Experimental study on mechanics and seepage of coal under different bedding angle and true triaxial stress state

The bedding and the true stress state have a great effect on the seepage and mechanical properties of coal. In this study, the seepage experiments of coal with different bedding angles were conducted by a true triaxial apparatus. The influences of bedding and stress states on the deformation, seepage, energy, and fracture cracks of coal are discussed. The results indicated that, when the intermediate principal stress is consistent with the bedding dip direction, there is a reversal effect of the intermediate principal strain. While the phenomenon that the intermediate principal strain is smaller than the minimum principal strain after the failure, when the minimum principal stress is consistent with the bedding dip direction. With the increasing in bedding angle (within the range of angle studied in this paper), the peak stress of coal decreases, while the overall permeability and residual strength of coal increases, and the sensitivity of permeability to stress increases. In the SS-A, the peak stress of θ = 30° and 60° were 11.52% and 28.96% lower than that of θ = 0°, while the residual strength increased by 15.11% and 32.25%, respectively. With the increase of bedding angle, the dissipation energy increases. When the bedding angle of coal is large, the main failure mode is shear slip along the bedding plane. However, in the near horizontal bedding, the crack of coal mainly presents X-type conjugate shear slip failure along π/4 angle. The study on the mechanical seepage of coal under different bedding angles and true triaxial stress is helpful to further understand the deformation and gas migration and evolution of coal under different types of coal seams and different mining methods.