Abstract: Taking the unloading as a dynamic process， the plastic zone generated from different excavation sections and in-situ stress conditions due to transient unloading was analyzed using modified Mohr-Coulomb criterion including tension cut-off and elliptic cap models in surrounding rock. The numerical results indicate that transient unloading will cause plastic deformation in the vicinity of excavation perimeter. The plastic deformation increases with the increase of depth and in-situ stress. Tensile stress is a key factor in accounting for the surrounding rock plastic deformation during transient unloading. The plastic deformation direction coincides with the maximum compressive principal stress. But it can induce serious compressive deformation in the minimum principal stress if the in-situ stress satisfies certain condition. The bigger the section curvature is， the smaller the plastic deformation becomes， so it is better to avoid linear boundary.

Key words: rock dynamics, plasticity, excavation, transient unloading, yield criterion