Analysis of the failure mode and softening behaviour of sands in true triaxial tests

This paper presents a theoretical analysis and a numerical study of the failure mode in sands based on a hypoplastic description of the material. A bifurcation analysis is performed to study the failure mode in sands under true triaxial stress conditions at the material response level. It is shown that either uniform failure or localized failure can occur, depending on the intermediate principal stress. Uniform bifurcation occurs in tests along stress paths with a lower Lode angle, including the stress path for a conventional triaxial compression test. Localized failure occurs in tests along stress paths with a higher Lode angle, including stress paths for biaxial compression and triaxial extension tests. Specimen responses under laboratory conditions are then studied numerically by 3D FE simulations of biaxial compression, triaxial compression, and triaxial extension tests. The effects of initial imperfection from a homogeneous state are taken into account by introducing a frequency distribution of the initial void ratio. This permits the softening behaviour and shear localization in sand specimens to be analysed for different degrees of initial heterogeneity.