عنوان مقاله [English]
Nowadays, in developing countries, due to city expansion and demand for trips within the city, creating the necessary infrastructures, such as an underground transportation network, is of great importance. But, constructing subterranean
structures and underground tunnels carry a high- risk potential due to its surface seismic responses above which the structures are located. However, insignificant information is available in literature to determine the seismic response of the ground surface due to twin side-by-side and parallel tunnels. In this paper, the effects of two long unsupported double tunnels on the seismic response of the ground surface are studied in different frequencies, using the boundary element method in the time-domain. The homogeneous medium is assumed to have a linear elastic constitutive behavior subjected to vertically propagating incident in-plane waves (SV waves). Comparing the results obtained through solving different problems with previous studies can show the results' suitable adaption. In addition, the results indicate that underground twin tunnels can have an effect on the vertical and horizontal components of the ground surface motion more compared to single tunnels. It is evident that the seismic amplification of the ground surface underlain by the shallow circular tunnels increases in long dimensionless periods. Moreover, the embedded depth and spacing distance of the double tunnels have significant influence on the amplification patterns on the ground surface. Creating more wave trapped zones by the double tunnels in comparison to the single tunnel can be one of the main reasons for increasing the seismic amplification of the ground surface. In addition, the interaction between tunnels and, consequently, the ground surface amplification values is decrease in the greater spacing distance of the double tunnels. Finally, some amplification coefficients are presented which could be used while introducing simple preliminary ideas to modify the standard design spectra in building codes and seismic microzonation studies.