A unified constitutive model for natural soils and a novel stress integration algorithm with the trust region method

Abstract

Constitutive models and stress integration algorithms are an essential part of computational modelling in geotechnics; selecting an appropriate constitutive model and a reliable stress integration algorithm are usually the key to successfully predicting the behaviour of geostructures. This paper develops a unified constitutive model, i.e., CASM-PSS, for natural structure clay and sand within the critical state framework, considering the influence of noncoaxial behaviour. By introducing the inclined yield surface, incorporating the influence of direction of stress increment on plastic flow rule, and introducing the super-subloading surface in the unified clay and sand model (CASM), the proposed model able to unified describe the mechanical behaviour of natural clay and sand. Novelty of the proposed model lies in its ability to consider the effects of anisotropy, destructuration, and direction of stress increment of natural soils. The CASM-PSS is then implemented into a finite difference software through the user-defined model platform (UDM), by a new semi-implicit integration algorithm integrated with the trust region method. Performance of the CASM-PSS is investigated and validated by a series of experimental and DEM evidence on soils under different loading paths, including drained and undrained conditions of triaxial compression and simple shear tests. Then, the newly proposed model is numerically applied to simulate centrifugal model testing and practical engineering problems, i.e., China Tsinghuayuan Tunnel, to identify effects of soil structure, anisotropy and noncoaxiality in boundary value problems.