Three-dimensional tunnel face extrusion and reinforcement effects of underground excavations in deep rock masses

The design experience and the shallow tunnel theory, which emphasize the plane strain assumption, two-dimensional (2D) strength theory, and indoor or back analysis-based parameters of rock mass, should not be directly adopted for deep tunnel design. This paper reports the development of a digital acquisition technique to quickly, automatically, and accurately obtain the in-situ strength parameters (e.g., GSI and D), followed by the establishment of a three-dimensional (3D) numerical model for deep tunneling for a typical tunnel in western China to study the 3D and nonlinear spatial effects and mechanisms during excavation with incorporation of the 3D Hoek-Brown (HB) strength criterion. The reliability and correctness of the developed methods were verified by the field monitoring data, and the results of our study showed that: (1) The deformations based on 2D strength criteria (e.g., HB and MC) and 3D criterion (e.g. GZZ) are close for shallow tunnel, but significantly different for deep tunnel because of the widely distributed plastic zone in deep tunneling, thereby indicating the obvious effect of intermediate principal stress (sigma(2)) on rock deformation; (2) The face extrusion deformation exhibits obvious nonlinear trends with an increasing buried depth, and the sigma(3) of the core rock reflects a tensile value in deep tunnel, thereby indicating the presence of an extremely unstable state. Reinforcement could increase the overall stress inside the core rock and reduce the stress unloading effect caused by the strong disturbances of deep excavations, thereby promoting a gradual change from tensile stress to compressive stress; (3) A uniform relationship was found between pre-extrusion and pre-convergence deformations, indicating that the pre convergence deformation of the surrounding rock could be controlled by constraining the pre-extrusion deformation of the core rock. This digital technique and such 3D HB-based (e.g., GZZ) numerical modeling may be promising for conducting the 3D forward and dynamic analyses of tunnel stability. Additionally, the core rock extrusion control can be treated as an active method to render the rock itself a natural support material and to realize its strength potential, thereby making a valuable contribution to the construction of deep and ultra-deep rock tunnels.

Automated summary

This study developed a digital acquisition technique and established a 3D numerical modeling to investigate the spatial effects and mechanisms of deep tunnel excavation. The results showed significant differences in deformations between 2D and 3D strength criteria in deep tunneling, as well as a clear relationship between pre-extrusion and pre-convergence deformations.