Nonlinear Stability Analysis of Shallow-Buried Bias Tunnel Based on Failure Mode Improvement

Based on the improvement of the Karl Terzaghi failure mode for calculating the surrounding rock pressure of loose rock mass, combined with the upper bound theorem of limit analysis, the formula for calculating the surrounding rock pressure of shallow-buried tunnel in a slope area under a nonlinear failure criterion is derived. The SQP algorithm in MATLAB (R2022a) software was used to optimize the solution, and the influence of slope top load, buried depth ratio, cohesion, and axial tensile stress on the surrounding rock pressure and the failure mode of shallow-buried tunnel was analyzed through examples. The results show that the surrounding rock pressure increases with the increase in the nonlinear coefficient, while it decreases with the increase in cohesion. When the ratio of horizontal support reaction force to vertical support reaction force (K) decreases, the surrounding rock pressure increases. The increase in slope top load, buried depth ratio, and axial tensile stress will reduce the pressure stability of shallow-buried tunnel-surrounding rock. With the increase in slope top load and buried depth ratio, shallow-buried tunnel failure tends to deviate toward the shallow-buried side of the slope. When the axial tensile stress increases or the cohesion decreases, the failure plane will extend outward, leading to an enlarged shear zone area and a higher propensity for the formation of a larger plastic zone in the surrounding rock of the tunnel. The research results can effectively overcome some limitations of the standard method, exhibiting a certain reference value for the formulation of the actual engineering scheme and the reasonable reduction in the cost of the project.