The research on blasting damage of medium has important engineering significance, wherein one of the main factors affecting the blasting effect of the medium is the free boundary of the medium. In this article, the blasting damage of media with one free boundary is studied and the damage evaluation based on fractal theory is carried out. Firstly, an experimental device is designed to simulate the state of the medium’s one free boundary. The distance between blasthole and the free boundary is the only variable considered in the experimental investigation. The fractal theory is used to analyze the fractal damage of the medium caused by the explosion, and MATLAB is used to extract three characteristic parameters (damage depth, damage width, and damage zone) of the binary image after the explosion. Finally, LS-DYNA is used to build a 2D plane strain model with the same size and boundary conditions, which simulates the damage evolution process of medium under a single free boundary blasting pattern. The fractal theory is then used to evaluate the fractal damage observed in the numerical simulation results. The experimental results and numerical simulation results of blasting fractal damage of one free boundary medium are in good agreement. The blasting fractal damage grows initially and subsequently decreases as the distance between the blasthole and the free boundary increases. Thus, the distance between blasthole and free boundary affects the efficiency of blasting energy destroying medium and the blasting failure pattern.

In hard rock deep‐hole blasting excavation, blastholes are rarely utilized due to the clamping effect of the lower rock, which affects excavation progress and restricts the development and application of deep‐hole blasting technology. Cut blasting is a key to improving excavation speed. In this paper, a new cutting method designating the two‐step cutting technology was presented. The blasthole was divided into upper and lower sections without changing the blasthole layout. The upper section was detonated first, creating sufficient free surface for the lower section, which was detonated secondly. This created a larger cavity and improved blasthole utilization. Results showed good blasting effects for two‐step cutting technology through theoretical analysis and engineering applications. The blasthole utilization rate was 96.1% when the upper and lower specific charge ratio = 0.78. This paper provides a good reference for resolving the low blasthole utilization problem in deep‐hole blasting of vertical shafts within a hard rock.

The blasting stress wave and blasting gas generated by explosive blasting are the two main motive powers of rock fragmentation. An experimental method based on water jet test is used to study the energy distribution ratio of blasting stress wave and blasting gas; the utilization efficiency of blasting energy under different borehole constraint conditions is also analyzed. It proves that the blasting stress wave does not cause the water jet, and the blasting gas is the only power of the water column jet. The results show that the energy of the blasting gas and blasting stress wave respectively account for about 64% and 36% of the total energy generated by the explosion of lead azide. The utilization efficiency of the blasting gas energy under strong, medium, and weak constraint conditions is 100%, 80%, and 52%, respectively. The borehole constraint condition is crucial for the effective utilization of blasting energy.

In blasting engineering, borehole utilization is a key metric for evaluating blasting performance. While previous studies have examined the effects of expansion space, cutting design, in-situ stress conditions, and rock properties on borehole utilization, research on the intrinsic relationship between extra-depth defined as the portion of the cut hole extending beyond the depth of auxiliary holes and borehole utilization remains insufficient. This gap in understanding has hindered the resolution of issues such as residual boreholes and unbroken rock at the borehole bottom in deep-hole blasting, thereby limiting improvements in borehole utilization. This study employs a simplified double-hole model for extra-depth cut blasting to conduct two-dimensional numerical simulations and three-dimensional cement mortar model experiments. It systematically investigates the blasting damage characteristics, fractal damage, and energy evolution under varying extra-depth as a single variable. Experimental parameters such as borehole utilization, cavity diameter, cavity volume, and fragment size distribution were obtained to comprehensively analyze the nonlinear effects of extra-depth on post-blast rock damage and its mechanisms. Both simulation and experimental results indicate that blasting damage parameters exhibit a nonlinear trend of initially increasing and then decreasing with increasing extra-depth. Appropriately increasing the extra-depth improves rock breakage efficiency, while excessive extra-depth reduces efficiency due to confinement effects at the borehole bottom. Adjusting the extra-depth can optimize the distribution of explosive energy between rock fragmentation and rock ejection.

Cutting is a key factor affecting the speed of blasting excavation. With the continuous advancement of deep-hole blasting technology, determining the optimal extra-depth of the cut relative to the non-cut blast hole is of paramount importance. By combining model experiments and numerical simulations, this study systematically investigates the effect of extra-depth on post-blast damage morphology, damage width, depth, area, and fractal damage. Furthermore, numerical simulations were employed to validate the experimental results from the perspectives of damage evolution and peak pressure at monitoring points. The main findings are as follows: The extra-depth of the cut has a significant nonlinear effect on the blasting outcome. As the extra-depth increases, blasting damage initially increases but decreases beyond a critical depth due to marginal effects, leading to the appearance of residual blast hole features. Under experimental conditions, when the extra-depth was 15 mm, the damage depth, width, and area reached their maximum values of 43.5 mm, 109.9 mm, and 6055.2 mm², respectively, indicating optimal blasting performance. The fractal damage of the specimen also exhibited a significant trend of initially increasing and then decreasing. The maximum fractal damage, obtained by deriving the fitting curve, was 0.75, corresponding to an extra-depth of 13.7 mm. The numerical simulation results are in good agreement with the experimental findings, showing a significant downward shift in the peak pressure points in the damage zone with increasing extra-depth. In summary, an appropriate extra-depth can achieve optimal borehole utilization, while an excessive extra-depth can lead to residual blast hole formation at the bottom of the cut, reducing effectiveness. This study provides theoretical guidance for optimizing the design parameters of extra-depth in deep-hole blasting.