A series of loading - unloading consolidation creep experiments are carried out on Nanning unsaturated expansive soils. Lots of stress-time curves of different moisture expansive soils and stress-strain isochronous curves are obtained under different stress levels. Through the various stages of creep deformation analyses

it is concluded that the creep deformation of expansive soil is a nonlinear creep

which contains viscoelastic deformation and viscoplastic deformation. Combined with empirical rheology and rheology model theory

the creep deformation of expansive soil is divided into two parts of linear and nonlinear ones. The creep compliances and the linear viscoelastic modulus decrease; while the linear viscoplastic modulus increase with time increasing. Considered linear viscoplastic model and nonlinear viscoplastic empirical model of expansive soil together

viscoplastic constitutive equation is proposed which can be used to describe the nonlinear rheologic characteristic of expansive soils. According to the experimental data

all parameters of this model under different stresses are determined by nonlinear fitting methods. It is shown that the regression curves calculated using this theory fit well with creep test results. The conclusions provide a reliable theoretical basis for the rheological model of expansive soils to further study.