Analysis of Tunnel Face Stability Based on Upper Bound Method of Limit Analysis

［Objective］ During tunnel excavation, the reformation of a free surface ahead of the tunnel face due to soil removal necessitates the application of appropriate support pressure to ensure soil stability. Thus, it is necessary to accurately analyze tunnel face stability during tunnel excavation and determine the required ultimate support pressure to maintain stability. ［Method］ The upper bound method of limit analysis is employed to develop a two-dimensional failure mechanism model for tunnel faces, which allows for optimized searches based on basic parameters of tunnel face. By deriving the calculation formulas for internal and external powers, the problem of tunnel face stability analysis is transformed into a nonlinear optimization problem for finding the extreme value. On this basis, the safety factor and ultimate support pressure of the tunnel face are calculated, which are compared with the calculation results of existing literatures and the OPTUM G2 numerical software, validating the accuracy and reliability of the method. Additionally, the effects of soil cohesion force, internal friction angle, and depth-to-diameter ratio on tunnel face stability are thoroughly explored. ［Result & Conclusion］ Tunnel face stability improves with increasing soil cohesion force and internal friction angle, indicating their positive influence on stability. However, this stability decreases with an increase in the depth-to-diameter ratio; when the depth-to-diameter ratio reaches a sufficiently large value, the shape of the tunnel face failure mechanism tends to stabilize, and further increases in the ratio have minimal impact on stability. Furthermore, increasing internal friction angle can mitigate the adverse effect of the depth-to-diameter ratio on tunnel face stability.