Combined study of creep and ageing under deviatoric loading in granular materials

Granular soils creep and age. Previous findings on the time-dependent phenomena under deviatoric stress are summarized and extended with the results of an experimental investigation. Multi-stage triaxial compression tests with creep phases at different deviatoric loading on medium-dense and dense samples of a uniformly graded silica sand confirm an increase in stiffness after creep phases. Contact maturing, contact homogenization, and stabilization of the soil structure are known causes for ageing reported in the literature. As other results found in the literature, the volumetric creep behavior can be dilatant, contractant or of negligible strain close to zero and depends on the trend of the volumetric strain resulting from deviatoric loading at the beginning of creep. By the triaxial tests it is shown that dilatant creep results in an increase of the radial strain due to grain rearrangements. The axial strain rates during creep and changes of the small-strain shear modulus (ageing) follow a power law with time. According to the experiments, the exponent of the proposed power law describing the development of strain and shear modulus at small strain during creep is independent of the density and stress state. The small-strain shear stiffness and the associated soil structure at the onset of creep determine the subsequent ageing behavior. A linear dependency was found between the related ageing rates and axial strain rates during creep, which can be used to predict ageing of granular materials in combination with rate-dependent constitutive models.