An internal-variable creep model for soils

Soil creep is such a process that the deformation of soil develops with time under a state of constant stress. Following the tradition of continuum mechanics, the time-dependent creep rate is always employed in the investigations of the soil creep. However, in practices, soils undergo complex loading-unloading cycles and are allowed to creep on the stages at different stress and strain levels. In this complex situation, the initial states (or configurations) of the creep stages are different and thus the traditional continuum mechanics is incapable in describing the soil creep. Different from the traditional continuum mechanics, the internal-variable theory is based on the concept of internal variable, which describes the internal structure of materials. In this study, an internal-variable creep model is proposed, in which the creep rate of soils is dependent on not only applied stress, but also irreversible strain, which is adopted as internal variable for soil deformation. A series of laboratory tests have been performed to verify the proposed creep model. The parametric relation between the proposed internal-variable and the traditional power law creep models is derived. A method to accelerate creep tests is proposed based on the proposed creep model. It is found that the proposed internal-variable creep model is capable in describing the soil creep under complex conditions.