Bidirectional permeability characterization of composite specimen of graded crushed coal bodies

In the gas drainage project of a coal mine, the permeability of the coal body around the borehole is a key factor affecting the drainage efficiency, which is significantly affected by the degree of coal body crushing. To study the permeability of the coal body with different degree of crushing around the borehole, a composite specimen with different Talbol index in the upper and lower positions were prepared by the grading theory, and a triaxial permeability test was carried out by using improved LFTD1812-3 permeability meter, to analyze the influence of the effective stress and equivalent Talbot index on the pore structure and permeability of the composite specimen. The results show that the permeability and porosity of the composite specimen are negatively correlated with the effective stress and positively correlated with the equivalent Talbot index. The permeability tends to be stabilized, which might be caused by the structural solidification of the coal specimen by continuous loading. The pore throat effect occurring in the interface of the composite specimen may lead to the bidirectional permeability observed in the composite specimen. This study provides insights into the gas seepage mechanism around boreholes and offers a theoretical foundation for the seepage of a composite specimen.