For some immersed tube tunnels, the horizontal slab contributes to the structural integrity. If a train running on the slab were subjected to an explosion, which then failed a large area of the horizontal slab, the sidewall might yield under the lateral earth and water pressures. In this study, a three-dimensional Finite Element (FE) model was established to analyze the problem. The dimension and structural characteristics of the model tunnel was taken from a published literature. Three different failure scenarios were analyzed to investigate the risk of side wall collapse. It was found that for a well-designed immersed tube tunnel with sufficient lining thickness and reinforcement, the possibility of sidewall collapse is very small.