# Theoretical Study of Large Uncoupling Coefficient Loading for Surface Blasting
## 1 Introduction  
Smooth blasting is one of the commonly adopted methods of controlled blasting. The objective of this technique is to cause as little damage to the surrounding rock mass as possible while leaving a smooth surface of the unexcavated rock face. Residual half-hole walls are an important indicator of successful smooth blasting [1; 2; 3; 4]. The currently adopted effective charge structures for smooth blasting are the air-spaced axial uncoupled charge and air-spaced radial uncoupled continuous charge. Air has high compressibility and wave impedance value relative to the rock; therefore, the blast wave generated by the explosionfirst compresses air in the gap, producing an air shock wave, which then impacts the wall of the drilled holes. The air-spaced charge serves three purposes: (1) Air-spaced charge has a buffering effect on the blast stress wave, which reduces the peak blast stress and improves the rock breaking effect. (2) Air-spaced charge increases the time duration of the explosive stress wave. This, on the one hand, reduces the impact pressure, which in turn reduces the energy of the drilled hole wall fragmentation. On the other hand, the cushioning of the air also increases the action time of the stress wave. (3) It increases the impulse of the stress wave acting on the rock. This allows uniform distribution of the specific impulse along the shell, which can effectively improve the quality of the crushing blocks and avoid the formation of large blocks [5; 6; 7; 8; 9; 10; 11]. [PERSON] et al. [12] stated that adopting a larger axial uncoupling coefficient for an axial air bedding charge in a blast hole affects the magnitude of the peak pressure acting on the blast wall and the loading time duration of the quasi-static pressure of the explosive gas. By increasing the axial uncoupling coefficient, the peak pressure decreases, whereas the positive pressure action time is prolonged. To achieve the desired effect of smooth blasting, three conditions must be satisfied when defining the structural parameters of the air-spaced charge:  
(1) To ensure that no compressive damage occurs to the blast hole wall, it is necessary that the initial peak radial stress acting on the wall is lower than the compressive strength of the surrounding rock. (2) To ensure the formation of radial cracks in the blast hole wall, the initial peak tangential stress acting on the blast hole wall must be higher than the tensile strength of the surrounding rock. (3) To ensure the formation of inter-hole penetration cracks, the spacing between the holes must be smaller than the length of the burst cracks. [PERSON] et al. [13] theoretically calculated the axial uncoupling coefficient, which plays a decisive role in establishing the structural parameters of the small aperture air cushion charge. Similarly, [PERSON] and [PERSON] [14] conducted a theoretical analysis and developed a method to determine the structural parameters of the axially uncoupled charge of the air and water cushions for small-aperture smooth blasting. [PERSON] [15] established a fracture mechanics model for smooth blasting and pre-cracking blasting with a blast hole diameter of 42 mm and a charge diameter of 25 mm. They analyzed the role of stress waves and quasi-static pressure of explosion-generated gas in the blast rupture process. [PERSON] and [PERSON] [16] studied the artificial neural network (ANN) technique to determine the near-optimum blasting pattern. This method is applied in Gol-E-Gohar iron mine. [PERSON] et al. [17] studied minimizing vibration while drilling and obtaining the optimal operating condition to enhance drill performance. Based on the deep neural network research, analysis of the relationship between blasting parameters and rock fragmentation was established by [PERSON] et al. [18], and sensitivity analysis was performed on blasting parameters. [PERSON] et al. [19] provided a powerful technique to optimize the blast parameters in open pit blasting operations based on the genetic algorithm.