A nonlinear viscoplastic double yield surface constitutive model for geologic materials

A new constitutive model is proposed to describe the stress-strain-time behaviour of clays and organic soils. This approach is based on the framework of overstress viscoplastic theory and can be applied to any elastoplastic (inviscid) model.The most innovative idea of the proposed model is the identification of two components of strain for each deformation mechanism. In particular, in both the elastic and the plastic regimes a fast and a slow strain mechanism are assumed to be present. To this aim the constitutive model is based on two yield surfaces based on overstress viscoplasticity theory: one is quasi-instantaneous (for the fast part of plastic deformations) and the other is viscous (for the slow part of plastic deformations). This assumption permits the reliable simulation of the mechanical behaviour of a wide range of clayey soils: from inorganic clay (a.e. kaolinite and bentonite), to organic clays and peats. A further interesting aspect of the proposed model is the capability of simulate both the normally consolidated and the overconsolidated regime within a unique constitutive approach. An extensive experimental program on three different peaty soils has been performed to validate the constitutive model. The experimental tests include a wide range of oedometric tests (with very different loading conditions), and a series of triaxial tests conducted on NC and OC peats. The model has been validated also by simulating the settlements of a real embankment founded on a thick layer of organic clay.