Mechanical responses and probabilistic sensitivity analysis of shield tunnel segment in water-rich soft strata

Lining segments of shield tunnels in water-rich soft strata suffer severe mechanical challenges from groundwater level fluctuations and fluid-solid coupling, which threatens tunnel structural stability. Taking the Liuyang River-crossing shield tunnel of Changsha Metro Line 3 as the research object, a 2D hydro-mechanical coupling finite element model was established via ANSYS to analyze the mechanical responses (pore water pressure, stress, displacement) of segments under four typical groundwater level conditions. Based on the ANSYS/PDS platform, the Monte Carlo method was adopted for reliability analysis, involving 9 types of random variables related to surrounding rock and segment parameters with 10,000 cyclic sampling calculations; probability sensitivity analysis was further conducted to identify key factors affecting segment internal forces. The results show that groundwater level is the dominant factor for segment mechanical behavior: peak pore water pressure, structural stress and horizontal convergence are positively correlated with it, while vertical settlement is negatively correlated; the segment outer ring is prone to tensile failure, with the crown and haunches as key control positions for vertical settlement and horizontal convergence respectively. Increasing segment stiffness elevates its internal forces, while higher surrounding rock elastic modulus reduces segment internal forces and improves reliability, and the rise of surrounding rock unit weight and permeability coefficient significantly increases segment load. Probability sensitivity analysis reveals that surrounding rock unit weight is the dominant factor for segment axial force and lining thickness is the primary factor for bending moment, and the effects of surrounding rock cohesion and internal friction angle can be neglected in engineering practice. This study clarifies the mechanical response law of segments under variable groundwater levels and identifies key sensitive parameters for structural reliability, providing valuable theoretical and engineering references for the optimal design and safety control of shield tunnel linings in similar water-rich soft strata.