Abstract: The field observations show that the existing widely-used constitutive models tend to underestimate the settlement of dams higher than 200 m. One of the main reasons is the separation of load-induced deformation and creep in such models and the neglecting of creep during construction in numerical simulations. In this study, a constitutive model, taking the stress, strain and time as the basic variables, is proposed in the light of experimental observations to model the loading deformation, creep and stress relaxation in a unified manner. A basic assumption used is the simultaneous occurrence of the load-induced plastic strains and creep strains. Both the change of stresses and the accumulation of time can result in expansion of the yield surface and thus produce plastic strains to obey, however, different flow rules. The model is used to reproduce the experimental results of a rockfill material and a gravelly soil used in an extremely high rockfill dam, and it is further used to study the deformation behavior of the dam. The numerical results show that neglecting the creep during construction may underestimate the settlement by more than 10%. Therefore, for the extremely high rockfill dams, it is necessary to use the unified models to consider the creep and loading behavior for rockfill materials so as to increase the precision of predictions.