The failures of rock masses are often resulted by sliding or extension of discontinuities. Based on the idea of effective shearing stress

the viewpoint of activation of surface discontinuities is put forward. Activated depths of surface discontinuities which surround a tunnel with circular cross-section are analyzed. Results demonstrate that the activated depths of discontinuities are influenced by their inclinations and shearing strengths; and shearing strength plays a very important role in them. Discontinuities with same inclinations result different activation depths at different zones around a tunnel. Fracture mechanics theories are employed to analyze the growth of wing cracks of surface discontinuities. It implies that the deepest possible location where a wing crack can initiate is just the place of critical activation point. So the possible maximum depth of disturbed zone surrounding a tunnel is just the activated depth of discontinuities.