Abstract: When the stress reaches a certain level, the rock would undergo accelerated creep failure after the attenuation and stable creep processes, but the traditional Nishihara model could not reflect the accelerated creep stage effectively. In order to solve this problem, a non-linear rheological element is introduced and connected in series to the traditional Nishihara model to form a new six-element model, which could fully reflect the three stages of the rock creep. Based on the improved constitutive equation of the Nishihara model and the Laplace transformation, the variations of the radial displacement of the surrounding rock of the shield circular roadway are obtained. Based on the measured data of the convergence deformation of the surrounding rock of the roadway, the viscoelastic-plastic creep parameters of the rock are obtained by inversion. The comparison between the calculation results of the radial displacement of the surrounding rock and the measured data shows that when $t\leq 25$ d, the surrounding rock of the roadway is in the stage of the accelerated deformation, and the rate of change of the radial displacement is increased with time; the surrounding rock of the roadway enters the stage of the stable deformation when $t>25$ d, and the rate of change of the displacement of the radial surrounding rock is gradually stabilized with the passage of time.

Key words: modified Nishihara model, nonlinear viscoelasticity, parameter inversion, roadway surrounding rock