The role of overburden stress on the seismic vulnerability of deep tunnels

Generally speaking, underground structures have shown to be less vulnerable to earthquakes than above-ground facilities. This occurred even during severe events and the reduced vulnerability seems to increase with the increase of the overburden stress. Focus-ing on tunnels, this correlation has been shown by Sharma and Judd (1991) who studied empirically, the dependence between the level of expected damage and the construction's depth. Their work, based on a database of 191 cases, showed that severe damage is restricted to depths less than 50 m while no appreciable damage is expected below 300 m depth. This trend does not appear evident in deep tunnels surrounded by weak rock. In the aftermaths of the 2008 Wenchuan earthquake in China, Li (2012) noted that whereas the damage suffered by tunnels in hard rocks was consistent with the findings of Sharma and Judd (1991), in weak rock collapses of tunnels were observed at depths as large as 500 m. For the same case study, slight damages were observed at 300 m depth. To clarify the role played by overburden stress onto the seismic vulnerability of deep tunnels in weak rock, this paper presents the results of a parametric study carried out using advanced , fully-nonlinear, incremental dynamic analyses. The same model of the tunnel has been subjected to different levels of seismic intensity by varying parametrically the overburden stress and the properties of the surrounding ground.