# Research on the Outburst-Rockburst Coupling Disaster Law Based on True Triaxial Unloading Tests
## 2 Test Methods and Principles  
In the test, the coal was collected from the coal and gas outburst mine with a serious rockburst in the No.3 coal seam. More specifically, the coal used in this test was collected from the coal mining working area in Section II, Floor III, Area III-IV, Level III of the No.3 coal seam. According to the determination method of physical and mechanical properties of coal and rock (GB/T 23561.9-2009 [28]), in combination with the characteristics of the test equipment, the specimen preparation size was determined to be 100 mm \(\times\) 50 mm \(\times\) 50 mm. Considering the development of pores and cracks in the raw coal, the internal damage state of specimens varied after preparation, which led to significant differences in their mechanical properties, thus affecting the regularity analysis of the test results. To ensure the reliability of the research on the outburst-rockburst coupling disaster law, an ultrasonic measurement test of coal specimens was carried out. The P-wave of coal specimens was measured with the Sonic Viewer-SX ultrasonic measurement system (OYO Company, Tokyo, Japan) to explore the damage state of these specimens. The P-wave velocity mainly ranged from 1900 m/s to 2100 m/s. After the specimens with large dispersions of wave velocity were eliminated, the coal specimens with similar mechanical properties were obtained to conduct true triaxial single-sided unloading mechanical tests. The ultrasonic measurement process of the specimens is shown in Figure 1.  
To restore the complex stress state, gas, and temperature environments of the coal body in outburst-rockburst coupling disasters, a true triaxial solid-thermal-gas coupling test device was adopted for the mechanical test. As shown in Figure 2a, the test device was composed of six components: a gas flow loading system, a double-cylinder servo loading system, a true three-axis clamping system, a temperature control system, an acoustic emission monitoring system, and a real-time data acquisition system [29]. This device can be employed to simulate the three-dimensional unequal stress loading of the coal body, with the maximum stress being 80 MPa, the maximum temperature being 85 \({}^{\circ}\)C, and the gas pressure being 10 MPa. Through this device, the true occurrence environment of the coal body can be restored to the greatest extent, and the occurrence process of outburst-rockburst coupling disasters can be reproduced. Acoustic emission monitoring technology can be utilized to reveal the energy release law and the damage and fracture mechanism of the coal body [30; 31]. To explore the mechanism of outburst-rockburst coupling disasters, the SH-II acoustic emission system (PAC, Newark, NJ, USA) equipped with the Nano30 probe (PAC, Newark, NJ, USA) was used to collect acoustic emission signals during the  
Figure 1: The ultrasonic measurement test of coal specimens. single-sided unloading of coal specimens in real time. Given the crack evolution in the coal body as the root cause of the disaster, ICT-3400 industrial CT (Dandong Aolong Ray Instrument Group Co., Ltd., Dandong, China) was adopted to scan the meso structure of coal specimens, with the minimum spatial resolution being 5 \(\upmu\)m, the DR sensitivity being 0.8-1.2%, and the maximum energy of high-frequency X-rays being 225 kV. CT scanning can convert the optical signals carrying X-ray attenuation information into electrical signals to form grayscale images [32; 33] and characterize the distribution of meso crack structures in coal specimens in the form of two-dimensional slices. In this study, four main disaster factors in the working area were selected to perform experimental research, including the crustal stress in the coal seam, gas pressure, temperature, and excavation unloading conditions.