Abstract: Up to now， the tensile failure in highly stressed rock after excavation， resulted from unloading， is a common phenomenon， but not well theoretically explained yet. Taking the unloading as a dynamic process， a plasticity model for transient unloading was developed to study the failure mechanism in the surrounding rock based on the theory of continuum elastoplasticity using the finite element analysis (FEA) with a commercial software COMSOL Multiphysics. The results show that excavation can trigger strong disturbance in surrounding rock due to the transient release of insitu stress. The stress redistribution is a dynamic process transiting from dynamic stress response to quasistatic stress condition. The transient unloading can cause tensile stress near the excavation surface. Although the duration of transient tensile stress is short， its amplitude is small and the affected domain is limited， the tensile stress may result in tensile failure because the uniaxial tensile strength of rock is far lower than its compressive strength. It is also denoted that the lateral inertia effect may be one reason for this phenomenon.

Key words: rock mechanics, excavation, transient unloading, dynamic, failure, plasticity