Deformation Damage and Energy Evolution Characteristics of Coal at Different Depths

As coal mining is extended to greater depths, it is found that the mechanical behavior of deep coal differs from the mechanical behavior of shallow coal, resulting in a significant increase in accidents. To explore the characteristics of deformation damage and energy evolution of coal at different depths, coal samples from depths of 300, 600, 700, 850, and 1050m at the Pingdingshan Coal Mine were acquired. The effects of depth on the in situ stress environment and physical properties of coal were considered, and triaxial compression experiments on 128 coal samples were conducted on this basis. The results show that as the depth rises up, the elastic modulus of coal gradually increases, reaching a maximum of 6.88GPa in the range of 700-850m. Poisson's ratio, the strength, and the volume strain at failure increase nonlinearly, reaching a maximum of 0.48, 129.7MPa and 0.81%, respectively, at the depth of 1050m. With increasing depth, the elastic energy and dissipated energy increase more rapidly, and the corresponding peaks of 1050m are 1.84 times and 3.1 times that of 300m, respectively. The dissipated energy causes a more significant internal structural change in deep coal, thereby leading to more prominent macroscopic failure in the deep. From the perspective of energy conversion efficiency, it is mainly the accumulation of elastic energy at the pre-peak stage, followed by the release of elastic energy and a rapid increase in dissipated energy at the post-peak stage. The efficiency of accumulation and release of elastic energy decreases significantly in deep coal, while the generation efficiency of dissipated energy increases sharply. These results provide a useful reference for the safe and efficient exploitation of deep coal resources.