Based on the unified strength criterion with parabolic failure envelope

the nonlinear improved expression of the two-parameter twin shear unified criterion is obtained. Elastoplastic analysis of pressure tunnels is presented by considering the seepage and intermediate principal stress. The results show that the intermediate principalstress parameter b in unified strength theory has significant effect on the radius of plastic zone. The radius of plastic zone is decreased along with the increase of the intermediate principal stress parameter b. An example is analyzed and contrasted with and without considering seepage. The radius of plastic zone with considering influence of seepage is larger than that without considering the seepage influence. The results also indicate that the parameter b and seepage have significant influences on tangential stress and radial stress. The results obtained by the original linear expression and the nonlinear improved expression of the two-parameter twin shear unified strength theory are large difference. In tunnel design

the unified strength theory parameter should be reasonably adopted according to surrounding rock material properties. So as to provide theoretical supports for tunnel design.