Underground structures are critical elements in transportation and utility networks. The worldwide growing need to further expand these networks has determined a renewed interest for studying the vulnerability of underground structures to earthquake loading. Fragility curves constitute a powerful tool for assessing the seismic vulnerability of structures. A few fragility relationships have however been specifically derived for underground structures. All of them use as a measure of strong motion intensity the Peak Ground Acceleration (PGA). However PGA shows almost no correlation with the damage potential of ground motion to a structure, particularly underground structures. In this paper empirical fragility curves are proposed for deep tunnels using as a measure of ground motion intensity the Peak Ground Velocity (PGV), which is known to be better correlated to damage.
The underground structures need be thought as critical elements when considering transportation and utility networks. The importance of these structures makes the vulnerability to seismic events extremely relevant. In the preliminary design stage, the seismic vulnerability assessment of underground structures can be done by using fragility curves. These curves can provide the designer with a tool for deciding whether a given problem needs to be studied with more refined analyses. This is a relevant issue in engineering applications considering the complexities associated with these analyses in terms of input data to be provided and of software requirements. The response of underground structures to earthquake loading is considerably different from that of above-ground facilities since the overall mass of the structure is usually small compared with the mass of the surrounding ground and the confining pressure provides high values of radiation damping.
The review of damage suffered by underground structures due to earthquakes (i.e. Dowding&Rozen (1978),Owen&Scholl (1981), Sharma&Judd (1991), Power et al. (1998),Wang et al. (2001), etc.) confirms that these structures are less vulnerable than above-ground facilities. In particular, Dowding & Rozen (1978) grouped damages of underground structures due to earthquakes into three main categories:
damage due to ground shaking;
damage due to fault dislocation;
damage due to earthquake-induced ground failures (e.g. liquefaction and landslides). Usually, most of the damage occurs due to the effects of fault dislocation or landslides. However, since the main focus of this paper is on seismic vulnerability of deep tunnels, the latter effects are not considered. Damage due to fault dislocation is only indirectly associated with seismic loading and it is usually concentrated in a relative small area that can be identified by geological and seismotectonic studies. Damage due to shaking occurs when one or more of the following conditions are met:
earthquakes with large magnitude
tunnels located close to active faults (< 15÷25 km)
poor rock mass conditions
poor construction conditions (i.e. without lining or lining with masonry bricks)
sharp variation of mechanical and/or geometrical properties in both the ground and the structure.