Exploration of fracture evolution in gas-containing coal under gangue influence

To investigate the influence of gangue interlayers on fracture evolution, this study takes gas-containing coal as its research object. First, fracturing experiments were conducted using a triaxial loading system designed for coal and rock. Samples with varying gangue interlayer thicknesses (0, 6, 10, 16, and 20 mm) and numbers of layers (0, 1, and 2) were loaded to failure with uniform axial pressure under a constant confining pressure of 2 MPa and different gas pressures (0.2, 0.4, 0.6, and 0.8 MPa). Subsequently, fracture-scanning experiments were performed on gas-containing coal using an industrial computed tomography (CT) scanning system for loaded coal and rock, obtaining various original CT images. With the help of VG Studio MAX three-dimensional (3D) reconstruction technology, two-dimensional CT slice sequences were converted into 3D digital models, allowing for the visual characterization of the spatial distribution of internal fracture networks in coal. Furthermore, the Avizo software was adopted to quantitatively extract mesoscopic structural parameters, such as fracture rate, 3D fractal dimension, and average coordination number. By closely combining the fracture mechanics theory and Mohr–Coulomb strength criterion, the fracture evolution characteristics in gas-containing coal were quantitatively analyzed. The results show that the failure mode of gas-containing coal samples exhibits significant shear failure. With an increase in gas pressure, shear cracks in pure coal samples further develop into composite forms, such as parallel and cross cracks. Fractures in the gangue interlayer samples are mainly concentrated in the coal part, with only one or two main fractures extending to and penetrating the gangue interlayer within the rock part. This indicates that the gangue interlayer is less damaged, and obvious faulting occurs in the 3D fracture network. In double-layer gangue samples, fractures are primarily distributed in the coal between the two gangue interlayers, generally showing that two fractures develop and merge, ultimately penetrating the sample. Simultaneously, transverse fractures form between the two main fractures, presenting an overall “A”-shaped fracture characteristic. Under the influence of gangue interlayers, the fracture rate, 3D fractal dimension, and average coordination number of gas-containing coal after failure increase with increasing gas pressure but decrease with increasing gangue interlayer thickness and number of layers. An increase in gas pressure promotes sample failure, enhances internal complexity, and improves pore connectivity. Conversely, an increase in gangue thickness and number of gangue layers inhibits fracture development and reduces the internal complexity and pore connectivity of samples. Increasing the gas pressure causes the Coulomb failure line and Mohr stress circle to approach each other, gradually reaching the failure condition of the sample, which promotes fracture development. Increasing the gangue interlayer thickness reduces the energy release rate of crack propagation, increases the critical fracture length, and consequently hinders crack propagation. This study provides a theoretical basis for understanding the failure mechanism of gas-containing coal with gangue interlayers and holds significant engineering importance for gas outburst prevention and control in coal mines.