Characteristics of borehole surface deformation and fracture evolution in progressively demaged coal samples containing double circular boreholes

The coordinated arrangement of gas pre-drainage boreholes and anti-impact and pressure relief boreholes can integrate the prevention and control of mine rock burst and gas compound disasters. However, the dense arrangement of these boreholes may lead to the occurrence of phenomena such as string holes and collapse holes, which could potentially impact the efficiency of mine gas extraction. To investigate the deformation around borehole surface and the evolution of fractures during the compression failure process in the coal and rock masses with dual borehole structure, the 3D-DIC observation experiment was conducted on the coal samples containing double circular holes under uniaxial compression conditions using a digital image observation system for coal rock deformation and failure. This experiment yielded some time-series images of sample surface deformation under various stress states. The three-dimensional full-field deformation of the sample surface was calculated using the three-dimensional surface reconstruction technology and VIC^3D analysis processing software. Based on the results of strain field analysis, the stress-strain curve was divided into five deformation stages using statistical index analysis methods, providing a detailed description of the deformation and fracture evolution characteristics around the borehole surface. The results show that under uniaxial compression, the grayscale of strain field in the coal samples with double circular holes shifts toward higher gray levels. The proportion of peaks in the strain field gray histogram changes slowly at first and then rapidly, with the peak stress serving as the boundary. The radial displacement curve around the sample holes presents a “从” shape, while the circumferential displacement curve presents a “人” shape. The left hole H₁ always accelerates counterclockwise, while the right hole H₂ always accelerates clockwise. The fractures around the holes are primarily characterized by counterclockwise dislocation, accompanied by a few instances of clockwise dislocation. The outer zone of strain localization primarily exhibits energy accumulation before the loading peak and energy release after the loading peak. This is accompanied by energy accumulation resulting from partial energy adjustment and transfer. During the loading process of the specimen, the factors such as the stress field environment, the geometry of fracture end, and the length of fracture undergo constant changes. This results in a reduction in the stress intensity factor K_Ⅰ at the tip of the fracture, thereby impeding the expansion of the fracture.