The structural integrity of immersed-tube tunnels that have a cross-section as shown in Fig. 1 depends on the lining, the center wall and the horizontal slab.  If a train running in tunnel-section I or II were subjected to an explosion, which then failed large area of the horizontal slab and also significantly weakened the sidewall, the sidewall might collapse under the lateral earth and water pressures.  In this study, a sophisticated three-dimensional Finite Element (FE) model will be established to analyze the problem.  It will identify the failure mechanism of such tunnels under the aforementioned scenario.  Structure protection using FRP wrapping will also be investigated.  Outcome of this study may not only serve the infrastructure protection in New York City area, it may also contribute to the general tunnel security literature, as the investigations on sidewall collapse due to loss of lateral support are very limited.