Classification and fractal characteristics of coal rock fragments under uniaxial cyclic loading conditions

Rock burst is a serious geological hazard in deep underground mines affecting progress of mining operations. Although rock burst is a complex process, a distribution law of fractal characteristics can explain the rock failure mechanism. Using a servo-controlled testing system, uniaxial cyclic loading tests on coal rock specimens were conducted to investigate the fractal characteristics of the fragments under different loading rates. To comprehensively characterize the coal fragments of different sizes, samples were divided into four groups of different size: particles, fine, medium-size, and coarse fragments. The distribution of the fragments under uniaxial cyclic loading conditions was then investigated based on the theory of fractal geometry, and the relationships between fractal dimensions and loading rates. Under uniaxial cyclic loading and unloading conditions, most of the fragments are irregular wedges and bulks, exhibiting obvious shape characteristics. Under various loading rates, the length-quantity fractal dimensions of the coal fragments ranged from 0.74 to 1.44, the width-quantity fractal dimensions range from 0.44 to 1.65, and the thickness-cumulative mass fractal dimensions range from 1.0 to 1.33. The coal rock's crushing size-mass fractal dimensions under different loading rates were 2.27, 2.30, 2.32, and 2.35, respectively. Under a small loading rate, the dimension-quantity fractal dimensions are relatively small, suggesting that the coal rock was less crushed, with large fragments differing greatly in length, width, and thickness. The results show that the coal rock fragments exhibit certain shape characteristics after the cyclic loading, like irregular shapes and wedges. Under a larger loading rate, the fragments showed greater fractal dimensions of both size and mass; the coal samples crushed more thoroughly with more uniform fragments in length, width, thickness and mass. The conclusions obtained in this study confirm the classification and fractal characteristics of coal rock fragments by uniaxial cyclic loading conditions in laboratory test and provide the basis for further study on the mechanism of rock burst. This study is helpful for us to make a thorough inquiry the danger degree of rock burst in coal mine by using fractal geometry, understand the effects of methane to coal and the evolution mechanism of cracks, and it can be applied to the research on occurrence mechanism and early warning of rock burst.