A creep constitutive model with variable parameters for thenardite

Results from the triaxial creep tests indicate that the relationship between steady creep rate of thenardite and deviatoric stress can be described with the Heard model. By combining the Kelvin model which reflects the primary creep property of rock with the Heard steady creep model, a Kelvin-Heard (K-H) creep model is constructed. However, the predicted curves from the K-H model are not in good agreement with the creep test data of thenardite. To improve the prediction precision, it is assumed that the viscosity coefficient of thenardite increases over time in primary creep stage, and an improved K-H model is proposed. Compared with the K-H model, the improved K-H model can more precisely describe the creep behavior of thenardite under a certain stress state. However, each of its parameters G, eta(infinity), a and b has different values under different deviatoric stress levels. These four parameters all decrease with the increase in deviatoric stress, which is considered to establish an improved K-H model with variable parameters for thenardite finally. Comparison with the test data shows that this model can precisely predict the creep behavior of thenardite when 25 MPa <= sigma(1) - sigma(3) <= 40 MPa.