Based on an expression with a fractional calculus form,a new four-element nonlinear viscoelasto-plastic rheological model was proposed,by using the Riemann-Liouville fractional calculus theory and drawing ideas from the classic element combination modeling.The constitutive equation and creep equation of the model were given, in which the creep strain rate at the first,second and third stages can be effectively controlled by adjusting creep parametersβ1andβ2,respectively.The prediction of the model on existed data was examined,and the result showed that the nonlinear rheological model can effectively describe three steps of rock creep with comparable prediction accuracy to existed theoretical models. It was found that in a lower stress level,the model can reflect the primary creep and steady creep,while if the stress level exceeds the long-term rock strength, the model will reflect the accelerated creep. Meanwhile, the use of the soft elements plays a role to reduce the element number and parameters in the model.
