## 6 Conclusions  
In this work, the triaxial pressure unloading confining pressure test was carried out to simulate the failure process of rock mass caused by unloading, analyze the mechanical characteristics of the deformation and failure process, and study the energy evolution and  
Figure 8: Variation curves of reliable elastic energy under different confining pressures.  
energy release law of rock under the unloading action combined with the energy theory. The conclusions are as follows:  
1. When the confining pressure increased from 5 MPa to 20 MPa, the crack closure stress \(\sigma_{cc}\), crack initiation stress \(\sigma_{ci}\), dilatancy stress \(\sigma_{cd}\), and peak stress \(\sigma_{p}\) were 6.34 times, 2.75 times, 1.93 times, and 1.66 times higher than the original, respectively. By comparing the increase in characteristic stress, it was found that the confining pressure had a great influence on the crack closure stress and crack initiation stress, but relatively little influence on the dilatation stress and peak stress.
2. Before the peak stress, the axial absorbed energy \(U_{1}\) increased exponentially, the elastic energy \(U^{e}\) was similar to \(U_{1}\), and the circumferential consumption energy \(U_{3}\) and dissipated energy \(U^{d}\) were small. After reaching the peak stress, the growth rate of \(U_{1}\) decreased slightly, \(U^{e}\) decreased rapidly, and \(U_{3}\) increased rapidly but only accounted for a small proportion of the total energy, while \(U^{d}\) grew almost exponentially and rapidly became the main part of the energy. Each energy eigenvalue increased linearly, reflecting an obvious confining pressure effect; axial absorption energy \(U_{1}\) increased the most, followed by elastic energy \(U^{e}\), while annular consumption energy \(U_{3}\) increased the least, and dissipation energy \(U^{d}\) increased slightly more than \(U_{3}\).
3. Under each crack-characteristic stress state, each energy characteristic parameter increased with the increase in confining pressure, as shown by the increase in slope in the linear fitting formula of the energy characteristic parameter. For example, the slope of the \(U_{1}\) fitting formula increased from 16.458 under the crack closure stress state to 42.149 under the peak stress state, indicating that the value of each energy characteristic parameter increased gradually before the peak stress, reflecting a strong confining pressure effect.
4. The post-peak elastic energy release process could be divided into three stages: \"slow-fast-slow\". In the first stage, the release rate was the slowest and the axial strain variation range was the smallest, at basically 0.05%. The second stage had the fastest release rate and the largest axial strain variation range. In the third stage, the release rate was slow, and the variation range of axial strain was small. The energy release process showed an obvious confining pressure effect.  
Conceptualization, G.Z.; validation, Y.D.; formal analysis and investigation, T.Q. and Y.D.; data curation, Y.D. and T.Q.; writing--original draft preparation, T.Q. and Y.D.; writing--review and editing, G.Z. and Y.D.; supervision, G.Z. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Scientific and Technological Key Project of \"Revealing the List and Taking Command\" in Heilongjiang Province: Study on geological model and ventilation model of intelligent mining in extremely thin coal seam (2021 ZXJ02A03).  
Not applicable.  
The authors declare no conflict of interest.