Model test on damage control of the composite cushion charging structure subjected to blast at the bottom of underwater borehole

The present investigation examines the effectiveness of a composite cushion charging structure, placed at the bottom of underwater boreholes, to mitigate bedrock damage induced by blasting excavation during cross-sea engineering. With reference to the embedded open caisson blasting project for a major cross-sea bridge, this research adopted a combined methodology of field sampling and controlled underwater explosion model tests. A piezoelectric ceramic detection system was employed to quantitatively analyze how the composite cushion, composed of iron-sand concrete layers with varying wave impedances, affects rock sample damage. Piezoelectric signals were processed via wavelet packet analysis to calculate the axial damage factor (DI) of the blast hole. Furthermore, by integrating fractal dimension theory with damage mechanics, a quantitative assessment was performed on the propagation behavior of cracks formed on the top surfaces of rock specimens under different test conditions. Results demonstrate that the implemented composite bottom cushion effectively mitigates blast-induced damage by significantly inhibiting macroscopic crack propagation, reducing surface crack density, and decreasing the axial penetration depth of cracks into the rock mass. Detailed analysis of axial damage reveals a clear trend, showing that the reduction in DI becomes more pronounced as the wave impedance of the cushion material increases. Specifically, within the borehole-affected zone (0–12 cm), the maximum achievable reduction in DI reaches 10.70%. At the bottom measurement point, reductions exceeding 90% are observed for the higher-impedance cushion configurations. In conclusion, this work confirms that employing a composite cushion charging structure in underwater drilling and blasting operations can significantly alleviate blast-induced damage to foundational bedrock. The findings also establish that axial damage to the rock mass can be effectively controlled by adjusting the wave impedance of the iron-sand concrete in the cushion layer. These outcomes offer valuable insights for optimizing blasting design and enhancing damage control in marine engineering projects.