Research on mechanism of directional slit-making and the whole process of “jet+detonation” micro-differential composite blasting

Directional pre-splitting technology is conducive to the precise caving of rock mass and the control of stope stress distribution. Existing directional fracture technologies mainly include directional water jet, directional hydraulic fracturing and convergence energy blasting, but these methods suffer from small fracture scales and poor directional effects. In this paper, a directional roof slitting by composite blasting(DRSBCB) based on “jet + detonation” was proposed. A continuous directional fracture surface was formed by utilizing the perforating charge's shaped charge jet at the microsecond level and the explosive's high-pressure splitting effect at the millisecond level, achieving precise fracture creation. The key equipment and process flow of the DRSBCB were introduced. The mechanical model of shaped charge jet and secondary blasting was established. The mechanism of the directional jet penetrating rock and the secondary blasting high pressure splitting was obtained. The theoretical calculation formulas of the directional perforation depth of shaped charge jet was provided, and the theoretical expression of the directional crack propagation length during secondary blasting process was given. The mechanism of the DRSBCB was revealed. The numerical model of jet penetrating rock was established by using LS-DYNA numerical simulation software. The jet channel expansion law and rock damage characteristics of the whole penetration process were obtained, and the evolution law of jet morphology, jet velocity and jet channel length in different periods was revealed. Based on the jet penetration model, the numerical model of secondary blasting was established, and the whole fracture propagation process of high pressure splitting in secondary blasting was clarified. The evolution law of crack length, crack propagation speed and rock damage with time was obtained. The characteristic of whole fracture propagation process of the DRSBCB was revealed. Additionally, the feasibility of the DRSBCB was verified through ground concrete target test, and industrial experiment was carried out in mine underground roadway. The directional fracture rate of 91%, 89% and 61% in the hole was obtained by borehole peeping. Through the research results of the paper, the feasibility of the directional fracture theory of the DRSBCB was proved. This approach offers a new technical method for precise control of coal and rock mass in coal mines.