Blast Vulnerability of Underground Tunnels

The unfortunate events of September 11th, 2001 have prompted transit agencies to consider potential terrorist attacks against underground tunnels as part of the design criteria for such structures. Due to the confining nature of those tunnels, and the potential to inflict physical and psychological harm to the public, the underground tunnels might be considered attractive targets by would-be assailants. The blast response of the tunnel structure is influenced by the medium surrounding the tunnel. For example, it has been found that significant damage can be caused by waterborne bombs due to the thrust induced by hydraulic forces. Furthermore, the properties of the surrounding soil will not only influence the tunnel response during a blast event, but might also affect the infiltration of water and debris into the tunnel network should the tunnel walls be breached. This article will discuss blast vulnerability of underground tunnels, blast damage assessment considering nonlinear soil-structure and fluid-structure interactions, and finally some of the blast mitigation strategies will be presented. A risk-based methodology is presented as a tool to facilitate and justify the selection of appropriate blast mitigation strategies.