Abstract: In order to solve the problem of nonlinear creep characteristics of surrounding rock in tunnel excavation under high ground stress,taking metamorphic sandstone as the research object,triaxial creep tests were carried out for two different loading methods of axial pressure and grading unloading confining pressure and axial pressure and fixed confining pressure to study the creep properties of metamorphic sandstone. The results show that the creep behavior of metamorphic sandstone shows obvious nonlinear characteristics,and the traditional linear model is difficult to accurately describe its deformation law,especially in the accelerated creep stage. Therefore,assuming that the change of viscosity coefficient with time conforms to the law of power function,an improved fractional nonlinear viscosity model is proposed. An improved nonlinear damage viscoelasticplastic body is proposed based on the Kachanvo damage variable theory by introducing a damage factor,and a nonlinear damage creep model is developed. This model consists of a series combination of an enhanced Maxwell model and the improved nonlinear damage viscoelastic-plastic body. There are four parameters in the stable creep stage and seven parameters in the unstable creep stage. Compared with the experimental data,the established nonlinear creep damage model generally has a high fitting degree with the experimental curve,especially in the accelerated creep stage,the prediction effect of the model is significantly better than that of the traditional model. The research results provide a theoretical basis for the stability analysis and engineering design of tunnel surrounding rock under high ground stress conditions,and have broad application prospects. 

Key words: fractional diffusion order,metamorphic sandstone,creep damage,accelerated creep,creep test