together cannot be typically reproduced by the classical elasto-plastic models available in commercial packages, such as the cohesive and frictional shear-induced destructuration or the contractive tendency associated with a strain-softening response. The final part of the paper is dedicated to the analysis of a boundary value problem involving an embankment construction on a fine-grained deposit