Seismic Response Assessment of the Ground Surface induced by Twin Tunnels subjected to Incident In-Plane Shear Waves

Due to the population growth and transportation problems in urban areas, the use of underground structures such as subways and tunnels are widely increasing in developing countries and populous cities. The passage of tunnels and subways under existing buildings reveals the important impact of these underground structures on the ground seismic response. In this paper, the effects of two long unsupported parallel tunnels aligned horizontally on the seismic response of the ground surface are studied using the boundary element method in the time-domain in this paper. The medium is assumed to have a linear elastic constitutive behavior subjected to vertically propagating incident SV in-plane waves. The proposed algorithm is validated by comparing it with the previous researcher's solution. Finally, the presented results have shown that the seismic interaction between twin tunnels with close spacing distance in comparison with a single tunnel could create more seismic amplification on the horizontal and vertical component of ground surface motion.
Underground structures due to population growth and transportation problems are one of the most substantial infrastructures of developing countries. Hence, in recent years many tunnels have been designed or constructed in urban areas in order to develop or extend underground transportation systems. Based on the experience gained from past earthquakes, it is evident that underground long structures, such as subway and tunnels and also surface structures above them have great potential for destruction and disruption during earthquakes. The shortage of considering the effect of underground cavities and tunnels, especially urban subsurface structures such as twin-parallel tunnels, on the seismic amplification of the ground surface is observed in building codes and seismic microzonation studies.
Methodology and Approaches: The effects of the two long parallel tunnels aligned horizontally on the seismic response of the ground surface are studied. The elastic medium of the proposed model has been used that subjected to vertically propagating in-plane SV incident waves. In order to analyze the seismic response of twin-parallel cavities, unlined tunnels and its effect on the ground surface, a computer code named SAMBE (Seismic Analysis of Multiple Boundary Element) is implemented based on time-domain boundary element method.
Results and The ground surface amplification potential above underground twin tunnels was strongly influenced by the wavelength of the incident wave, depth, spacing ratio and also tunnel shapes of the twin tunnels. Moreover, the conducted study can be a step forward in seismic design of buildings and seismic microzonation of the regions with underground structures.