Evaluation of a constitutive model for unsaturated soils: stress variables and numerical implementation

Abstract

This paper presents an evaluation of a constitutive model for unsaturated soils based on the BBM (Alonso et al. 1990). The focus of the paper is on the stress variables used and on the numerical algorithms adopted. Conventional stress variable approach (net stress and suction) as well as the approach that takes into account the degree of saturation (Bishop’s stress and suction) are examined. To solve the constitutive stress–strain equations, two stress integration procedures have been implemented, an explicit stress integration scheme with automatic substepping and error control techniques (Sheng et al. 2003) and a fully implicit stress integration scheme based on the Backward-Euler algorithm with substepping (Pérez et al. 2001). A triaxial test on compacted Barcelona clayey silt performed by Barrera (2002) is analysed to examine the effects of stress variables and of numerical algorithms. Relevant features of different stress variable formulations to simulate experimental data are discussed. Particular attention is given to the strong dependency of Bishop’s stress on the shape of the soil water retention curve (SWRC). SWRC shape changes can lead to the appearance of an inflection point in the stress path (plane p′-s) that may result, even for a simple test, in the elastic prediction of the stress path crossing the yield surface several times.