Seismic design and Assessment of Urban Tunnels- A literature review on modelling studies

Advancements in tunnelling technologies and ease of implementation of drilling methods in addition of other political and security issues made the construction of underground structures as an important alternative for answering the demands of population growth and the limitations of surface spaces in urban areas. Underground roads and highways, various types of tunnels and urban subway networks are the examples of underground structures being constructed and rapidly implemented in different countries. Meanwhile, for reducing negative effects to the environment, shortening the routes and improving traffic efficiency, urban tunnels should have high level of safety standards in design, construction and operation. Tunnels are considered major national projects and infrastructure investments, and huge costs are incurred around the world to build these structures. In countries located in highly active seismic zone, such as Iran, seismic researches for such important underground structures should not be ignored. The safety of such structures should be provided with respect to all loading demands and hazards issues associated with the site, including seismic loads. Reviewing seismic events in the past shows that underground structures have suffered less damage than above ground structures against seismic loads. However, in recent years, major earthquakes such as the 1995 Kobe earthquake in Japan, the 1999 Chi-Chi earthquake in Taiwan, the 1999 Kocaeli earthquake in Turkey, and the 2008 Wenchuan earthquake in China have caused underground structures to experience significant damage. There is evidence to conclude that the structural vulnerability of a tunnel in seismically active areas is an important issue but is either not yet well understood or not well assessed at the time of construction, emphasising that dynamic analysis of these structures against seismic loads is necessary. Earthquakes are likely to significantly affect tunnel performance by causing severe damage or excessive deformation of the tunnel structure. To understand the seismic-induced behaviour and performance of urban tunnels, this paper provides the state of the art in modelling studies of seismic design and assessment of tunnels. The review includes an investigation in seismic responses of real tunnels reported during past seismic events, the probable mechanisms caused damages in tunnels and physical and numerical methods used until now to either investigate those mechanisms or implemented in new designs. As an introduction, the seismic performance of tunnels affected by previous seismic events discusses first, emphasising the effective parameters in evaluation of tunnel seismic response and the relationship between the parameters, and the damage levels caused during earthquakes. Subsequently, the paper continues with a comprehensive literature review on the experimental methods used to investigate seismic-induced response in tunnels including physical testing, centrifuge tests, shaking table tests, and static tests. Analytical, quasi-static and numerical methods of dynamic analysis of tunnels and the accuracy of these methods are discussed then in details referring to some examples. The paper also reviews the effects of soil heterogeneity in the seismic response of tunnel and application of the random field for dynamic analysis of underground structures.  Examining the achievements and challenges remained in the field, the paper concludes with the existing gaps in the field to stimulate readers for doing more relevant researches.