Numerical Investigation of Bench Blasting in Hard–Soft Interbedded Rock Masses: Implications for Blasting Design in Heterogeneous Rock Masses

Uneven energy distribution and suboptimal fragmentation in bench blasting of hard–soft interbedded rock masses are critical challenges in open-pit mining. In this study, a five-hole bench blasting numerical model is developed using the discrete element method (DEM) to systematically investigate the effects of hard ore layer position, dip angle, and thickness on blasting performance. Numerical results indicate that while hard–soft layering has limited influence on overall bench fragmentation, it strongly controls block size distribution. Hard ore layers located in the upper or lower parts of the bench tend to form concentrated zones of large blocks, whereas those in the middle part achieve more uniform fragmentation, reducing the oversized block rate by approximately 57% and 45% compared with upper and lower locations, respectively. The dip angle of hard ore layers exhibits a nonlinear effect on the oversized block rate, reaching a maximum at 20°, and layer thickness is positively correlated with large-block occurrence. Based on these findings, a refined blasting strategy for hard–soft interbedded rock masses is proposed. Numerical simulations demonstrate that introducing satellite holes and implementing staged charging reduce the oversized block rate by 13% and 36%, respectively. Field bench blasting trials further indicate that top air-deck charging is beneficial for improving fragmentation uniformity in heterogeneous rock masses. These results provide a scientific basis for optimizing bench blasting parameters under complex lithological conditions.