Seismic ground amplification by unlined tunnels subjected to vertically propagating SV and P waves using BEM

• The seismic amplification pattern of the ground surface underlain by a tunnel is evaluated.
• The time-domain BEM formulation is developed for seismic ground response analyses.
• Variations of amplification values and the characteristics period of the medium versus tunnel depth have been shown.
• The amplification of the ground surface underlain by a tunnel is increased in long periods.
• Some preliminary simple equations are presented which can be used in the building codes and seismic microzonation guidelines.

Recent researches have revealed that the seismic ground response above tunnels can be different from the free-field motion during earthquakes. Nevertheless, to the best of the authors׳ knowledge, neither building codes nor seismic microzonation guidelines have yet considered this matter. In the present study, the seismic response of a linear elastic medium including a buried unlined tunnel subjected to vertically propagating incident SV and P waves are addressed. For analysis purposes, a numerical time-domain analysis is performed by utilizing a robust numerical algorithm working based on the boundary element method. It is observed that the amplification of the ground surface underlain by a tunnel is increased in long periods. The variation of the amplification factor and characteristic period of the medium versus the buried depth of the tunnel are depicted as the major results of this study. Some simple and useful relations are proposed for estimating the seismic microzonation of the areas underlain by tunnels. These relations can also be used for the preliminary seismic design of structures located on underground structures.