## 4 Discussion
### Triggering Mechanism of coal Burst under Different Mining-Induced Stress Disturbed  
Many researchers pointed that the triggering mechanism of coal burst are different under quasi-static loading, cyclic loading and transient loading. Figure 14 shows the schematic diagram of the coal burst mechanism under different mining disturbances. The mechanical bearing system comprises a roof, a floor surrounding rock, and a coal body. The left side is the bearing stress-strain curve of the surrounding rock, and the right side is the bearing stress-strain curve of coal in front of the working face. Its stiffness and strength are much smaller than that of surrounding rock. In this figure, coal was assumed as a softened material with non-linear behavior, and the roof and floor were abstracted as the whole surrounding rock [26]. It can be seen that the mechanical interpretation of the whole process of coal burst evolution (gestation-trigger-appearance-end) under different mining disturbances is:  
(1) A series of classical theories of coal burst have been summarized based on laboratory experiment and field research, including rock strength [27], rigidity [28], energy accumulation and dissipation [29], burst liability [30], and stability [31]. The above burst theories can basically be classified as the research category of static load. When the strain rate caused by the addition and unloading of the mechanical load-bearing system is less than \(10^{-5}\) s\({}^{-1}\), the process is a quasi-static process. In deep mining, the stress level of the roadway is very high, resulting in the concentration of the surrounding static load stress caused by the excavation of the roadway, and the stress level is close to or even exceeds the critical stress, resulting in the impact damage of the coal body. This type of coal burst is called the high static load dominant type. Figure 13: Characteristics of coal burst pit: (**a**) static load; (**b**) cyclic load; (**c**) transient load. (2) With the great progress of research coal burst theory, such as shearer cutting, cyclic hydro-fracturing, blasting, drilling and seismic waves from adjacent coal burst events, have been proven as a very critical external factor to trigger coal burst [32]. The cyclic loading can well reproduce the periodic loading, which corresponds to the complex and repeated excavation stages during mining operations and underground constructions [33,34]. When the strain rate caused by the addition and unloading of the mechanical load-bearing system is between \(10^{-5}\) s\({}^{-1}\) and \(10^{-4}\) s\({}^{-1}\), the process is a cyclic disturbance. Such as mine earthquake, especially the far field source, its mode of action is equivalent to cyclic loading and unloading. Due to the heterogeneity of coal and rock materials, each loading and unloading caused by mine vibration load will cause permanent deformation of the coal body. When the action time of dynamic load is long enough, the burst condition similar to the stress state at point S\({}_{2}\) under quasi-static load can be started for the coal body at point S\({}_{1}\) under superimposed dynamic load. Therefore, the dynamic load-induced effect of the dynamic and static load superposition-induced coal burst is called the seismic fatigue load dominant type. (3) When the strain rate caused by the loading and unloading of the mechanical bearing system is greater than \(10^{-4}\) s\({}^{-1}\), the inertial force and related dynamic response cannot be ignored. Under the action of transient loads, such as the instantaneous dynamic load caused by fault slip and roof fracture near the working face, its action mode is equivalent to applying an instantaneous stress increment \(\Delta\sigma\). When the area S\({}_{123}\) > S\({}_{374}\), for the stress state of the coal body at 1 point before the peak, under the action of superimposed dynamic load, the impact condition similar to the stress state at 4 points after the peak under quasi-static load can be started.