Study of the mechanical response and numerical simulation of uniaxial compression of coal based on size effect

With the intensification of coal mining, coal pillars have become weak links in mining systems, resulting in an increasing frequency of coal pillar rock burst disasters. The impact mechanisms, conditions, and failure characteristics of coal pillars vary depending on their size. For this reason, it is important to investigate the mechanical properties of coal pillars under different size conditions to prevent and control coal pillar rock bursts. In this work, the mechanical response of nine coal samples under uniaxial compression is analysed. Different coal samples undergo stress and fracture evolution during the entire failure process, which can be determined via discrete element software. The results show that (1) the stress-strain curves of coal samples are strongly influenced by their size. When the sample diameter is 25 mm, the coal sample curve presents a three-stage failure process. As the diameter increases, the stress-strain curve evolves gradually into a four-stage failure process. (2) The failure characteristics of the coal sample involve primarily shearing failure, which transitions from 'single slash' shear failure to 'double slash' conjugate shear failure as the diameter increases. A coal sample with a diameter-to-height ratio greater than 1:1 has primarily tensile failure characteristics. (3) Size significantly influences the energy and damage ageing evolution of coal samples during uniaxial compression. The height and diameter of a coal sample affect its dissipative energy growth starting point. The damage stage and damage rate of coal samples vary depending on their size. (4) Discrete element numerical simulations reveal that the characteristics of crack propagation are determined by the distribution of internal stress in coal samples. Macroscopic failure characteristics are determined by the types of main cracks in coal samples.