Analysis on settlement factors of shield tunnel foundation for operating metro

The metro tunnel settlement would directly affect the normal train operation and the comfort of passengers; severe uneven settlement can easily cause tunnel structural diseases. The relevant studies mainly focused on long-term settlement prediction and single factor impact analysis; it is necessary to analyze the multiple factors that influence the tunnel foundation settlement. A three-dimensional finite element numerical model of one shield tunnel section of Zhengzhou metro 1^st line was constructed and the vertical load of metro train was simulated as artificial excitation force to extract the dynamic stress and static deviator stress data. Introducing the cumulative plastic strain calculation formula under cyclic load, the layered summation method was used to calculate the tunnel foundation settlement. The influences of factors, such as the properties of soil where the tunnel is buried, the depth of track bed void, and metro operation velocity, on the tunnel foundation settlement were further analyzed. The results show that the numerical simulated foundation settlement and development law are consistent with on-site measured results, which verifies the accuracy of finite element simulation. The tunnel foundation settlement decreases with the increase of sand content in the surrounding soil layer; the size of the track bed void would exacerbate the stress concentration and vibration amplification in the tunnel and track bed slab, and then increase the dynamic deviatoric stress of soil layer under the tunnel and foundation settlement. Increasing the metro train velocity would increase the lateral influence range of foundation settlement. The sensitivity of tunnel settlement to various factors is obtained with orthogonal experimental method, range analysis method, and variance analysis method. The descending order of sensitivity is soil layer properties, train velocity, and track bed void, in which, soil layer property has a highly significant level and vehicle speed a has significant level. This study can provide basic information for the deformation monitoring, safety assessment, and reinforcement treatment of operating metro tunnels.