Study on differences in rockburst prevention mechanism and pressure relief effect between strike and dip blasting for hard roofs

To reveal the differences in rockburst prevention mechanism and pressure relief effect between strike and dip blasting for hard roofs, the stress evolution law of surrounding rock, the structural response characteristics of the roof, and the rockburst prevention mechanism under two blasting layouts are systematically studied by combining 3DEC numerical simulation and theoretical analysis, with the Xinjulong 6305 working face as the engineering background. It is shown by the results that strike blasting mainly acts on the coal pillar of the mining roadway and the lateral roof area. The peak stress of the coal pillar is gradually reduced with the optimization of blasting parameters, and the optimum is achieved at a blasting depth of 45 m and an angle of 80°, where the peak stress is reduced by 20.49%. Dip blasting mainly acts on the overlying strata ahead of the working face. The peak advance abutment pressure first decreases and then tends to stabilize with the increase of blasting height, and under the optimal parameters, the peak value is reduced by 20.60%. On this basis, a mechanical model of the lateral cantilever beam and a mechanical model of the dip hinged structure are established, respectively. By strike blasting, the lateral cantilever beam is cut off and its length is shortened, so that the roof stored energy is reduced in proportion to the fifth power of the cantilever length. By dip blasting, the key stratum is divided through a fan-shaped weak surface, so that the roof breaking interval is reduced and the energy released in a single event is reduced in proportion to the third power of the breaking interval. Significant differences are found between the two blasting methods. Strike blasting is suitable for high static load control conditions characterized by a large lateral hanging roof along the gob side and obvious stress concentration in the coal pillar. Dip blasting is suitable for dynamic load control conditions characterized by high advance abutment pressure and strong periodic weighting. It is demonstrated by field engineering practice that satisfactory pressure relief and rockburst prevention effects can be achieved through reasonable selection of the blasting method. The research results can provide a theoretical basis for the design of blasting for hard roofs in deep mines.