Condition evaluation of urban metro shield tunnels in Shanghai through multiple indicators multiple causes model combined with multiple regression method

Many existing condition evaluations of metro shield tunnels are developed based on the experts' judgments or single deformation/defect measurements. In addition, the influence of factors such as buried depth and age on the condition of shield tunnels was not quantified in previous research. This study proposed a comprehensive condition index, the Tunnel Serviceability Index (TSI), through the Multiple Indicators Multiple Causes (MIMIC) model combined with a multiple regression method. The contributions of the deformation/defect variables and the influencing factors to the TSI were evaluated. The observed deformation/defect variables included average relative settlement, differential settlement, average convergence ratio, water leakage area, cracking length, and spalling area. The influencing factors included the operation age, burial depth, and so on. The relationship among the TSI, deformation/defect variables, and influence factors was quantified, and an expression for the TSI was obtained. The paired-sample t test demonstrated that there were no significant differences between the experts' ratings in our group's previous research and the tunnel condition evaluation used by the MIMIC model in this study at the significance level alpha = 0.05. Among the six observed deformation and defect variables, approximately 95% of the TSI deterioration was caused by the increase of convergence ratio, relative settlement and differential settlement. Finally, the case of Shanghai Metro Line No. 1 was investigated to evaluate its condition.