Modelling stress-induced anisotropy in multi-phase granular soils

Abstract

In this paper, we outline a constitutive model capable of describing anisotropy and many other features of the behaviour of multiphase granular soils, together with the computational framework that enables its numerical implementation. The constitutive model is formulated within the framework of bounding surface plasticity. It can simulate monotonic and cyclic loading for a wide range of stress and saturation states, it includes enhanced descriptions of wetting and drying processes, of anisotropy and of changing compressibility. These features are captured using a single set of parameters by using a combination of isotropic and kinematic hardening. The model is formulated based on the concept of effective stress for unsaturated states that guarantees smooth transitions between unsaturated and fully saturated states. Furthermore, we present unified formulations for saturated and unsaturated states in which the isotropic hardening law and the critical state line are described in a bi-logarithmic space defined by the logarithms of the mean effective stress and void ratio. Moreover, the constitutive model is coupled with a soil water characteristic model that allows consideration of the hysteretic nature of the saturation degree changes upon wetting/drying reversals. The paper describes the numerical implementation, which includes several smoothing techniques to enhance the constitutive model’s performance in numerical modelling during transitions between kinematic hardening and isotropic hardening and drying/wetting reversals. The numerical implementation also includes automatic error control and sub-stepping techniques, suitable for explicit integration algorithms, that give users additional control over the accuracy and speed of the analysis. Lastly, several examples are provided to demonstrate the range of application of the computational framework.