Influence of stratification and assessment of fragility curves for mountain tunnels

Abstract

Mountain tunnels are generally constructed in the stratified rock mass. For the satisfactory seismic performance of mountain tunnels, adequate evaluation of the effect of stratification is absolutely necessary. This study reports the influence of stratification on the seismic behaviour of mountain tunnels through numerical investigation using discrete-element software UDEC (universal distinct element code). Tunnel liner forces are compared for a single-layer and a two-layer geological set-up considering both harmonic excitation and earthquake time histories. It is observed that tunnel supports are more vulnerable to damage when the tunnel passes through a stratified medium. For assessment of the vulnerability of tunnels in the stratified deposits of rock mass under earthquake motions, seismic fragility curves have been derived for different damage states with varying grades of concrete. It is observed from the fragility curves that the probability of damage may significantly be reduced by using higher-grade shotcrete for the tunnel liner, especially in stratified rock deposits.