Abstract: Cyclic loading significantly affects coal seepage; however, the quantitative characterization of seepage channels in fractured coal under cyclic loading is inadequate, and a matrix structure model for the fractured coal is absent. Therefore, seepage tests involving pore pressure and confining pressure cyclic loading were conducted on fractured coal with different grading conditions. The results indicate: 1) Under both pore pressure and confining pressure cyclic loading, the initial permeability of the coal mass varies with grading. As the Talbol power exponent increases, permeability successively increases, exhibiting clear stratification. 2) The sensitivity of effective stress to permeability K under cyclic loading is represented by effective stress variation Δσ^s_e . It is observed that the value of Δσ^s_e decreases in an “L-shaped” manner as permeability K increases during pore pressure cyclic loading. When K≤0.75×10⁻¹² m², the smaller Talbol power exponent corresponds to a lower the value of Δσ^s_e  is. 3) A method to calculate the average seepage pore area of graded coal is proposed, and a tortuosity model is developed to describe the evolution of tortuosity in fractured coal under cyclic loading. This research provides a theoretical basis for understanding the permeability evolution mechanism of fractured coal under cyclic loading and establishing a seepage model.

Key words: permeability, cyclic loading, effective stress, sensitivity coefficient, tortuosity model