A Practical Approach for Seismic Risk Assessment of Underground Structures: a Case Study of Iranian Subway Tunnels*

Earthq Sci (2019)32: 64–71 64 doi: 10.29382/eqs-2019-0064-03 A practical approach for seismic risk assessment of underground structures: A case study of Iranian subway tunnels* Arsham Moayedifar1 Hamid Reza Nejati1,* Amin Nazerigivi1 Mohammad Khosrotash2 1 Rock Mechanics Division, School of Engineering, Tarbiat Modares University, Tehran, Iran 2 Donya Tunnel Saze Consulting Engineering Company, Tehran, Iran Abstract Active geological and young faulted zones Keywords: seismic response; fragility; risk assessment; under- have made Iran’s territory one of the most seismological ground structures; urban tunnel active areas in the world according to recent historical earthquakes. Some of the deadliest earthquakes such as Gilan 1990 and Kermanshah 2018 caused tens of thousands 1 Introduction fatalities. If such violent earthquakes affect strategical structures of a country, indirect losses would be more Tunnel and underground structures are increasingly concerning than direct losses. Nowadays there is no doubt being used as a vital component of infrastructures in a about the vital role of tunnels and underground structures in wide range of applications such as urban transportation, urban areas. These facilities serve as nonstop functional water tunnel, power house caverns, etc. One of the most structures for human transportation, water and sewage major hazardous events which could put these infrastruc- systems and underground pedestrian ways. Any external tures in danger is an earthquake occurrence which is more hazard subjected to underground spaces, such as earthquake could directly affect passenger’s lives and significantly of a concern in shallow urban tunnels constructed in soil decrease whole system reliability of public transportation. (Hashash et al., 2001). According to previous empirical Commonly two earthquake levels of intensities, Maximum observations (Dowding and Rozen, 1978; Owen and Design Earthquake (MDE) and Operating Design Earthquake Scholl, 1981; Sharma and Judd, 1991; Power et al., 1998; (ODE) were used in seismic design of underground structures. Kaneshiro et al., 1999 and Jiang et al., 2010), underground However, uncertain nature of earthquakes in terms of structures may have more capacity to withstand earthquake frequency content, duration of strong ground motion, and loads than surface ones. However, in the recent decades level of intensity indicate that only the two levels of occurrence of some violent earthquake such as 2004 Indian earthquake (ODE and MDE) cannot cover the all range of Ocean, 2008 Wenchuan, and 2011 Christchurch signifi- possible seismic responses of structures during a probable cantly affected tunnel functionality. Furthermore, variation earthquake. It is important to evaluate the behavior of tunnel under a wide range of earthquake intensities. For this purpose, of rock mass geo-mechanical properties and unpredictable a practical risk-based approach which is obtained using the characteristics of a probable earthquake cause an enormous total probability rule was used. This study illustrates a uncertainty in the seismic response evaluation of the framework for evaluation seismic stability of tunnels. Urban underground structure. These uncertainties play significant railway tunnels of Tehran, Shiraz, Ahwaz, Mashhad, Isfahan roles in probable seismic response of structures. Therefore, and Tabriz were considered as study cases. Nominal value of it is important to obtain a reliable evaluation of tunnel seismic risk for three main damage states, including minor, stability with consideration of the influential uncertainties. moderate and major were calculated. In such cases, risk-based evaluation approaches are more applicable than conventional methods. As mentioned, most portion of the previous researches * Received 22 June 2019; accepted in revised form 11 December on seismic behavior of tunnels restricted to empirical 2019; published 20 April 2020. reports of damaged tunnels affected by earthquakes which * Corresponding author. e-mail: [email protected] © The Seismological Society of China and Institute of Geophysics, are more qualitative than quantitative. On the other hand, China Earthquake Administration 2019 most engineers design tunnel support based on free-field Earthq Sci (2019)32: 64–71 65 procedure (Hashash et al., 2001) or performing full opening and tunnel section shape. dynamic time history analysis with respect to design Furthermore, several researchers were developing earthquake criteria limited to two earthquake levels of analytical and numerical approaches to evaluate seismic intensities: maximum design earthquake (MDE) and behavior of underground structures (e.g., St. John and operation design earthquake (ODE). Therefore, based