Keywords: seismic analysis, soil-structure interaction, body waves, Rayleigh waves, buried structures, twin tunnels.

In this paper, numerical methods based on finite elements are used to compute circumferential or hoop stresses on twin tunnels when they are subjected to seismic excitation. Several types of seismic waves are considered: shear and compression (body) waves travelling vertically, and harmonic Rayleigh waves propagating horizontally. The soil system considered is a layer on a very stiff half-space, modelled by means of four-node isoparametric elements. The tunnel section is modelled by beams, for the case of body-wave excitation, or by isoparametric finite elements for Rayleigh waves. Different numerical tools have been considered: FLUSHP and FLAC-2D for propagating body waves as well as the computer program ELISEO, written by the authors, for Rayleigh waves.

In our analysis, and for harmonic Rayleigh travelling waves, the free-field propagation problem is first solved and transmitting boundaries adapted to time domain analysis were used for each frequency considered. From this free-field analysis, a system of forces acting on one of the vertical edges of the finite-element mesh is applied to one of the vertical edges of the model, allowing the analysis of the problem in the time domain, which would facilitate taking into account the non-linear soil behaviour.

After the seismic response (maxima hoop stresses) of the twin-tunnel system was compared to that of only a single tunnel, it became possible to understand the seismic interaction of the two tunnels. All numerical analyses show that the seismic interaction between the tunnels may be important, depending basically on four key parameters: seismic wave type, tunnels depth, distance between the tunnel axes and, finally, soil stiffness.

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