Effects of stress histories and drainage conditions on inception of strain localization in unsaturated soils

A diagnostic method to detect the inception of strain localization, which is based on the singularity of a total acoustic tensor, was developed for variably saturated soils. The effective stress approach was used to establish coupling between the multiphase fluid flow and deformation fields, while isotropic hardening/softening due to changes in suction and plastic volumetric strain were accounted for. To this end, several fundamental testing conditions were investigated, including Conventional Triaxial Compression (CTC) and Extension (CTE), Plane Strain Compression (PSC) and Extension (PSE) under drained, constant water content, and undrained loadings. It was found that, for the silty soil and stress histories considered, suction can completely prevent the onset of strain localization in undrained CTE tests and partially prevent the onset in constant water content and drained CTE tests. Furthermore, while suction can significantly affect the inception of strain localization in all PSC tests, it has almost no effect on the onset of strain localization in all PSE tests. Finally, suction affects more significantly the orientation and mode of shear bands at the inception of strain localization in extension tests than in compression tests.