Abstract: In order to characterize the rheological properties of surrounding rocks after the excavation of circular tunnel, theoretical solutions of radius, stress and displacement of the plastic zone for surrounding rocks after the excavation of circular tunnel are derived by using the Drucker-Prager yield criterion and considering the viscosity and dilatancy characteristics of plastic zone, assuming the constitutive model of surrounding rocks as the Nishihara model. When the dilatancy angle is 0, the solutions change into viscoelastic-viscoplastic solutions based on the Nishihara model and Mohr-Coulomb criterion under the assumption of constant volume. The effects of dilatancy angle on the radius of plastic zone, tunnel-wall displacement and stress field are analyzed. The solutions for viscoelastic-viscoplastic displacement and viscoelastic-plastic displacement are compared and analyzed. The results show that before the surrounding rock reaches the steady state, the effects of dilatancy angle on the stress field and the radium of plastic zone are relatively smaller while the effects of dilatancy angle on the tunnel-wall displacement is relatively larger. The stress field and the radium of plastic zone for stable surrounding rocks are independent of the value of dilatancy angle, however, the tunnel-wall displacement of steady-state rock increases obviously with the dilatancy angle. When the dilatancy angle is large, the viscosity of plastic zone should be considered; otherwise the tunnel-wall displacement of steady-state surrounding rocks will be underestimated. The research results are of certain reference value for practical engineering.

Key words: viscoelastic-viscoplastic, Nishihara model, dilatancy, Drucker-Prager yield criterion, theoretical solution