The determination methodology for optimal stemming length of deep-hole blasting for hard roof cutting in coal mines

Deep borehole blasting of coal mine roadway roof is significant for roof cutting-based gob-side entry retaining and hard roof pre-cracking. It not only needs to ensure roof cutting effect, but also ensures that no punching occurs and the roadway surrounding rocks are stable after blasting. Therefore, it is crucial to determine the reasonable length of borehole sealing. Given the lack of theoretical basis for the current coal mine roof blasting borehole sealing length, the borehole sealing parameters in projects involving hard roof cutting in some parts of China were first statistically analyzed, and based on the sealing mechanism of the blasting boreholes and the force characteristics of the clogging material in the process of blasting, it established a mathematical model of deep borehole blasting, and proposed a method of calculating the sealing length based on the constraints of the blasting “no pushing out of borehole blockages”. In addition, based on Livingston’s blasting funnel theory and combined with the charging structure, the top plate deep borehole charging blasting funnel model was established, and the borehole sealing length calculation method based on the “maximum internal blasting effect” of explosives was obtained. The nonlinear dynamic analysis software LS–DYNA was used to analyze the effective stress distribution and crack development in the borehole surrounding rocks after blasting, and to determine the reasonable distance between sealing body and the last section of explosives and the plug based on the “maximum energy utilization of explosives”. According to the above research results, a three-way criterion for determination of optimal sealing length of deep blasting boreholes for hard roof cutting in coal mines based on “no pushing out of borehole blockages”, “maximum internal blasting effect” and “maximum energy utilization” is proposed. Taking the roof cutting of 22010 working face of Chaochuan coal mine as the engineering background, the optimal sealing length was calculated based on the three-dimensional criterion for sealing length determination, and the degree of stability of the roadway top plate and the reasonableness of the sealing length were analyzed based on numerical simulation with the effective stresses of the rock units on the surface of the roadway top plate as the indexes, and the on-site experiments were also carried out. The test results show that the sealing length calculated based on the present ternary criterion not only avoids pushing out of borehole blockages, but also generates a slit surface from the borehole depth of 7 m to the bottom of the borehole through the blast borehole, cutting off the target rock layer. The research results can offer a theoretical basis for determining the optimal sealing length of the roof cutting and pressure relief blasting borehole in the coal mine roadway.