Effect of strain localization on the response of granular materials subjected to monotonic and cyclic triaxial tests

This paper discusses about the influence of strain localization on the critical state of sand determined by triaxial tests. Bearing this goal in mind a comprehensive experimental study has been carried out on three different granular materials, two homogeneously distributed quarzitic sands and a steel spheres assembly tested in axisymmetric condition. The analysis is logically structured in two parts, the first reporting the outcomes of a classical phenomenological analysis developed by subjecting reconstituted specimens to conventional axisymmetric compression and extension tests in a standard triaxial apparatus, the second reporting similar tests performed with x-ray scanning with results processed by Digital Image Correlation and porosity measurement tools. Aim of this second segment is to detect the deformation mechanisms taking place at the particle scale. All experimental results confirm the natural tendency of granular media to concentrate deformation within narrow portions of the specimens, even though the exterior appearance of the specimens might suggest that uniform deformation is taking place. An important consequence is represented by the limitation on the classical inference of critical state parameters from triaxial tests. In fact, while a clear dependency of the critical void ratio on the followed stress path is seen at macroscopic level, a unique critical state locus is obtained when porosity is measured locally in the soil portions where shear strains concentrate. The analysis of the strain fields, extended to cyclic tests involving alternated compression and extension, shows the role of the principal stress orientation on the yielding mechanisms and the sharp transition from one pattern to the other produced by the reversal of loading.