Mechanical behavior of fracture in coal bodies containing double boreholes based on digital images

The interaction between gas extraction boreholes is a key factor influencing the deformation and failure of the coal-rock around the boreholes. To deeply study the failure characteristics of the coal-rock containing double boreholes, this paper carries out the uniaxial failure test of the intact coal specimen, the specimen containing single boreholes, and the specimen containing double boreholes. It adopts the digital image technology (DIC) to analyze the deformation and failure law of the specimen. Greyscale histograms, greyscale characteristics parameters, and texture characteristics parameters of the surface strain field are introduced to quantitatively characterize the strain field evolution during uniaxial compression of the specimen. The results showed that the interaction between the double boreholes significantly reduced the specimens’ mechanical properties and aggravated the specimens’ failure. The stress superposition effect of double boreholes changes the distribution characteristics of strain field, aggravates the degree of strain concentration around the hole, and weakens the mechanical properties of double hole specimens. Compared with the complete specimen, the maximum compressive strength of the double-hole specimen is reduced by 38.31%. The variation characteristics of greyscale and texture parameters are closely related to the deformation and fracture process of the specimens, When the stress concentration occurs, the gray entropy of the double-hole specimen increases by 63.52%. When macroscopic cracks appear on the surface of the specimen, the fluctuation range of the texture correlation curve of the double-hole specimen reaches 27.8%. The texture characteristic parameters change abruptly at the onset of crack initiation and penetration, leading to the emergence of mutation regions P1 and P2 in the texture characteristic parameter-time curves, which are used to identify the deformation and fracture state of the coal-rock. The increase in the number of boreholes leads to a delay in the appearance of the mutation regions and an increase in the time interval. By studying the greyness characteristic law during the deformation and failure process of the coal around the double boreholes, it can provide reference for the characterization of deformation and failure characteristics of samples, which is of reference value for the optimal arrangement of the gas extraction drilling boreholes in deep coal seams.