# Effects of Water-Soaking Height on the Deformation and Failure of Coal in Uniaxial Compression
## 3 Results and Discussion
### Local Strain Characteristics
In order to further analyze the influence extent of water-soaking height on the nonuniform deformation and failure of coal specimen from the perspective of local strain, the nonuniform deformation index \(S_{i}\) was introduced to quantitatively evaluate the nonuniform failure characteristics of coal deformation (as the Equation (3)). \[S_{i}=S_{d}/S_{e} \tag{3}\]  
where \(S_{e}\) is the overall strain before the fracture expansion of the specimen, \(S_{d}\) is the axial strain difference before crack propagation of the specimen (It is shown in Figure 11). The overall deformation of the coal specimen is in a relatively uniform phase before crack propagation. It is reasonable to use the overall strain at this stage as an indicator to evaluate the uniform deformation of the specimen. At this stage, there is no obvious dislocation on the surface of the coal specimen. The strain rosette attached to different positions on the surface of the coal specimen will not be damaged due to the displacement of the surface of the coal specimen, so the strain data can be continuously detected. Therefore, the overall strain and local strain difference before the crack expansion stage can reflect the nonuniform deformation characteristics of coal specimens well. According to five groups of coal specimens with different water-soaking heights, the water-soaking interface was distributed at 25 mm, 50 mm, and 75 mm height of coal specimens. Through the previous local strain analysis, the uneven degree of strain at the coal specimens of 25 mm and 75 mm is relatively large. Therefore, in order to unify the comparative analysis, the local strain difference value was made by the axial strain at heights of 25 mm and 75 mm. The uneven deformation index \(Si\) can be calculated, and the specific calculation results are shown in Table 3.  
Table 3 shows that the nonuniform deformation index of coal specimens with the 25 mm water-soaking height was the largest, and the corresponding strength was the smallest among three partial water-soaking groups (25 mm, 50 mm, 75 mm), which indicated that the nonuniform deformation of the specimen exacerbated the damage of the specimen and reduced the strength of the specimen. Furthermore, the axial strain difference was analyzed combined with the overall stress and the acoustic emission energy. Figure 12 shows local strain difference-time, stress-time, acoustic emission energy-time curve. Figure 12 shows that the axial strain difference increased with loading and increased significantly in the stage of stable crack expansion. The axial strain difference first reached the maximum before the specimen reached the peak strength. A sharp increase of the axial strain difference on the specimen surface can be used as a precursor to the instability of the specimen. The axial strain difference of partially soaking coal specimens was more significant during the loading process. The large axial strain difference of the specimen represents that its nonuniform deformation is obvious. The specimen became unstable and destroyed quickly after the occurrence of a larger axial strain difference. The acoustic emission mode and local axial strain evolution law were very good to show that the inhomogeneity of the coal specimen with 25 mm height water soaking was the largest, which led to the weakening of its strength. The coal specimen with 25 mm water-soaking height had the lowest strength among the coal specimens with partial water soaking due to the obvious nonuniform deformation characteristics. Although the water content of the coal specimens increased with the increase of the water-soaking height, the strength of the coal specimen did not decrease as the water content increased. Nonuniform deformation will reduce the strength of coal specimen to some extent. Figure 11: Analysis of axial strain difference index.