## 2 Experimental Study
### Experimental Results
#### 2.2.1 Strength Characteristics
Based on the results of the two unloading paths in Figure 4 and Table 2, it can be found that, as the gas pressure increased, the radial stress at the time of sample failure increased, and the unloading ratio decreased. On the other hand, as the initial radial stress increased, the radial stress at the time of sample failure increased, and the unloading ratio increased. After fitting the radial stress at the time of failure under the two unloading paths, it can be seen that the radial stress at the time of sample failure had linear function relationships with increases in the gas pressure and initial radial stress. The fitting curves are shown in Figure 7.  
After the gas adsorption in the coal, the free gas expanded the volume of the coal and provided a force that opposed the radial stress. As a result, the effective confining pressure decreased. Furthermore, the gas molecules could also increase the distance between the coal particles, and the cohesive force  
Figure 6: Reduction ratio of peak stress under different initial conditions: (**a**) \(\sigma_{3}\) = 4 MPa; (**b**) P = 2 MPa. Figure 7: Fitting curves of radial stress at failure: (**a**) \(\sigma_{3}\) = 4 MPa; (**b**) P = 2 MPa. decreased. Meanwhile, some gas molecules existed in the form of chemical adsorption. The mechanical properties of the coal decreased under the combined effects [29; 32; 33; 34]. As the gas pressure increased, the effective confining pressure of the sample decreased, and the radial deformation ability increased. Therefore, with less unloaded radial stress, the sample was more likely to yield and be destroyed. As the initial radial stress increased, the effective confining pressure increased, and the effect of the gas on the mechanical properties was weakened. Therefore, the high confining pressure limited the ability to deform radially, and a larger unloading ratio was needed for the failure of the sample. After fitting the unloading ratio under different unloading paths, it could be seen that the unloading ratio was linearly related to increases in the gas pressure and initial radial stress, as shown in Figure 8.  
Comparing the results listed in Table 2, it can be concluded that the influences of the unloading paths on the failure of the samples were obviously different. Under the same initial conditions, the radial stress at the failure of a sample increased and the unloading ratio decreased under the AUCP path. The sample was significantly more susceptible to failure under accelerated unloading conditions. The acceleration of the unloading radial stress caused the effective confining pressure to decrease more rapidly, and the mechanical and nonmechanical effects of the gas appeared sooner. The sample responded faster, and the stress state changed more quickly. The rapid decrease in the effective confining pressure led to a rapid release in the energy stored in the sample in the radial direction. Under the AUCP path, because of the acceleration of the unloading, the deviatoric stress increased more quickly, and the sample did not have sufficient time to fully develop cracks. Under the same conditions, the work done on the sample was the same. However, the unloaded radial stress at the time of failure under the AUCP path was smaller. Therefore, the release of energy under the AUCP path was more violent and impactful. After reaching the bearing limit, the energy was suddenly released, and the sample was destroyed. Although the accelerated unloading path had significant effects on the failure of the samples, the influence degrees were different with different gas pressures and initial radial stresses. Under the AUCP path, when the radial stress was 4 MPa, the reduction magnitude of the unloading ratio decreased from 4.25% to 1.25% with an increase in the gas pressure. On the other hand, the reduction magnitude increased from 1.25% to 3.4% with an increase in the initial radial stress.