Abstract: Based on the triaxial creep curves of silty mudstone, the linear Burgers model is selected to describe the characteristics of decaying creep and steady creep of the rock. When the nonlinear viscoplastic component is connected with the Burgers model in series, a new nonlinear viscoelasto-plastic constitutive model is obtained, namely nonlinear Burgers model, which can describe the accelerative creep of the rock well. The Levenberg-Marquardt algorithm is adopted to identify the model parameters, and the Burgers model parameters of the rock under different stress levels are obtained. Comparing testing curves with fitting curves, it is shown that the linear and nonlinear Burgers models are suitable to describe the three creep stages of rocks. The identification results show that the model parameters identified by the axial strain are different from those identified by the radial strain at the same stress level. Therefore, the creep model parameters of silty mudstone should be selected by considering the anisotropic characteristics and the differences of the creep strain and the creep rate between the axial and radial directions of the rock. Under the first seven stress levels, the Burgers model parameters identified by the axial strain should be chosen as the creep parameters of the rock. Under the eighth stress level, the Burgers model parameters identified by the radial strain should be chosen as the creep parameters of the rock. Under the failure stress level, the nonlinear Burgers model parameters identified by the radial strain should be chosen as the creep parameters of the rock.

Key words: creep, constitutive model, nonlinear Burgers model, parameter selection, anisotropy, stress level