Keywords: tunnel, internal forces, seismic, soft soil, non-linearity, excavation.

Underground structures experience a lower rate of damage due to earthquake waves comparing to surface structures. Nevertheless, several examples of recorded damage to underground structures during recent earthquakes demonstrate the need for a deeper consideration of seismic forces in the original design. For example, the collapse of Daikai subway in Kobe (Japan) in 1995, the damages of highway tunnels in central Taiwan in 1999 and the collapse of the Bolu tunnel in Turkey in 1999.

This paper presents a three-dimensional numerical modeling to study the influence of non-linearity on the seismic induced response of a tunnel embedded in soft soils. In particular, it concerns the influence of tunnel installation on the overall tunnel seismic response. This issue requires a nonlinear analysis since the excavation induces a perturbation in the stress distribution adjacent to the tunnel. Furthermore, numerical simulations aim to simulate a TBM tunneling process model, with pressure at the tunnel face and thus require three-dimensional analysis. The tunnel lining is modeled using shell elements embedded in non-linear soil. This analysis is conducted using non associated Mohr-Coulomb criterion for the soil.

After a brief review on the closed form analytical solution proposed in the literature, three-dimensional numerical calculations assuming elastic behavior for soil are compared to analytical solution of Wang [1]. Results show a good agreement between the two approaches. Numerical simulations are then extended to analyze the effect of tunnel installation and the tunnel depth (comparison for shallow and deep tunnels) on the nonlinear seismic induced response of soil-tunnel system.

Results indicate that elastic analysis is not sufficient to determine the seismic induced response of the soil-tunnel system. For shallow depths, the influence of tunnel installation is of major importance on the global response. Indeed, this process leads to a more realistic estimation of the seismic induced thrust force in the tunnel lining and settlement. In particular, it is noted that elastic analysis leads to a significant overestimation of settlement. The influence of tunnel depth shows that in case of deep tunnel, the influence of tunnel installation is not significant and may be neglected.

1

J.N. Wang, "Seismic Design of Tunnels: a State of the Art Approach", Parsons Brinckerhoff Quade & Douglas. Inc., Monograph 7, 1993.

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