Seismic response of tunnels revealed in two decades following the 1999 Chi-Chi earthquake (Mw 7.6) in Taiwan: A review

Abstract

The 1999 Chi-Chi, Taiwan, earthquake (Mw 7.6) caused damage to 49 tunnels within 60 km of the epicenter, including the collapse of three mountain tunnels, ending the traditional perception that tunnels, and especially mountain tunnels, were earthquake-resistant. A series of studies that involved seismic damage investigation, physical model and numerical simulation experiments, numerical analyses and field monitoring have been carried out to investigate the damage mechanism, influencing factors and the seismic response of tunnels, and these are reviewed in this manuscript. The results of such studies over the last two decades demonstrate that tunnels suffer less seismic damage than surface structures. Most of seismic damage to tunnels involves failure of the adjacent ground or the movement of a fault that is crossed by the tunnel. Tunnel damage that is caused by ground motion occurs where the seismic effects are amplified, such as at the slope next to mined sections and portals, in shallow overburden tunnels, and at particular combinations of tunnel depth and geological conditions. Results of physical model-based tests using shaking tables and centrifuges reveal that tunnels exhibit rocking responses along with ovaling deformation or racking distortions under seismic excitation; also residual earth pressures remain on the tunnel side walls and internal forces of lining exist after shaking. The structural responses of a tunnel under non-uniform excitation are larger than that under uniform excitation. Beyond investigations of the damage mechanism and influencing factors, physical model-based tests provide experimental benchmarks for correcting numerical model settings and calibrating the characteristic parameters in numerical simulations, and validate the application of such models and simulations in the investigation of tunnel seismic responses. A fully dynamic analysis with complete descriptions of the topography and geological characteristics of a site, and the engineering characteristics of the tunnel and surrounding ground, is nowadays probably the most effective way for the seismic simulation of tunnels, and the design and evaluation of the same. An understanding of the state-of-the-art with respect to the seismic response of tunnels demonstrates the importance of three-dimensional geological models for tunnel seismic analysis; the evaluation of regional ground motion models that can capture the expected range of possible ground motions at tunnel site, and determinations of both the static and dynamic characteristics of geological conditions that are associated with site-scale vulnerabilities. Since residual earth pressures and internal forces remain on tunnel after an earthquake, the seismic damage to tunnels and loads and strain of the surrounding ground may accumulate. Methods for evaluating potential seismic damage to existing tunnels must be further developed, and more field monitoring of the seismic responses of tunnels is required. • Seismic response of tunnels revealed in past two decades is extensively reviewed. • Ground motion-induced tunnel damage occurs where the seismic effects are amplified. • Seismic damage to tunnels and loads and strain of surrounding ground may accumulate. • 3D geological models and more field monitoring are important for fully dynamic analyses.