Progressive failure mechanism and damage characterization of brittle single pre-existing hole coal specimens under uniaxial loading

Large-diameter relief drilling is the most direct and effective technique for preventing and controlling dynamic coal and rock disasters such as rockburst. To investigate the destructive effects of a pre-existing hole on brittle coal masses, this study employs experiments and numerical simulations to examine both surface strain and crack monitoring, as well as internal stress and damage evolution in single pre-existing hole coal specimens. It reveals the progressive failure mechanism of such coal specimens and introduces fractal geometry theory to characterize the quantitative evolution of damage. The results indicate that the presence of an internal hole significantly reduces the strength of the coal specimen. The primary cause of failure in the single pre-existing hole coal specimens is structural defects induced by the large-diameter hole, with the failure mode characterized by a combination of tensile and shear high-strain concentration zones forming an “X”-shaped fracture surface. Axial compressive-shear stresses drive the compression and shearing of internal coal particles, while radial stresses promote the separation of these particles, forming an “X”-shaped tensile-shear failure surface. Tensile damage plays a primary role in crack initiation during the early loading stage, while in the later stages, increased compressive-shear damage becomes the primary mechanism causing tensile-shear failure of the specimen. Fractal dimension was used to quantitatively characterize the development and spatial complexity of the fracture network in the single-hole coal specimen. The fractal dimension increases rapidly near the hole in the specimen when combined tensile-shear cracks form, marking a critical stage in crack propagation.