Sand ejecta kinematics and impulse transfer associated with the buried blast loading: A controlled laboratory investigation

An experimental test facility has been developed for performing laboratory studies on shallow buried blast loading. The shock tube based test facility offers an alternative method for generating blast wave in a controlled and repeatable manner, without the use of explosives. In this paper, we consider a spherical expanding blast wave of moderate shock strength with relatively low driving pressure. A confined dry sand bed prepared with a constant density, is exposed to a blast wave from the embedded shock tube. The principal objective of this paper is to understand the various events involved during the interaction of leading blast wave with the soil medium, followed by the expansion of the entrained gas. The process initiates with the formation of stress wave in the sand media, followed by the gas bubble expansion and terminates with sand ejection. The variation in the output of the sand ejecta is investigated with the help of high speed photography. The velocity of the sand ejecta front is found to decrease with the increase in the burial depth (DoB). Further, the impulse (using vertical pendulum) and peak pressure (using transducers) imparted to the rigid target are evaluated. The target is located at different stand-off distances (SoD) above the top surface of the sand bed. It is found that the peak pressure values are influenced by the presence of dome-cap of the ejecta, expanding vertically upwards generating a point-load impact. Irrespective of the depth of burial (DoB = 32 mm-64 mm), maximum impulse is observed around the zone of bubble expansion (close to the sand bed surface). Sand ejecta does however have a greater influence on the impulse at higher SoDs (> 40 mm). Moreover, the maximum momentum transfer is observed when SoD to DoB ratio is 2.5. In conclusion, the shock-driven sand test facility is found to be a simple and efficient tool to study the complex dynamics of sand ejecta, including the post- impact on the target structures.