Use of fragility curves to assess the seismic vulnerability in the risk analysis of mountain tunnels

The high uncertainties involved in the design and construction of mountain tunnels have driven the development of risk analysis procedures to control the risk level within an acceptable range. Since underground constructions have proven to perform better than above-ground structures during the past earthquakes, the assessment of the seismic risk of this kind of infrastructures is generally disregarded. Yet, post-earthquake investigations have shown that tunnels are exposed to seismic risk because they are vulnerable to ground shaking. For this reason, a comprehensive risk analysis should also include the seismic risk assessment. Fragility curves, which express the conditional probability to manifest a certain level of seismic damage given a certain seismic intensity measure (e.g. Peak Ground Acceleration), represent a suitable tool that can be implemented in the risk analysis of mountain tunnels for a rapid seismic risk assessment. The currently available fragility curves for rock tunnels are based on expert-opinion or empirical approach, without a properly consideration of the system variability (e.g. tunnel type and rock characteristics, depth of the construction). This article proposes a comprehensive numerical methodology to construct fragility curves for mountain tunnels subjected to transversal seismic loading. The proposed numerical technique, which is based on fully-nonlinear dynamic analyses accounting for the nonlinearity of both ground and tunnel support, allows to consider the features of the tunnel (e.g. type of support, geometry, type of rock mass and depth of construction) as well the variability of the seismic input motion in a more systematic way. Different sets of fragility curves for tunnels bored in fractured rock have been defined in this article. Finally, two simplified case studies have been presented aiming to illustrate the potential uses of the fragility curves within the risk analysis of mountain tunnels. In the first case study the fragility curves are used to evaluate the probability of seismic damage of several tunnels to assess the functionality of the road network in the post-seismic phases. In the second case study, the fragility curves are used to optimize the selection of the tunnel support with regard to the seismic action. In particular, fragility curves are used to evaluate the level of seismic risk associated to the selection of two different types of final lining.