پاسخ لرزه ای تونل دایره ای در سنگ مفصل دار Seismic Response of Circular Tunnels in Jointed Rock

1. Introduction Underground space has been reported to be more resistant to seismically induced damage compared with above-ground structures. However, recent earthquakes have illustrated that even underground structures are susceptible to damage under strong seismic excitations (Asakura and Sato, 1996; Hashash, 2002; Wang, 1993). Considering the importance of underground facilities in urban societies, underground spaces should be designed to withstand strong earthquakes. Because underground tunnels are surrounded by ground, they are not allowed to freely vibrate and primarily conform to the movement of ground. Analytical pseudo-static solutions have been presented to estimate the tunnel response subjected to shear deformation (e.g. Wang, 1993; Penzien and Wu, 1998). The solutions assume that the tunnel is surrounded by elastic, massless, and continuous medium. The procedure assumes that the response of tunnel lining under dynamic motion can be decoupled from that produced under static condition. For a realistic simulation, the static stress imposed on the tunnel due to overburden and the tunneling process has to be modeled (Hashash et al., 2010). However, extensive comparisons showed that the increment of moment and forces due to seismically induced deformation for the massless condition with zero gravity and for the case where the in-situ static stresses are applied are nearly identical (Ahn, 2013). Numerical pseudo-static analyses, where the tunnel is assumed to be surrounded by massless ground, have been widely performed (Hashash and Park, 2001; Hashash et al., 2003). Dynamic analyses were also performed to simulate the dynamic soil-structure interaction (Asakura and Sato, 1996; Sedarat et al., 2009; Cilingir and Gopal Madabhushi, 2011). Hashash et al. (2010) reported that for single box tunnels, the results of pseudo-static and dynamic analyses are almost identical. Argyroudis and Pitilakis (2012) also reported that the difference between the pseudo-static and dynamic analyses is not significant. Effect of jointed rock on tunneling has been a topic of interest (Barton, 1995). Researchers used the discrete element method to investigate how the joints influence the tunnel (Bhasin and Høeg, 1998; Hao and Azzam, 2005; Vardakos et al., 2007). Studies on the influence of jointed rock mass on blast induced vibration propagation have been performed. Hao et al. (2001) used the measured motions under in-situ blasting to identify the effect of the joint layout on propagation of stress waves. Li and Ma (2010) studied the interaction between the blast wave and arbitrarily positioned rock joint. Ma and Brady (1999) investigated the performance of an underground excavation in jointed rock under repeated loading. Limited study has been performed to investigate the effect of rock joints on seismic response of tunnels, possibly because the shear deformation in jointed rock is small compared with soil profiles and typically does not lead to structural failure. However, although a tunnel collapse is unlikely to occur in jointed rock, it should be investigated whether localized concentration of the force or moment at the joint-tunnel intersection may cause cracks to develop. The cracks may influence the long-term sustainability of the underground infrastructure. Another concern is the response of nuclear facilities that are built underground, for which even small-scale cracks are not allowed. In this study, a series of discrete element analyses (DE) were performed to evaluate the influence of rock joints on the seismic response of tunnels. A comprehensive set of joint parameters, which include tunnel-joint intersection location, joint spacing, joint dip, and mechanical properties, were used in the simulation. We also perform parallel continuum analyses to compare with DE and to quantify the difference.