Study on crack propagation mechanism of coal under the different orientation between prefabricated slots and true triaxial stress

In engineering, the interrelationship between hydraulic slots and in-situ stress affects the efficiency of the relief stress of coalbeds. In this research, true triaxial experiments were carried out on coal with prefabricated slots parallel to the σ2 (FP-σ2) and the σ3 (FP-σ3). The results show that the modulus of elasticity and Poisson's ratio of the specimens change dynamically with the increase of cycles, and the peak strength and peak strain of the FP-σ3 specimen is larger than the FP-σ2 specimen. The cracks of FP-σ2 specimens were mainly distributed in the X-Z plane, while the cracks of FP-σ3 specimens were more uniformly distributed inside the specimens. For FP-σ3 specimens, a crack's stable expansion orientation of the slot exists, which influences the competitive extension of cracks in the slot-plane with the σ1-σ3 plane. In this orientation, the cracks are distributed in a complex manner. The effects of the horizontal stress difference Kn and the horizontal deflection angle ω of slot on the spatial distribution of cracks were further analyzed by numerical simulation. It can change the slot's horizontal deflection angle ω to make the cracks' distribution uniform. The optimisation method of hydraulic slotting in deep coalbeds is discussed based on research results.