Measurement of Overbreak and Underbreak Thickness and Optimization of Perimeter Hole Blasting Parameters in Roadways

In tunnel blasting operations, inappropriate peripheral-hole design parameters frequently lead to overbreak and underbreak phenomena, adversely affecting excavation efficiency and jeopardizing the structural integrity of surrounding rock formations. Using a tunnel project at the Guangxi Fozi Lead-Zinc Mine as the case study, this research initially employed a handheld 3D laser scanner to acquire excavation-face geometric data, with multiple tunnel sections selected for overbreak and underbreak quantification. Subsequently, an LS-DYNA numerical model was developed to simulate the contour blasting excavation process with high fidelity. Critical parameters including hole spacing, explosive charge per blasthole, and smooth blasting layer thickness were systematically optimized. Comprehensive qualitative and quantitative analyses were performed to evaluate damage zones across various tunnel sections. Field validation of the optimized design was conducted, incorporating the coefficient of determination R² to quantitatively assess excavation face smoothness and its engineering significance. The results demonstrate that: (1)The numerical model effectively captures the damage progression in surrounding rock mass under blasting loads, revealing significant overbreak primarily in the charged sections and underbreak near the blasthole collar and bottom under current blasting parameters; (2)Excessive hole spacing and charge quantity substantially increase average overbreak thickness, while smooth blasting layer thickness(40~60 cm range) exhibits comparatively minor influence; (3)Implementing decked charging structure instead of continuous charging with optimized parameters(60 cm spacing, 1.5 kg per hole, 40 cm smooth blasting layer) reduces maximum overbreak depth from 38.5 cm to 27.4 cm, decreases maximum underbreak depth from 44.5 cm to 20.9 cm, and improves tunnel contour matching coefficient from 0.339 to 0.841. The optimized blasting parameters successfully mitigated overbreak and underbreak in surrounding rock mass while significantly improving the contour smoothness of the tunnel excavation faces.