# Research on the Outburst-Rockburst Coupling Disaster Law Based on True Triaxial Unloading Tests
## 3 Analysis of Mechanical and Acoustic Characteristics of Outburst-Rockburst Coupling Disasters  
### Stress-Strain Characteristics  
Five groups of typical specimens under different gas pressures were selected to plot the whole stress-strain curve in the unloading direction, as shown in Figure 4. As can be seen from the figure, there were two sudden drops in the stress in the stress-strain curves of all five groups of outburst-rockburst coupling disaster tests with different gas pressures. However, the strain at corresponding moments exhibited two patterns. When the gas pressure was 0.6 MPa and 0.9 MPa, the strain produced under the first sudden stress drop was smaller than that under the second one. However, when the gas pressure was 1.2 MPa, 1.5 MPa, and 1.8 MPa, the strain produced under the first sudden stress drop was larger than that under the second one. During these tests, many bubbles were observed in the bubbling water at the gas outlet of the device during the two sudden stress drops. This suggested that the structural instability of specimens was accompanied by abnormal gas emission during the two sudden stress drops. When the gas pressure was 0.6 MPa and 0.9 MPa, the instantaneous gas emission volume and velocity during the second sudden stress drop were higher than those during the second one. When the gas pressure was 1.2 MPa, 1.5 MPa, and 1.8 MPa, the instantaneous gas emission volume and velocity during the first sudden stress drop were higher than those during the second one, but the instantaneous gas emission volume decreased significantly after an instant. In summary, when the gas pressure was 1.2 MPa, 1.5 MPa, and 1.8 MPa, the first sudden stress drop contributed to the \"small structure\" instability failure under the action of gas, namely the coal and gas outburst, while the second one contributed to the \"large structure\" instability failure under the action of three-dimensional stress, namely the rockburst.  
The instability point state of the coal specimens was analyzed under different gas pressures. The analysis results revealed that the stress unloading degree during the structure instability failure of the coal specimens was inversely proportional to the initial gas pressure state. When the gas pressure increased from 0.6 MPa to 1.8 MPa (0.6 MPa, 0.9 MPa, 1.2 MPa, 1.5 MPa, and 1.8 MPa), the instability failure stress of the specimens was unloaded by 11.59 MPa, 10.67 MPa, 9.78 MPa, 8.08 MPa, and 5.98 MPa, respectively, and the unloading surface stress under the latter four gas pressures decreased by 7.94%, 8.34%, 17.38%, and 25.99%, respectively, compared with the corresponding previous stress level. This can be explained that with the increase in the gas pressure, the larger the gas content in the coal body and the more significant its deterioration effect. Due to the strong attraction between  
Figure 3: The stress path in the outburst–rockburst coupling disaster test.  
gas and carbon molecules, a large volume of gas may play an adsorption role and reduce the cohesion between carbon molecules in the coal body, thus exerting the deterioration effect. Meanwhile, a large volume of free gas in the coal body would perform work to promote the development of cracks under the synergistic effect of unloading. Therefore, the stress unloading degree of the coal body required for outburst-rockburst coupling disasters decreased with an increase in the gas pressure.