Movement Law and Length Optimization of Stemming Structure in Tunnel Blasting
To determine the stemming length of a borehole,it is important to improve the blasting fragmentation effects and control the adverse effects of blasting.In this study,a stress analysis based on a one-dimensional axial motion model was first performed to investigate the movement law and length optimization of the stemming structure in tunnel blasting.The calculation method for blast loading was optimized by considering the effects of blasting cavity expansion,crack propagation,and stemming structure movement on gas pressure.The mechanism of additional sliding friction resistance was revealed by considering the influence of shock compression of the stemming structure on the sliding friction resistance.Subsequently,a new time-sharing piecewise calculation method for calculating the macro movement process of the stemming structure was established,and the distribution law of the sliding friction resistance decreasing from the bottom to the top was revealed.Moreover,the influences of rock mass type,charge structure,and stemming length on the movement law of the stemming structure were analyzed.Furthermore,the optimal stemming lengths of the cut,breast,and trim holes that allowed part of the stemming structure to rush out of the borehole under different rock mass conditions were calculated using the optimization principle.The results show that the movement process of the stemming structure is not constant and is significantly affected by the rock mass type,charge structure,and stemming length.Under common tunnel blasting conditions,the optimal stemming lengths range from 0.3 to 0.6 m.Specifically,for good rock mass classified as Ⅰ orⅡ,the range is 0.5-0.6 m;for medium rock mass classified as Ⅲ,it is 0.4-0.5 m;for poor rock mass classified as Ⅳ or Ⅴ,it is 0.3-0.4 m.In addition,for a good rock mass,the optimal stemming lengths of the easer and breaking holes are equal,and both are larger than those of the perimeter hole.For medium and poor rock masses,the optimal stemming lengths of the easer and perimeter holes are equal,and both are smaller than those of the breaking hole.The reliability of the results was verified using engineering cases.The findings provide a basis for calculating the stemming length in tunnel blasting.