To achieve precise excavation boundary control and promote planar cracks propagation in underwater blasting operations, this study systematically investigates fracture initiation mechanisms through the deployment of multi-point bidirectional shaped charges submerged in aqueous environments. Based on the unique energy transmission dynamics of underwater explosions, compact-volume shaped charges were engineered with forward-positioned air cavities adjacent to the liners, optimizing jet formation efficiency and directional fracture control. The actuation sequence and planar fracturing mechanism of the underwater shaped charge were comprehensively analyzed through integrated 2D and 3D numerical simulations and concrete splitting experiments. The results demonstrate that the designed actuation sequence proceeds through distinct phases: blast shockwave propagation, head shockwave formation, shaped jet generation, hydraulic wedge action, and bubble pulsation effects. The synergistic interaction of these mechanisms establishes an optimized chronological sequence that, when coupled with pre-cut borehole grooves, induces a tensile stress concentration zone along the predetermined fracture plane, facilitating preferential crack initiation and controlled propagation. Field validation tests successfully generated planar fractures in concrete targets using a remarkably low linear charge density (39 g/m), with fracture patterns exhibiting exceptional alignment with the designed splitting plane and strong correlation with numerical simulation. These findings confirm both the operational feasibility of the charge configuration and the predictive accuracy of the computational model. The developed methodology offers significant practical value for boundary-controlled blasting applications in underwater environments and in terrestrial settings requiring stringent blast-impact mitigation.

In the blasting excavation project of the Pinglu Canal channel, the adjacent bridge pile foundation is in a repeated blasting vibration environment for a long time, and its structural damage accumulation effect directly affects the safety performance of the whole life cycle of the bridge. Clarifying the damage evolution mechanism of pile foundations under repeated blasting vibration is a key scientific premise for ensuring the coordinated development of efficient canal channel excavation and the safe operation of adjacent bridges. Based on the channel blasting project for the Luyang New Village Bridge in Pinglu Canal, combined with a finite element numerical model that considers the cumulative damage of the concrete structure, the influence of on-site blasting construction on the pile foundation during construction is simulated and analyzed. Based on the reliability analysis of the model, the cumulative damage variation law of pile foundation under different blasting times is simulated and analyzed. The damage variable prediction model based on the peak vibration velocity of pile foundation is established, and the cumulative blasting times of pile foundation safety under the given blasting center distance are obtained. The results show that the maximum damage variable value of the pile foundation appears on the horizontal side of the explosion source. When the explosion center distance is less than 20 m, the damage to the pile top area is significantly aggravated. The safe blasting times for pile foundations are 2~3 times longer when the blasting center distance is 20 m.