Experimental and numerical studies of the blast-induced overbreak and underbreak in underground roadways

Overbreak and underbreak are the crucial problems during the blasting excavation of underground tunnels owing to their effects on the construction costs and operational safety. A critical challenge facing overbreak and underbreak control is the difficulty in developing guidelines with respect to various and complex engineering conditions. In this study, a series of field measurements of overbreak and underbreak using the FocusS 150 laser scanner were performed in a deep roadway of the Kaiyang phosphate mine, China. The distribution and extent of the overbreak and underbreak surrounding the roadway contour were accurately analyzed in accordance with the collected point cloud data. Subsequently, a simplified three-dimensional model was established to simulate the blasting excavation of pre-stressed roadway using the explicit dynamic analysis code LS-DYNA. A comparison of numerical and measurement results revealed that the proposed model was a reliable tool to simulate the overbreak and underbreak induced by blasting excavation. Thereafter, the influences of uncontrollable geological factors such as in situ stress conditions and controllable blasting factors including contour hole spacing (S), charge concentration (β) and decoupled coefficient (ζ) as well as stemming were further numerically investigated. The simulation results indicated that the lateral pressure coefficient significantly affected the distribution pattern of the overbreak and underbreak, while the stress magnitude contributed to their extents. Moreover, a comparison of the simulation findings and the field measurement data indicated that the minimal extents of the overbreak and underbreak corresponding the optimal contour blasting results were obtained at S = 0.70 m, β = 0.9 kg/m and ζ = 2.5, respectively. Furthermore, the contour blastholes stemmed with sand created smaller damage to the periphery rock mass of roadway and enhanced the utilization efficiency of explosive energy. The research findings of this study provide important implications for similar blasting excavation projects.