Seismic evaluation of 3D tunnels in near fault ground motion using domain reduction method

Abstract

Underground tunnels are an essential part of transportation and utility networks, and their vulnerability to earthquakes has significant socio-economic impacts. Tunnels have suffered substantial damage in past earthquakes, including the 2008 Wenchuan (China) earthquake, the 1995 Kobe (Japan) earthquake, the 2004 Niigata (Japan) earthquake, and the 1999 Chi-Chi (Taiwan) earthquake. Past studies on the 2D plane strain analysis of tunnel cross-sections subjected to vertically propagating seismic waves provide essential insights on the racking and ovaling of the tunnel cross-section during an earthquake. However, during the recent 2008 Wenchuan (China) earthquake, tunnels located near earthquake faults suffered damage not just to the cross-section but also in the longitudinal direction, which previous 2D studies cannot explain. To numerically assess the 3D response of structures to near-fault earthquake ground motion, the required simulations are computationally expensive due to the large numerical domains encompassing both the earthquake fault and the structure. Domain Reduction Method (DRM) developed by Bielak et al. 2003 is an efficient modular two-step methodology employed for reducing the computational costs for such simulations. This method was developed for finite structures, and in this thesis, modifications are proposed to this method for infinitely long tunnels. The accuracy of the modified DRM in capturing the soil-tunnel interaction and the complete 3D response of the tunnel is verified by comparing the numerical results with available analytical solutions for a deep tunnel embedded in a homogenous space subjected to inclined P wave. The modified DRM is then used to analyse the seismic response of long deep and shallow tunnels to incident harmonic P and SV waves. For deep tunnels, the effect of wave propagation direction on the acceleration and stress response of the tunnel is analysed. For shallow tunnels, in addition to the wave inclination, the effect of the tunnel depth on the response of the tunnels is analysed. These parametric studies provide insights into the variation of both the magnitude as well as location of maximum stresses along the tunnel surface with respect to different parameters. The results from this study would be beneficial for the safe and economical design of tunnels in seismically active regions.