Theoretical and numerical analysis of the failure mechanism and jacking force during vertical tunneling using a CEL approach

Over the past few decades, a significant surge has occurred in the domain of underground space construction and development, especially in China. The vertical tunneling method has gained popularity among emerging methods for building water inlets and sewage outlets in horizontal tunnels. While this method has been previously studied, a significant knowledge gap persists concerning the failure mechanism above the standpipe. To address this issue, in this paper, the Coupled Eulerian-Lagrangian (CEL) model is employed to simulate the vertical tunneling process. By conducting a sensitivity analysis, a balanced compromise is obtained between precision and computational efficiency, revealing that a simulation velocity of 0.4 m/s and a mesh density of 0.12D could facilitate a balance. Based on the CEL results, the failure mechanism during vertical tunneling is categorized into four phases: local soil compression, extended shear failure, progression of the shear face across the ground surface, and soil redistribution. The shape of the failure zone changes from a small conical frustum to a large conical frustum across these phases. The corresponding jacking distance, which characterizes the differences between these phases, is determined. To facilitate jacking force calculations during vertical tunneling, the upper diameter of the conical frustum is considered, and an empirical shear failure method is proposed. Four constituents of the jacking force can be calculated directly instead of inverse calculation from measured values. Additionally, it is compared with prior research, such as the shear failure method, and the results emphasize the superiority of the empirical shear failure method in generating precise jacking force predictions. By applying the empirical shear failure method, the impact of the construction time, the external diameter of the standpipe, and the standpipe length on the jacking force are analyzed. The conducted validation reinforces the credibility and effectiveness of the proposed approach within the broader scope of vertical tunneling analysis.