Isolation bearings have been a widely applied seismic strengthening technique in above ground structures. Whereas, the sliding isolation bearings were seldom used in underground structures. This study aims to explore the feasibility of sliding isolation bearings reducing the seismic response of underground structures. The collapse mechanism of underground structures was firstly analyzed by taking the Daikai Station as an example. Numerical results demonstrated that the collapse of the structure was due to the poor ductility of the intermediate columns. Therefore, the sliding isolation bearing could be installed between the columns and the beam to reduce the lateral deformations of columns. In order to determine an appropriate coefficient for sliding bearings, static analyses for the capacity of columns were conducted. Moreover, the performances of a beambearing-column system were also investigated. Finally, seismic responses of the underground structure retrofitted with bearings were studied. Numerical results presented that the responses of both columns and the whole structure were reduced remarkably. Moreover, the frictional coefficient of bearing influencing the seismic responses of underground structures was discussed. And some interesting conclusions were also obtained for the seismic design of underground structures.

Introduction

The construction of underground structures including subway stations, underground malls, tunnels, and underground parking stations has gained a rapid development in China during recent years. Taking the subway stations as the example, more than 2000 stations along the total of 3000 km long tunnels have been constructed till 2015 (Chen et al., 2016), and up to 373 stations were constructed in 2016. However, almost all the cities with underground structures in China are in the strong earthquake prone area. Seismic investigations showed that underground structures were at greater risk during an earthquake (Sharma and Judd, 1991; Yashida and Nakamura, 1996; Wang et al., 2000; Wang et al., 2009; Shen et al., 2014; Lee et al., 2016). Therefore, it is of urgent need to focus on the seismic safety and seismic design of underground structures. Seismic isolation technology is an approach that adding an isolation system between structures and the foundation, and is an effective way to reduce earthquake energy transferring from ground to structure (Providakis, 2009; Li and Li, 2011). This technology as an approach to earthquake protection has been used more than 100 years (Buckle and Mayes, 1990) in above ground structures. Seismic isolation technology has been introduced to underground structures aiming to reduce the damage of underground structures during an earthquake (Xin et al., 2014; Li, 2012) in recent years. The isolation layer was applied as the buffering to mitigate the constraint of the surrounding ground, and then the earthquake-induced structural deformation and forces of underground structures were reduced (Xin et al., 2014). The isolation layer was always used for the seismic design of tunnels (Suzuki, 2000; Kim and Konagai, 2001; Konagai and Kim, 2001; Hasheminejad and Miri, 2008; Kiryu et al., 2012; Chen and Shen, 2014; Wang et al., 2017). Moreover, seismic investigations and damage features of tunnels from the Wenchuan earthquake also illustrated that isolation layers could be applied to improve the seismic performance of tunnels (Li, 2012). Studies proved that seismic isolation technology was an effective way of reducing damages to tunnels. Alternatively, seismic reduction technology is another approach to protecting structures surviving during an earthquake, and has also been applied in the underground structures. For example, flexible joints were utilized between the segments of tunnels to mitigate the earthquake-induced stress concentration (Ding et al., 2006; Yu et al., 2013; Do et al., 2015; Kawamata et al., 2016; Yu et al., 2017).