Effects of bedding directions on coal blast-induced fracture and fragmentation

Blast-induced fracturing is an important method to improve coalbed methane reservoirs and enhance coal seam permeability for gas extract, which is inevitably affected by directions of coal beddings. To study the influence of bedding directions on coal blast-induced fracturing, high-speed photography was used to conduct coal blasting experiments. From the whole process of “fracture-fragmentation” in coal blasting, the characteristics of coal blast-induced crack initiation and propagation, fracture mode, fragment throwing, fragment distribution, and fracture surface roughness under different bedding directions (horizontal, inclined, vertical) were analyzed, revealing effects of coal bedding directions on blast-induced fracture and fragmentation. Crack initiation: Coal bedding direction affects the time of crack initiation and gas ejection, with horizontal bedding > inclined bedding > vertical bedding, coal with horizontal bedding can fully utilize gas energy, resulting in high energy utilization efficiency, while coal with inclined and vertical beddings has leakage of gases along the bedding, resulting in low energy utilization efficiency; Crack propagation: The coal blasting with horizontal bedding forms a complex crack network, which is caused by multiple crack branching between blast-induced cracks and the beddings; however the coal crack network with inclined and vertical beddings is single, because the cracks mainly propagate along the beddings; Fracture mode: In the borehole section, coal with horizontal bedding produces matrix and bedding plane tensile fracture, while coal with inclined and vertical beddings mainly experiences bedding plane tensile fracture, below the borehole, coal with horizontal bedding is mainly characterized by shear fracture, while coal with inclined and vertical bedding is mainly characterized by bedding tensile fracture due to the bedding penetrating from top to bottom, tensile fracture is mainly contributed by the action of gases, while shear fracture is mainly contributed by the action of blast stress waves; Fragment throwing: Coal fragments under horizontal bedding are scattered and thrown at a high speed, however throwing direction of coal fragments under inclined and vertical bedding planes is basically perpendicular to the bedding plane, and the speed is relatively low; Fragmentation distribution: Coal with horizontal bedding has a high degree of fragmentation, and the average angle of fragments near the borehole is small, indicating dense radial cracks, however the coal with inclined and vertical beddings has a low degree of fragmentation, and the average angle of fragments near the blasthole is large, indicating sparse radial cracks; Fracture surface roughness: The fracture surface near the borehole with horizontal bedding passes through the beddings, and the roughness is high, however the coal with inclined and vertical beddings produces fracture along the bedding plane near the borehole, and the roughness is low. Summarizing the whole process of “fracture-fragmentation” in coal blasting, it is found that the borehole perpendicular to the coal bedding produces the best fracture and fragmentation degree. The research results provide experimental basis for precise coal blasting, promoting the development of coalbed methane mining, and enhancing underground gas extraction.