Numerical simulation of zonal disintegration of surrounding rock at arched tunnel under progressive excavation and different internal frictional angles

Zonal disintegration is a phenomenon that the alternated regions of fractured and relatively intact rock mas- ses appear in the surrounding rock at great depth,which is completely different from the traditional understanding on the failure characteristics of the tunnel surrounding rock. A full understanding on the development of zonal disinte- gration in different surrounding rock can provide a foundation for its control and application. Excavation of a tunnel will lead to motion of rocks in the vicinity of the working face toward the tunnel. Thus,the stress state of planes or- thogonal to the tunnel axis is not a plane strain state,but a three-dimensional state. Using the heterogeneous strain-softening model where the Weibull distribution function was introduced based on the strain-softening model in FLAC3D ,the effects of the internal frictional angle on the pattern of zonal disintegration were studied,and the distri- bution of the shear strain increment,minor principal stress and major principal stress were investigated when the tun- nel nearly intersected the numerical model where a U-shaped tunnel was progressively excavated. The following re- sults were found. The low internal frictional angle leads to the serious failure of the tunnel surrounding,a full devel- opment of shear bands,and an apparent zonal disintegration. Zonal disintegration appears after the tunnel surround- ing rock is subjected to a certain failure. The mechanism of the zonal disintegration can be summarized as follows: under stresses and excavation,at planes going through the tunnel axis,the randomly distributed defects develop into ( conjugated) shear bands,and at planes orthogonal to the tunnel axis,the isolated annular fracture zones can be formed. At planes going through the tunnel axis,the present results are qualitatively consistent with the previous ex- perimental results. According to the present research,at different planes orthogonal to the tunnel axis,zonal disinte- gration is different,which means that it is not necessary to choose the unified tunnel support parameters. For exam- ple,at some positions where zonal disintegration is obvious,the lengths of rock bolts should be extended,and vice versa.