Unloading is the same with that of redistribution of surrounding rock’s stress when excavating; and the unloading velocity affects stress field

so the problem of unloading is a dynamic one essentially. Assumed the unloading ruled by a simple function characteristic of the time parameter and based on the elastic mechanics and the Hamilton principle

the regulation of the stress field nearby the tunnel is obtained. The stress fields around the carvity in different depths are generalized. Especially when the in-site stress is larger than the rock strength，the characteristics of the failure nearby the tunnel are analyzed considering the shear failure and tensile failure. The results show that the main mode of the shear failure

intact and tensile failure occurs from the tunnel. In other words

the characteristics of the shear failure

intact and tensile failure is one of the likely failure modes. The situation is concerned with the unloading time