# Effects of Water-Soaking Height on the Deformation and Failure of Coal in Uniaxial Compression
## 3 Results and Discussion
### Acoustic Emissions Records  
In order to study the failure characteristics of coals with different soaking heights, this paper analyzes the AE energy and AE cumulative energy characteristics by combining the stress-strain curves of coal specimens during compression process. During the uniaxial compression process, the development, expansion, and damage of fractures of coal will release stress-strain energy, which can be captured by AE [32; 33; 34]. The stress-time, acoustic emission energy-time, and acoustic emission cumulative energy-time curves of five groups of coal specimens with different water-soaking heights are shown in Figure 9:  
The evolution of acoustic emission of coal specimens with different water-soaking heights was similar: At the beginning of the loading, there was a quiet period of acoustic emission. And when the coal specimens was about to be destroyed, the acoustic emission event surged and there was an active phase of acoustic emission [17]. Combined with the stress-strain curves of coal specimens  
Figure 9: Stress–time, acoustic emission energy–time, and acoustic emission cumulative energy–time curves. (**a**) 0 mm water-soaking height; (**b**) 25 mm water-soaking height; (**c**) 50 mm water-soaking height; (**d**) 75 mm water-soaking height; (**e**) 100 mm water-soaking height. with different water-soaking heights, the development of stress is divided into compaction closure stage, linear elastic stage, fracture expansion stabilization stage, unstable fracture expansion stage, and post-peak destruction stage [21]. In the compaction closed stage and the linear elastic stage, little acoustic emission energy was generated only when the internal microcracks of the test piece were closed. In these two stages, the five groups of coal specimens were at AE quiet period. In the fracture expansion stabilization stage, the coal specimens in the 25 mm water-soaking height group and the 50 mm water-soaking height group produced more acoustic energy than the other three groups. The different soaking heights made the softening degree of different layers of coal specimens different, so that the deformation of different horizons was not synchronized during the loading process. The stress concentration was caused by the local position of the uncoordinated deformation, then the instability of the coal specimen was aggravated. In the unstable fracture expansion stage and the post-peak failure stage, the acoustic emission events of the five coal specimens were active, the acoustic emission energy was enhanced, and the cumulative energy quantity increased rapidly. Overall, the failure process and acoustic emission evolution characteristics of the five groups of coal specimens had a good correspondence. Before the linear elastic phase, the coal specimen was basically in a quiet period of acoustic emission. At the beginning of the crack propagation stage, the acoustic emission events of the 25 mm and 50 mm soaking height coal specimens were more active than those of the other three groups. In the unstable fracture expansion stage, the acoustic emission events of the five coal specimens occurred intensively, the acoustic emission energy became larger, the cumulative energy of acoustic emission increased rapidly, and reached the highest point after the peak damage. Many scholars [35; 36] have studied the relationship between rock homogeneity and acoustic emission mode, and the results show that the mode of acoustic emission depends on the homogeneity of the rock. The more heterogeneous the material, the more obvious the precursor information before the main rupture occurs. Considering the characteristics of the above five groups of acoustic emission vs. time, it shows that the 25 mm soaking height and 50 mm soaking height coal specimens had obvious acoustic emission activities at the initial stage of crack propagation compared with the other three group coal specimens, which indicated that the heterogeneity of the 25 mm and 50 mm coal specimens was significantly higher than that of the other three groups.