## 2 Experimental Study
### Experimental Results
#### 2.2.1 Strength Characteristics
When the confining pressure was the same, as the gas pressure increased, the gas pressure in the pore and crack structure of the coal increased, and the effective confining pressure of the coal decreased. In addition, an increase in the gas pressure led to greater gas adsorption and coal matrix swelling. Meanwhile, an increase in the gas pressure also had a certain compression effect on the coal matrix. Furthermore, as the gas pressure increased, the chemical action of the gas on the coal was enhanced. Under the combined physical and chemical effects, the strength of the coal was reduced. After fitting the peak strengths of the samples under the two loading paths, it could be found that the peak strengths of the samples decreased linearly with an increase in the gas pressure and increased linearly with an increase in the confining pressure. The fitting curves are shown in Figure 5.  
Comparing the strengths of the samples under different loading paths, it can be concluded that under the same initial conditions, the peak strengths of the samples under the ATC path increased by 11% to 27%, while the residual strengths did not show an obvious difference. Meanwhile, it can be found that the magnitude of the decrease in the peak stress and the reduction ratio of the peak stress were significantly improved. Under the ATC path, the elastic energy stored in the sample was higher than that under the CTC path, and the energy release was greater and more intense. During the loading process, a part of the work done on the coal was applied to the development of cracks, and the others were stored in the coal in the form of elastic energy. Under the ATC path, the axial loading was accelerated, and the sample did not have sufficient time to respond. The cracks in the coal were not fully developed, and more work done on the coal was stored in the form of elastic energy. After reaching the bearing limit, the energy was suddenly released. The sample was destroyed by a more violent and impactful action. When the confining pressure and gas pressure were 10 MPa and 2 MPa, respectively, the residual strengths of samples under different loading paths were basically the same. The ratio of the peak stress reduction under the CTC path was 17.15%, while the ratio doubled to 34.41% under the ATC path. The relationships between the reduction ratio of the peak stress and the gas pressure/confining pressure under different loading paths are shown in Figure 6. Although the relationships are not obvious, an analysis can clarify that under the same conditions, a larger reduction ratio for the peak stress will result in a more violent and impactful action at failure. Figure 5: Fitting curves of peak strength under different initial conditions: (**a**) \(\sigma_{3}\) = 4 MPa; (**b**) P = 2 MPa. Despite the influence of the ATC path on samples, the influence degree of the ATC path on coal differed with the initial conditions. On one hand, as the gas pressure increased, the magnitude of the peak stress reduction ratio decreased from 21% to 11%, indicating that the influence degree of the ATC path on coal was weakening. On the other hand, when the gas pressure was 2 MPa, the magnitude of the peak stress reduction ratio rapidly increased from 11% to 100% with an increase in the initial confining pressure, indicating that the influence degree was significantly improved. The times required for the failure of samples were also differed significantly according to the results listed in Table 1. Under the two loading paths, the time required for failure decreased as the gas pressure increased, and increased with an increase in the initial confining pressure. A comparison of the times under the different loading paths showed that under the same conditions, the time required under the ATC path was significantly shorter and was approximately half of that under the CTC path. Under the ATC path, the failure of the sample was more abrupt.