Dynamic and static analysis of mechanism of loosen zone in surrounding rock of tunnels

Considering the dynamic and static actions on surrounding rock during excavation process, the loosen zone is divided into two parts, broken zone and plastic softening zone. The transient unloading in surrounding rock will induce dynamic action on the rock near cavern periphery, leading to tension crack zone. Then, under the static action with the original stress in rock, the tension crack zone tends to be broken zone because of tension failure and shear failure. Under the stactic action with the original stress in rock, the deformatiom increases and the plastic softening zone is formed. Based on the elastodynamics theory, the stress redistribution and displacement field in surrounding rock induced by excavation are determined. A method to detemine the radius of the broken zone is put forward. On the basis of stress distribution in broken zone and the residual strength theory, the supporting force that the broken zone acts on the plastic softening zone is deduced. By means of the formula for calculating the stress in the plastic zone, an equation to calculate the radius of looen zone is deduced. The calculated results are in conformity the in-situ monitoring with data.