An Investigation on Seismic Response of the Ground Surface due to the Metro Tunnel–Case Study: Line 7, Tehran

In order to develop the underground structures such as subways, metro tunnels and other buried facilities in big cities, the dynamic interaction of these structures and their environment should be considered. In this study, all metro lines in Tehran are investigated and the 7thline with 28 km long and its environmental conditions in all length have been used as a case study. This line is selected because it passes through all kinds of geotechnical alluvium and also the plan and profile of this line are suitable for interaction with the buildings. A numerical method in this study was analyzed with finite difference code, FLAC, in dynamic cases. The models are performed for 8, 10,12,13,15 and 20 meter overburden and three different alluviums are used.In dynamic interactions, previous studies showed high amplification of displacement in the vicinity of the tunnel in the surface. The parameter of acceleration is also investigated in some physical model tests. Both acceleration and attenuation of acceleration are reported in different locations and under the different seismic motions. For choosing the critical sections in selected line, the buildings with overburden less than 20 m and the maximum surface distance up to 5D are selected. In this study, two conditions of problem such as free-field, and tunnel field are analyzed. In tunnel-filed models, results show that the presence of tunnel can increase the surface acceleration up to 33% in initial part of line with 8 m overburden. This result is accrued under Chi Chi earthquake with the dominant frequency of 1.68. This increment of acceleration in critical cases can change the design acceleration of the building to 0.46 g instead of 0.35 g that is proposed by Iranian Code. By considering the results of all cases, it can be seen that the most effective case happened when the dominant frequency of motion is so close to the natural frequency of medium. The frequency content of medium is predicted by 1D propagation of wave with equivalent linear method in frequency content. The region between 0.5D to 1.5D from the center of the tunnel on the surface is most affected by the interaction. The effect of overburden of tunnel is also studied that shows that the deeper tunnel has less effect on surface but the influence zone on surface is greater. Both amplification and attenuation happened in the models for accelerationof the surface. The output data show that the variation of acceleration in surface depends strongly on the frequencycontentofinputmotion,thesurface distance from axis of the tunnel and overburden of the tunnel. The most important factor that can describe the behavior of the surface is the variation of wave propagation around the tunnel. The presence of the tunnel under SH wave based on the mode of deformation can amplify the motions around the tunnel and not top of it in the surface. The tunnel in the medium can also prevent the propagation of waves to top of it and this may cause attenuation of motion. The color contour of the acceleration distribution around the tunnel and near of the surface during the dynamic analysis of model can better describe these phenomena.