on Zahrah, 1987, Wang, 1993, Penzien, 2000 and Hashash et qualitative and insufficient available data (Wang, 1993) al., 2001, Tsinidis et al., 2013, Abate et al., 2016). Free- which have been reported on seismic response of field deformation describes ground strains due to seismic underground structures or using just two intensity levels of load in the absence of support and excavated area. These MDE and ODE, a reliable seismic assessment of deformations calculated without considering the soil- underground structures with respect to many sources of structure interaction (Hashash et al., 2001). Newmark uncertainties cannot be obtained. (1968) and Kuesel (1969) proposed a simplified method To bridge this gap in this article, a risk-based approach for calculating these strains. St. John and Zahrah (1987) relies on probabilistic tools aimed to obtain a reliable developed Newmark approach to calculate free-field axial procedure for evaluation seismic response of underground and curvature strains. In order to obtain more reliable structures with consideration of uncertainties associated results by considering interaction between support and the with earthquake occurrence have been used. surrounding media, various closed-form solutions have been suggested by Schwartz and Einstein (1980) and 2 Seismic response of underground Penzien (2000). structures Aforementioned researches on the seismic response of underground structures are limited to analytical analysis, numerical simulations and empirical reports. There are Although tunnel and underground structures have scarcely available experimental studies on evaluation of been considered as naturally resistant structure to seismic underground structures due to seismic load. This is loads, occurrence of some violent earthquake such as 2004 because of costly and time consuming procedure of Indian Ocean, 2008 Wenchuan, and 2011 Christchurch physical modeling and seismic excitation. significantly affected structural stability of underground structures. Accordingly, it is more of a concern to obtain a reliable seismic evaluation in order to prevent catastrophic 3 Influential uncertainties consequences such as collapse. So far most of the seismic evaluations of underground All over the world underground structures were structures have carried out on the basis of empirical constructed within a discontinuous, inhomogeneous and observations such as Dowding and Rozen (1978), anisotropic media consisting of wide variety of soil and Owen and Scholl (1981), Sharma and Judd (1991), Li rock that have significant spatial variability. There are also (2012). Sharma and Judd (1991) showed that severe some other sources of uncertainty which could affect damages might happen in condition that consist of ① excavation phase such as water leakage, squeezing, and distance to epicenter less than 25 (km), ② earthquake swelling masses. Thus, tunneling process in term of magnitude more than 6 (Richter) and ③ earthquake inten- design, excavation and post excavation phases is highly sity more than 0.55g. Owen and Scholl (1981) showed that influenced by uncertainties. These uncertainties can be divi- the response of circular tunnels to seismic excitation can ded into two categories: ① epistemic uncertainty and ② be described by three following deformation types: ① aleatory uncertainty (Feng and Hudson, 2015). If an axial compression or extension, ② longitudinal bending, uncertainty source decreases by increasing level of know- and ③ ovaling. Dowding and Rozen (1978) collected data ledge, it is an epistemic uncertainty otherwise it would be from 71 rock tunnels which expressed that a tunnel an aleatory uncertainty. For instance, exact prediction of affected by an earthquake with PGA less than 0.19g might time of occurrence for next earthquake in Tehran is an experience no damage, the same tunnel subjected to an aleatory uncertainty and geo-mechanical characteristics of earthquake with PGA between 0.25g and 0.4g could rock mass are epistemic uncertainty. Figure 1 shows that experience some minor damages and also there would be increasing level of knowledge from complete ignorance serious damages in cases which earthquakes occurred with could reduce level of uncertainty nevertheless, there intensity more than 0.5g. Li (2012) investigated the always be amounts of irreducible uncertainty. relation between tunnel damages and site to source Aleatory and epistemic uncertainty associated with distance, faulted zones, rock mass properties, depth of structure, surrounding media around tunnel and probable 66 Earthq Sci (2019)32: 64–71 Maximum uncertainty Complete Present state of ignorance information Certainty Uncertainty Epistemic uncertainty Aleatory variability Irreducible Imprecision uncertainty Complete Degree of knowledge Complete Present

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