Energy evolution characteristics of coal failure in triaxial tests under different unloading confining pressure rates

Rock damage is considered as a state of instability phenomenon under the driving of energy, the law of energy evolution in the process of deformation and failure can reflect the essential characteristics of rock mass deformation and failure. The loading and unloading tests were carried out on coal samples to explore the laws of energy evolution under different unloading confining pressure rates. The experimental results show that before the peak stress, the coal sample mainly experiences the energy storage and dissipation, whereas it mainly experiences the energy release and dissipation after the peak stress. It is found that when the unloading confining pressure rate is high, the duration of unloading confining pressure is short, the internal cracks of coal sample extend and circumferentially deform insufficiently, and the powers for energy dissipation and overcoming confining pressures are small. Moreover, most of the energy release when the coal sample ruptures, which make the coal sample damage seriously. Meanwhile, when the unloading confining pressure rate is larger, the damage rate of the coal sample increases faster before and after the rupture, which results in the sharp increase of the damage curve. Therefore, when the high-stress coal is excavated with a high construction speed, it is easy to cause a sudden release of a large amount of accumulated energy, causing serious disasters.