Figure 22
ABSTRACTThe evaluation of the seismic behaviour of underground structures represents  one of the most actual seismic geotechnical and structural engineering research  topics about the study of the complex phenomena of soil-structural interaction. In the last decades, different types of simplified and numerical approaches  have been developed for the correct analysis of the seismic vulnerability of  these important infrastructures and a series of laboratory tests for the  seismic behaviour characterization of the soils (resonant column test, etc.)  and of the coupled soil-structure system (centrifuge test, etc.) have been  conducted, especially after the recent strong earthquakes where the underground  structures have been subjected to significant damages. In the same way, in the  last few years, the International Codes are beginning to pay attention to the  concepts of the seismic design of these structures.Despite the significant development of  knowledge, described above, still remain open several uncertainties of the  correct reproduction of the underground  structures behaviour under seismic load. In this paper, the evaluation of the  seismic behaviour of Mercato Santa Anida metro station was conducted through  the application of two different seismic input, considering the soil-structure  interaction effects. The results of the nonlinear Time History analysis are  analysed in terms of bending moment acting on the concrete retaining walls of  two different significant sections of the metro station.KeywordsUnderground structures; Soil-structure  interaction; Finite element analysis; Earthquake; Seismic vulnerability.          INTRODUCTIONUnderground structures can be grouped into three broad categories [1], each having  distinct design features and construction methods: bored or mined tunnels, cut  and cover tunnels and immersed tube tunnels (Figure 1).Unlike surface constructions, underground  structures were considered, for a long period, practically invulnerable to  earthquakes. This consideration about underground structures safety, however,  has been changed after some of them suffered serious damages caused by  earthquakes, including the 1995 Kobe (Japan), the 1999 Chi-Chi (Taiwan) and the  1999 Kocaeli (Turkey) earthquakes [3].Damaging effects of earthquakes on  underground structures can be classified into two main groups:damages caused by vibratory  motion (shaking) of the ground;damages due to ground failures.This study has the purpose of determining the most important aspects of the  soil-structure interaction effects on underground structures subjected to  seismic loads, taking as case study the new Lima Metro line 2 project.