Explosion loads in immersed tunnels

Certain goods have properties that may cause severe effects in case of an accident during transport. The most striking events are the occurrence of fire and explosions. Since tunnels are isolated environments, many casualties and severe damage are to be expected in case of an explosion. Although the probability for such an event is rather small, restrictions apply for tunnels concerning transport of hazardous goods. As a result of these restrictions alternative routes have to be used, often resulting in long detours that sometimes cross built-up areas, which is also undesirable. Therefore, at some locations it is desired to ease the restrictions for tunnels. For a recent immersed tunnel project requirements concerning explosion loads were stated. A static pressure of 500 kPa and a suction of 300 kPa should be taken into account. The nature of the requirement was not clear however. During the tender phase, BAM Infraconsult concluded that large thicknesses for the slabs and walls of the tunnel are required. The concrete elements, of which an immersed tunnel is composed, should float during transport, while in the final situation a large deadweight is favorable in order to prevent uplift. These are contrary demands that result in a very delicate balance. The requirement for the explosion load interferes with this balance and makes it difficult to comply with all demands efficiently if traditional design methods are applied. This research is at first initiated in order to evaluate the recently stated statically defined requirement concerning explosion loads. Secondly, a technical feasibility study into the design of an explosion resistant immersed tunnel is performed. In order to investigate the effect of explosion loads, a simplified analytical model was developed whereby the structure is divided into several elements that are schematized as single degree of freedom mass spring systems. Furthermore, the dynamic module of the finite element code Plaxis was used to perform more advanced calculations. Both models were used in the evaluation of the requirement and technical feasibility study. It is concluded that the representative explosion load is due to a LPG Boiling Liquid Expanding Explosion (BLEVE). The order of magnitude that has to be taken into account is obtained from recent research into this topic by TNO. Although the static requirement of 500 kPa pressure and 300 kPa suction is already hard to comply with, the order of magnitude is too small compared to the representative dynamic BLEVE Load according to TNO. The representative load according to TNO is used for the technical feasibility study. Several solutions, with respect to reduction of the load, dissipation of energy and application of alternative materials are considered. Exploring calculations were performed for promising solutions in order to judge the technical feasibility. It is concluded that the application of separate tubes provides an efficient structural solution. Since the vehicles transporting the dangerous goods are isolated from the regular traffic the probability of an accident decreases and the consequences of an explosion will be less severe. The costs involved will be high compared to a regular tunnel. There are possibilities with respect to traffic management of the special tubes that make this alternative more attractive in an economical sense however. If an explosion resistant tunnel is desired it is recommended to consider this solution.