Study on the microscopic mechanism of strength softening and multiple fractal characteristics of coal specimens under different water saturation

Coal resources are often subjected to the combined effects of stress fields and seepage fields during the mining process. Therefore, studying the micro-mechanics and strength softening laws of coal specimens under water saturation in deep mining has important practical significance. This article is based on experimental research conducted on coal specimens from the Wanfu Coal Mine in Shandong Province. The following conclusions were drawn: (1) The peak value of the T2 spectrum amplitude of the coal specimens presented a three-peak shape with increasing immersion time, indicating the generation and development of new pore structures. The spectrum area was positively correlated with the mass increment and satisfied three water absorption stages. (2) A relationship curve was established between stress, pore structure, and water absorption time of coal specimens, and the critical points of two water saturation during the process of coal specimen strength softening were determined to be 22.51% and 81.29%, respectively, according to the relationship between the three. (3) Based on changes in the coal specimens' pore structure and mineral composition with increasing immersion time, a microscopic pore structure model of coal specimens' water absorption was established. (4) The vibration counts of coal specimens under different water saturation exhibited multiple fractal characteristics. By analyzing the parameters of the multifractal spectrum, it was concluded that the secondary expansion of primary cracks was the main type of failure during the process of coal specimen destruction.