Study on fractal characteristics of creep damage in broken coal gangue with different particle size groups based on SEM scanning and digital image technology.

During the advancement of an underground coal mining face, after the overlying strata break, they are supported jointly by the coal ahead and the collapsed coal gangue in the goaf. The intensity of the roof pressure, stress distribution, and stability are related to the size and degree of damage of the collapsed coal gangue blocks. This paper simulates the deformation, damage and failure of the broken coal gangue in the goaf under the compression of the overlying strata by using the creep test of broken coal gangue. Creep tests are carried out on the broken coal gangue blocks of different particle size groups to study the damage characteristics of the broken coal gangue blocks after the creep test, and then determine the particle size group of the broken coal gangue that is most conducive to the stability of the overlying strata. Combined with SEM microscopic images, digital image processing and box-counting method, the fractal dimension of the fractures is calculated and the damage variable is defined to reveal the control mechanism of different particle size groups on the development of micro and mesoscopic fractures and the stability of the goaf strata. The test results show that the fractal dimension and damage variable of the broken coal gangue with a particle size of 50-60 mm are optimal. The research results can provide a theoretical basis for the directional blasting of the working face roof and the optimization of the particle size of the fallen gangue, which is helpful to shorten the activation time of the overlying strata and quickly tend to stability, and provide scientific support for the safety control of the working face roof.