Influence Mechanism and Control Measures of Smooth Blasting Contour Formation in Jointed Rock Mass Tunnel

The flatness and damage zone of the tunnel excavation contour is critical to the safety and economy of the project. Considering the seriousness of over-under-excavation and the difficulty of contour formation in the blasting excavation of joint-developing rock mass tunnels, a numerical model was first established by combining compression-shear and tensile-shear failure criterion. Then the analysis of the near-zone stress field distribution and the crack evolution path of the blasthole under different joint characteristics was carried out. Results reveal that the inclination, thickness, and strength of the joints have a significant effect on the crack evolution between the blastholes. The over-under-excavation was most severe when the angle between the joint and blasthole was 45 degrees, while the effect was minimal when the angle was 0 degrees or 90 degrees. The weaker the joint strength and the greater the width, the stronger the blocking effect and the tensile failure effect on the blasting stress wave. Finally, combined with a major engineering example, the smooth blasting scheme of the jointed rock tunnel was optimized and successfully verified. Based on the influence mechanism of joint characteristics, the contour blasting parameters and the technology of large jump delayed initiation and peripheral guiding empty hole were proposed, which significantly improved the smooth effect of the contour surface of the jointed rock mass tunnel.