Effects of Long–Short Inclined Blasthole Configurations on Overbreak and Damage Control in Contour Blasting

Due to the limited drilling accuracy of the drilling jumbo, the unavoidable outward inclination of perimeter blastholes is a key factor causing overbreak and damage to the remaining rock mass. However, effective methods to control this issue remain limited. In this study, a combined approach of laboratory blasting experiments and numerical simulations was adopted to investigate the crack propagation characteristics, stress response, and damage evolution of the surrounding rock under equal-length blastholes and long–short hole combined arrangements. The results indicate that the long–short hole configuration effectively guides interhole crack propagation along the designed contour. This approach can significantly reduce overbreak and decrease both the peak strain and damage level on the remaining rock mass side. Further analysis reveals that the short-to-long hole ratio (SLR) has a significant influence on blasting performance, and an SLR of 0.5 produces the lowest overbreak and retained-rock damage volumes among the investigated cases under the present model conditions. These findings demonstrate that a properly designed long–short hole combination can effectively control overbreak and surrounding rock damage while maintaining excavation efficiency, providing a practical approach to mitigate overbreak induced by outward inclination in engineering practice.