Abstract: To investigate the mechanical properties and failure characteristics of bedding coal under different stress paths, mechanical tests were conducted using the MTS816 rock mechanics testing system. Coal samples with bedding angles of 0°, 30°, 45°, 60°, and 90° were subjected to tests under three stress paths: conventional triaxial loading, increasing axial pressure unloading confining pressure, and constant axial pressure unloading confining pressure. The study analyzed the influence of stress paths and bedding angles on the strength, deformation, and failure characteristics of the coal samples. The results indicate the following: Under the path of conventional triaxial loading, the stress-strain curves of the coal samples show a nearly linear relationship before reaching the peak point, followed by a rapid drop in stress after reaching the peak. Under the path of increasing axial pressure unloading confining pressure, the axial strain increment ratio exhibits a linear increasing trend as the unloading ratio increases. The circumferential strain increment ratio and volumetric strain increment ratio display a three-stage change characteristic, involving low-speed growth, rapid growth, and stable growth. Under the path of constant axial pressure unloading confining pressure, the axial strain increment ratio, circumferential strain increment ratio, and volumetric strain increment ratio exhibit a two-stage change characteristic, involving low-speed growth and rapid growth. The peak strength and axial peak strain of the coal samples exhibit a V-shaped trend with increasing bedding angle under different stress paths. They initially decrease and then increase, reaching the maximum value at 0° and the minimum value at 60°. As the unloading ratio increases, the deformation modulus of the coal sample initially stabilizes and then rapidly deteriorates. The Poisson’s ratio first slowly increases and then exponentially increases. The critical unloading ratio initially increases and then decreases with increasing bedding angle. When the bedding angle is the same, the critical unloading ratio under the path of increasing axial pressure unloading confining pressure is lower than that under the path of constant axial pressure unloading confining pressure. Under the path of conventional triaxial loading, the coal sample undergoes brittle shear failure. However, under the paths of increasing axial pressure unloading confining pressure and constant axial pressure unloading confining pressure, the coal sample exhibits a mixed tensile-shear failure mode. When the bedding angle is 60°, the angle of the main shear fracture surface of the coal sample is almost consistent with the bedding angle.

Key words: stress path, bedding coal, mechanical properties, fracture characteristics, unloading ratio