Support Countermeasures for Large Deformation in a Deep Tunnel in Layered Shale with High Geostresses

Large deformation is a crucial factor in deep-buried tunnels constructed through soft rocks with a high geostress. Taking the deep-buried Baozhen tunnel through layered shale as an example, this study investigated its large deformation characteristics and the support countermeasures. Two study sections located in horizontally bedded carbonaceous shale and inclined bedded arenaceous shale were considered. First, the large deformation characteristics of normal sections were explored. Subsequently, a series of laboratory tests were conducted to investigate the mechanical properties and failure behavior of the shales in these sections. Based on the tested constitutive parameters considering the contact properties, the feasibility of optimization schemes related to the two sections was verified using the UDEC numerical software. Finally, the field monitoring results from the test sections, including the tunnel convergence, deep deformation, surrounding rock pressure and normal pressure on the secondary lining, were presented to further investigate the control effects of the optimization schemes. Both the numerical simulation and field monitoring results showed that the asymmetrically large deformation of the Baozhen tunnel could be attributed to the combined effect of the layered soft rock and the shear stress under high geostress. The invert rockbolts not only prevented the invert arch from heaving, but also helped reduce the global convergence of the tunnel. Tunneling through inclined bedded rocks was associated with a significant unsymmetrical-loading effect, and an asymmetric arrangement of the rockbolts could effectively deal with this issue; thus, the normal pressures acting on the surface of the support structures were more uniformly distributed. The research results provide references for the support design of deep-buried tunnels in layered rock masses.