Response of immersed tube tunnel against internal blast load and its damage mitigation: A numerical study

During service life, immersed tube tunnels may face threats of unforeseen explosive incidents, e.g., hazardous chemical detonations and car bomb attacks. However, owing to the difficulty in effectively predicting the internal blast loads, the current practice usually employs equivalent methods in internal blast load predictions for structural response analysis, which may lead to inaccurate response predictions. This study introduces an internal blast load model to handle the complex physical processes in immersed tube tunnels structures subjected to internal explosions. Based on the validated blast load model, a refined finite element (FE) model of the Hong Kong-Zhuhai-Macao Bridge immersed tube tunnel (HZMB ITT), incorporating the influence of the surroundings, was developed to investigate the dynamic response and failure mechanisms under internal explosion conditions. The results demonstrate that the proposed blast load model can accurately characterize the internal blast loading on the tunnel structure. The surroundings constrains the dynamic response of the tunnel, thereby mitigating blast-induced structural damage. However, due to inertial effects, the medium may separate from the tunnel lining, preventing full transmission of stress waves to the surroundings. As a result, blast-induced stress waves reflect and superimpose within the tunnel structure. As the charge mass increases, blast-induced tunnel damage becomes progressively more severe, manifesting as concrete spalling on the outer surface, fracturing and collapse of the middle and side walls, and failure of the bottom plate. Using the validated FE model, the influence of the charge mass and explosion position on the damage mode and service performance of the tunnel was studied, and a prediction method for the residual bearing capacity of blast-induced pre-damaged structures was established. Considering the constraint effect of the surroundings and the design requirements of HZMB ITT, the effectiveness of the backfill layer modification and node reinforcement measures was proposed and verified. These measures enhance the blast resistance of tunnel structure without compromising its resistance to static, seismic, or other design loads.