Abstract: The deformation and failure patterns of the surrounding rocks of tunnels in mixed ground composed of soft and hard strata are complex at different depths. Under the gravity stress operated in extensional basins,the lateral pressure coefficient is generally small and basically within 1.0. This paper takes the TBM excavation of circular caverns in horizontal and vertical superimposed mixed ground as an example. It uses the finite difference numerical simulation software FLAC^3D to study the displacement,principal stress and failure characteristics of different superimposed mixed ground along the tunnel axis under different depths in extensional basins. The results show that the deformation of surrounding rock mainly occurs in soft rock. With the increase of depth,the stability of hard rock becomes worse,especially the displacement of the top arch in working face increases obviously. When the buried depth is more than 800 m,the large deformation as nodal segments of soft rock along the tunnel axis begins to appear. With the increase of depth,the difference in principal stress between soft rock and hard rock becomes smaller. The larger the depth is,the larger the distribution range of plastic zone is. The tensile failure is the main deformation in small depth,while the shear failure is the main deformation in large depth. The transition depth(critical depth) of the two states is about 800 m. Therefore,the limit value of deep burial in extensional basin is theoretically verified.