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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. [ABSTRACT FROM AUTHOR]

The samples collected from Banxi coal mine in Chongqing, Southwest China, have been used to study on damage of coal based on permeability change rate (PCR) and load-unload response ratio (LURR) under cyclic loading by using thermal-hydrological-mechanical (THM) coupling experimental equipment. Research results show that (1) the LURR is applied to describe the samples response to cyclic loads based on loading/unloading elastic modulus. The LURR is larger at the initial cyclic, with the cyclic loading and unloading, it decreases sharply first, and then slowly decreases to 1 value, and then slowly rises away from 1 value. The increase of LURR after stabilization can be used as the precursory information to judge the failure and instability of coal and rock. (2) A new damage characterization method based on PCR can well characterize the three stages of coal-rock damage under cyclic loading, that is, first accelerates damage and then remains smooth, at last also accelerates damage. The evolution of damage variable based on PCR is similar to the general trend of damage variable based on LURR. As gas monitoring is a routine monitoring indicator for coal mines, so it is a practical, innovative, and economical method to characterize the damage variables based on PCR. [ABSTRACT FROM AUTHOR]

Anisotropy is a very typical observation in the intrinsic bedding structure of coal. To study the influence of anisotropy of coal structure and stress state on the evolution of permeability, a newly developed multifunctional true triaxial geophysical apparatus was used to carry out mechanical and seepage experiments on bedded coal. The permeability and deformation of three orthogonal directions in cubic coal samples were collected under true triaxial stress. It has detected the significant permeability anisotropy, and the anisotropy is firmly determined by the bedding direction and stress state of coal. Based on the true triaxial mechanical and seepage test results, the coal with bedding was simplified to be represented by a cubic model, and the dynamic anisotropic (D-A) permeability model was derived by considering the influence of bedding and stress state. The rationality of the permeability model was verified by the experimental data. Comparing the permeability model with Wang and Zang (W–Z) model, Cui and Bustin (C–B) model and Shi and Durucan (S–D) model, it is found that the theoretical calculated values of the D-A permeability model are in better agreement with the experimental measured values, reflecting the superiority of the D-A permeability model. Based on incorporating the model of D-A permeability under the concept of multiphysics field coupling, the numerical simulation experiments of coal seam gas extraction with different initial permeability anisotropic ratios were carried out by using COMSOL multiphysics simulator. The influence of initial permeability anisotropy ratio on gas pressure distribution in coal seam during gas extraction was explored, which provides theoretical guidance for the optimization of borehole layout for gas extraction in coal mine. [ABSTRACT FROM AUTHOR]