Assessment of damage to an underground box tunnel by a surface explosion

Recently, external terrorist activities have become one of the most influential events on tunnel structure safety because of the absence of proper mechanisms to detect these events in time to take preventive action. The present study used ANSYS/LS-DYNA software to investigate the damage behaviour of an underground box frame tunnel caused by a surface explosion of a sedan, van, small delivery truck (SDT), and container carrying 227, 454, 1814, and 4536 kg, respectively, of TNT charge weight. The Arbitrary Lagrangian Eulerian (ALE) technique was used to simulate and monitor the propagation of the blast pressure waves into the soil. The validation results indicated that the pressure waves propagated into the soil as hemispherical waves, and the peak pressure values closely matched the predicted values of the technical design manual TM5-855-1, except for large distances. Therefore, an equation was derived to calculate the values of the peak pressure at large distances for each explosion case. Intensive parametric studies were conducted to evaluate the interaction between the explosive charge weight, the tunnel lining thickness and the burial depth, which has a significant effect on tunnel safety. The assessment of the damage levels using the single degree of freedom (SDOF) approach proved that the tunnel experienced little damage when the explosive charge is a sedan or van with a lining thickness of 250, 500 or 750 mm at burial depths of 4, 6, or 8 m. However, tunnel collapse occurred when the lining thickness was 250 mm, and the tunnel was subjected to an explosion of an SDT or container at all investigated depths, as well as the case for a lining thickness of 500 mm at a depth of 4 m for the container explosion. The tunnel lining with a thickness of 750 mm appeared to be highly resistant to the explosion of an SDT or container for all the investigated depths, and the best resistance was achieved at a depth of 8 m, which should be considered by designers to ensure the safety of an underground box tunnel when subjected to an incredible surface explosion.