Mechanical properties and energy evolution mechanism of coal prone to ejection

In this paper, true triaxial loading and unloading tests under different principal stress conditions were conducted to study the mechanical properties, failure characteristics, and energy evolution laws of coal bodies that are prone to ejection under different loading and unloading paths, aiming to reveal the ejection mechanism of the surrounding rock of roadway that is prone to ejection after excavation and unloading. The results are as follows. Firstly, the damage of coal samples that are prone to ejection was more severe under high stress single-sided unloading than under loading; when the axial pressure is 90% of the peak value, a large shear crack appeared on the surface of the coal samples, the axial strain rate is high, its own damage degree is large, and the ejection phenomenon was obvious; when the axial pressure was 80% and 70% of the peak value, respectively, the coal samples did not undergo overall macroscopic damage, the axial strain rate is low, and the degree of damage is small, only generated tensile cracks on the free face, and the ejection phenomenon was not obvious. Secondly, the increase of the second principal stress had a reinforcing effect on coal samples that are prone to ejection within a certain range, the internal composite cracks of coal samples that are prone to ejection first increase and then decreased, and the failure mode changed from shear failure to tension-shear composite failure, and finally developed into splitting failure; the intensity of ejection first increased and then decreased. Thirdly, in the process of high stress unloading failure, elastic energy was converted into dissipated energy and released instantly, and the proportion of elastic energy increased sharply, causing the fragments being ejected from the mother body with energy; there were obvious cross mesh cracks in the free face, and the ejection phenomenon was obvious. Fourthly, for coal samples that are prone to ejection under high stress after unloading, the tension-shear cracks rapidly expanded and connected, resulting in tension-shear composite failure, and the ejection phenomenon was obvious; Fifth, tension-shear cracks appear in ejectable coal samples of after unloading, RA increased sharply and AF decreased continuously, AE showing the characteristics of high energy, high meter, high amplitude and high frequency. The study results can provide reference for the on trol of micro impact phenomena in unloading surrounding rock during excavation of similar underground engineering.