Study on Influence of Charge Structure on Contour Forming Effect of Hard Rock Tunnel

The contour forming effect of a hard rock tunnel is of significant importance in enhancing the stability of surrounding rock and reducing support costs. This paper aims to optimize the single-hole charge and charge structure for tunnel contour holes. The theoretical range of charge parameters for blasting contour holes in tunnels is proposed initially. Subsequently, blasting tests were conducted on a hard rock tunnel using single-hole charges of 1200 g, 900 g, 750 g, and 600 g respectively, with all tunnel contours analyzed by a laser scanner. Finally, a fracture mechanics model was employed to simulate the contour blasting of the hard rock tunnel with different single-hole charges. The results indicate that when charging parameters are within a reasonable range, smaller single-hole charges result in fewer cracks between blasting holes and less damage to the surrounding rock. Although decreasing the charging amount from 1200 g to 600 g reduces over-excavation volume from 6.53 m³ to 2.02 m³, it also increases under-excavation volume caused by hole position error from 0.15 m³ to 0.26 m³ along the tunnel contour. Furthermore, compared to the damage mechanics model, the fracture mechanics model proves superior in simulating contour blasting for hard rock tunnels as evidenced by good agreement between calculated results and experimental data regarding half hole numbers on the tunnel's contour.