Abstract
Tunnels in seismically active areas are vulnerable to adverse effects of earthquake loading. Recent seismic events have shown that there is a need to validate current design methods to better understand the deformation mechanisms associated with the dynamic behaviour of tunnels. The research described in this paper consists of physical and numerical modelling of circular tunnels with dynamic centrifuge experiments and complementary finite element simulations. The aim is to develop an understanding of the effects of tunnel depth on the seismic behaviour of tunnels. Tunnels with different depth-to-diameter ratios were tested in dry, loose silica sand. Accelerations around the tunnel and earth pressures on the lining were measured. A high-speed digital camera was used to record soil and lining deformations. Particle image velocimetry analyses were carried out on the recorded images to measure the deformations. Complementary dynamic finite element simulations were also conducted with a code capable of managing contact simulations at the soil–lining interface. Measurement of centrifuge experiments and finite element analyses show that the tunnel shifts from a static equilibrium to a dynamic equilibrium state as soon as the earthquake starts. The nature of the dynamic equilibrium, however, is difficult to predict using conventional analysis methods.
Introduction
Tunnels in seismically active areas are likely to be subjected to dynamic loads that may cause them to deform beyond their safe design limits. Recent events, such as the Kobe Earthquake in Japan (1995), Duzce Earthquake in Turkey (1999), Chi-Chi Earthquake in Taiwan (1999), Bam Earthquake in Iran (2003), and Wenchuan Earthquake in China (2008), demonstrated the appropriateness of this statement: some tunnels were seen to suffer damage beyond the limits of possible refurbishment. One of the main reasons for the inadequate design of these tunnels is the lack of experimental data and field evidence, which is needed to verify the predictions of available design methods. Such methods range from simplified pseudo-dynamic elasticity solutions to complex numerical soil–structure interaction models. Comprehensive reviews of seismic design methods are given by Wang (1993) and Hashash et al. (2001).
Summary and conclusions
Results of both centrifuge experiments and FE analyses show that the dynamic behaviour of circular tunnels can be split into three stages: transient stage, steady-state cycles, and residual (post-earthquake) stage. During the transient stage, which lasts for the first few cycles, the tunnel structure reaches a dynamic equilibrium configuration. Most of the residual forces that remain after the shaking stops are built up during this period. The transient stage is followed by the steady-state cycles, during which both the earth pressures around the tunnel and the forces in the tunnel lining oscillate around a mean residual value. After the shaking stops, residual stresses are left in the tunnel lining.
