Seismicity Acceleration Spectrum of Ground Surface under the Effects of Urban Metro Tunnels with Circular Cross Section in Alluvial Soils

Urban subways in the ground would change its seismic movements. Designing ground surface structure in far filed is related to the horizontal component of acceleration at the ground surface in that area. Therefore, an attempt has been made in this study to change the frequency of soil-tunnel systems and to calculate the maximum horizontal acceleration of the ground surface due to tunnel presence through changing in tunnel placement depth, its diameter and its lining thickness as well as changing the soil properties. The maximum horizontal acceleration of the ground surface has been calculated in the presence of the tunnel for soil type 4 based on code 2800. The relationship between the maximum horizontal acceleration of the ground surface and the frequency of the soil-tunnel systems will result in the production of a horizontal acceleration spectrum. In the frequency range studied, spectral accelerations in models without a tunnel have mainly higher values compared to those models that have a tunnel buried in the soil. Amplification and deamplification of the ground surface acceleration is dependent on the period of soil-tunnel systems, the properties of the model under study and the position of the point studied at the ground surface. 9 acceleration records of known influential earthquakes, 33 models (in presence of tunnel) and 7 models (in absence of tunnel) have been used in this study. Overall, about 360 nonlinear dynamic analyses have been carried out. These analyses have been made for multilayered soil while the nonlinear effects of the soil and its interaction with the surrounding structures have been considered. This study has been conducted on a Delhi subway tunnel. ANSYS and Plaxis2D software packages have been used for the study. The ANSYS software has been used for modal analysis, obtaining the frequencies and mode shape of the soil-tunnel systems, while the Plaxis2D software has been used to analyze the time history and obtaining the acceleration values of key points of the model. Elasto-plastic Moher-Coulomb model has also been used to model the soil. In most cases (11 out of 14 cases), amplification of ground acceleration is observed at a depth of 20 m. 9 of the 14 ground acceleration amplification are related to a tunnel with a smaller radius (3.13 m). Whatever the tunnel center image is taken away from the ground surface, we will have higher number of points in which acceleration amplification of the ground surface occurs.