Numerical seismic analysis of shield-driven tunnel crossing soil-rock interface under transverse excitation

Abstract

When a tunnel crosses the interface between a soil stratum and a rock stratum, the seismic responses of two free-fields would cause discrepant reaction of the hosted tunnel. In order to analyse the seismic performance of this particular portion of the tunnel, a three-dimensional finite element (FE) model was established based on an actual construction in Nanjing. An artificial boundary was adopted to eliminate the reflection and refraction of seismic motion at the lateral boundary of the FE model. The soil was simulated using visco-elastic constitutive model, while the tunnel and the rock were assumed to be elastic. The lining was simplified into segmental equivalent rings by the principle of stiffness equivalence. Beam elements were used to imitate bolts placed between adjacent segments. Nanjing synthetic earthquake motion was inputted from the bottom of the model, and the shaking direction was perpendicular to the tunnel axis. The acceleration data were analysed to explain the discrepant seismic responses of the tunnel buried in different strata. The seismic performance of the tunnel was evaluated through three observed data: the sectional deformation, the extension of joints and axial force of bolts. Results indicate that: 1) the discrepant seismic responses of the tunnel buried in different strata are revealed by the acceleration data, and the acceleration responses of the tunnel is highly correlated to hosting medium; 2) there are noticeable localized behaviors of the tunnel observed near the interface of the two strata such as elevated bolt forces, great extensions of joints and huge sectional deformation; 3) the amplified areas of the seismic responses of the tunnel at the soil side and the rock side are 3 times and 1 time the tunnel diameter, respectively.