Seismic analysis of twin tunnels by a finite/infinite element approach

The two-dimensional (2D) dynamic response of horizontally aligned, cylindrical twin tunnels subjected to vertically incident seismic waves was simulated by a finite/infinite element approach. A rational approach was proposed for imposing the earthquake forces using free-field recorded data. First, the earthquake motion recorded on the free field was transferred to that on the boundary of the near field using the one-dimensional (1D) wave theory. Second, the motion data were computed as the equivalent seismic forces and applied on the near-field boundary in the frequency domain. In this study, the data from the 1999 Chi-Chi earthquake recorded at the TAP003 station were adopted. The accuracy of the procedure for imposing the free-field earthquakes was first verified. Then, a parametric study was performed to investigate the effects of spacing between the twin tunnels, depths of tunnels, and stiffness of tunnel lining under seismic excitation. The results of the analysis indicate that although the depth and spacing of the twin tunnels have less effect on ground response, they are important parameters for the seismic responses of the tunnels.