Impact of underwater explosions on concrete bridge foundations

Abstract

In the event an underwater improvised explosive device (IED) were placed near a bridge, Explosive Ordinance Disposal (EOD) units would typically mitigate the threat by conducting a controlled detonation of the bomb. The controlled detonation must be executed a safe distance from any critical infrastructure to ensure the survivability of the structure. This thesis implements the Dynamic System Mechanics Advanced Simulation to characterize a safe detonation distance by determining the critical scenario contributing toward bridge failure. Efforts were also made to determine the parameters critical to modeling bridge foundations. To characterize the most critical scenario, trinitrotoluene was detonated at varying horizontal standoff distances and at varying water depths. The interaction of the underwater explosion (UNDEX) with a bridge foundation modeled from an actual bridge was observed. Intermediate depths were the most damaging to the foundation when the bomb was detonated near the surface of the water and when the bomb was located at the sand-water interface. Subsequently, EOD units should aim for controlled detonations in shallow or deep water. Two parameters, load and rebar reinforcement volume fraction, were varied to observe their impact on the foundation's response to an UNDEX. The damage to the foundation was minimal as the load fluctuated, indicating that these loads do not need to be properly modeled. When the reinforcement was placed entirely in the X-, Y-, or Z- dimension, the rebar perpendicular to the shockwave proved to be the most critical. Changes in reinforcement volume fraction are also not important in short simulations. As the simulation is extended, the dependence of the damage on the volume fraction increases. As such, foundation models do not need to accurately model the load, but must properly model the reinforcement perpendicular to the shockwave and the volume fraction, if the simulation is long. Understanding which foundational components are critical to its survivability allow bridge foundations to be grouped based on these components, such as reinforcement volume fraction.