SUMMARYIn this paper a micro-polar continuum approach is proposed to model the essential properties of cohesionless granular materials like sand. The model takes into account the influence of particle rotations, the mean grain size, the void ratio, the stresses and couple stresses. The constitutive equations for the stresses and couple stresses are incrementally non-linear and based on the concept of hypoplasticity. For plane strain problems the implementation of the model in a finite element program is described. Numerical studies of the evolution of micro-polar effects within a granular strip under plane shearing are presented. It is shown that the location and evolution of shear localization is strongly influenced by the initial state and the micro-polar boundary conditions. For large shearing the state quantities tend towards a stationary state for which a certain coupling between the norm of the stress deviator and the norm of the couple stress tensor can be derived.

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The paper presents a simple constitutive model for normally consolidated clay. A mathematical formulation, using a single tensor-valued function to define the incrementally nonlinear stress-strain relation, is proposed based on the basic concept of hypoplasticity. The structure of the tensor-valued function is determined in the light of the response envelope. Particular attention is paid towards incorporating the critical state and to the capability for capturing undrained behaviour of clayey soils. With five material parameters that can be determined easily from isotropic consolidation and triaxial compression tests, the model is shown to provide good predictions for the response of normally consolidated clay along various stress paths, including drained true triaxial tests and undrained shear tests.

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As shear occurs along a soil-structure interface, a localized zone with a thickness of several grain diameters will develop in soil along the interface, forming an interfacial layer. In this paper, the behaviour of a soil-structure interface is studied numerically by modelling the plane shear of a granular layer bounded by rigid plates. The mechanical behaviour of the granular material is described with a micro-polar hypoplastic continuum model. Numerical results are presented to show the development of shear localization along the interface for shearing under conditions of constant normal pressure and constant volume, respectively. Evolution of the resistance on the surface of the bounding plate is considered with respect to the influences of grain rotation.

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.