Numerical study on the fragmentation of rock under single free face explosion of variable diameter decoupled charge

In the conventional radial uncoupled charge single free surface blasting, the bottom rock mass is difficult to be fully broken, which affects the blasting effect and restricts the tunneling efficiency. This difficulty adversely impacts the blasting outcome and limits the efficiency of excavation. To address this issue, this paper proposes a solution that involves modifying the charge structure to implement a variable diameter decoupled charge, and it analyzes the theoretical feasibility of this approach. The variably diameter decoupled charge and radial decoupled charge single-hole blasting model was established and compared using LS-DYNA. Furthermore, the effects of various parameters on the rock-breaking efficiency of variable diameter decoupled charges were analyzed. The results show that, in comparison to radially decoupled charges, variable diameter decoupled charges exhibit a greater explosive mass at their base. This enhancement leads to an increase in the effective stress on the surrounding rock, thereby effectively addressing the issue of inadequate fragmentation of the rock mass at the base of radially decoupled charges. Simultaneously, the directional effect of stress wave superposition and the balancing effect of the cavity on internal pressure contribute to an increase in the effective stress and reflected tensile stress of the overlying rock mass. This phenomenon ensures that effective fragmentation of the overlying rock mass can still be achieved, even with a relatively small amount of explosive charge. Under the condition of maintaining the same charge weight and borehole diameter, increasing the length and radius of the expanding section of the explosive significantly impacts rock fragmentation, whereas reducing the radius of the contracting section has a minimal effect. In engineering applications with a common borehole diameter of 4.2 cm, when the length of the expanding section of the explosive charge is half of the total charge length and the radius of the expanding section ranges from 1.65 cm to 1.70 cm, more effective rock fragmentation at the bottom can be achieved, resulting in an overall favorable fragmentation outcome.