Effect of twin-parallel tunnels on seismic ground response due to vertically in-plane waves

•The amplification patterns of the ground surface caused by twin tunnels are evaluated.•The time-domain BE technique is developed for seismic ground response analyses.•Variations of amplification values versus depth and distance of twin tunnels have been shown.•The amplification patterns are essentially controlled by wavelength of the incident waves.•Some site coefficients are presented for modification of seismic microzonation studies.

In this paper, the effects of two long unsupported parallel tunnels on the seismic response of the ground surface are studied, using the boundary element method in the time-domain. The medium is assumed to have a linear elastic constitutive behavior subjected to vertically in-plane propagating incident SV and P waves. The ground surface amplification pattern above underground single and twin tunnels are examined based on the several effective parameters such as wavelength of the incident wave, buried depth, and spacing distance of the twin tunnels. It is evident that the seismic ground response above tunnels may be different from that of free-field motions during the seismic events. Moreover, the embedded depth and spacing distance of the twin tunnels have significant influences on the amplification patterns on the ground surface. Finally, some amplification coefficients are presented, which could be used while introducing simple preliminary ideas for modification of the standard design spectra in building codes.