# Effects of Water-Soaking Height on the Deformation and Failure of Coal in Uniaxial Compression
## 3 Results and Discussion  
### Water Soaking Experiment  
In order to study the influence of the soaking height on the water absorption law of coal specimens, the water-soaking tests of the coal specimens under five different soaking heights were carried out. The results are shown in Figure 5. In the Figure, the abscissa is the water-soaking time of the coal specimen, and the ordinate is the water content of the coal specimen. The water content is calculated using the following formula (ISRM, 1979 [28]):  
\[w_{t}=(m_{t}-m_{0})/m_{0}\times 100\% \tag{1}\]  
where \(w_{t}\) is the water content, \(m_{t}\) is the mass of the wet coal specimen at time \(t\), and \(m_{0}\) is the mass of the dry coal specimen  
The results showed that the evolution of water content of coal specimens with 25 mm, 50 mm, 75 mm, and 100 mm water-soaking heights was almost the same. The moisture content of coal specimens shows a nonlinear growth trend and can be divided into three stages [19]: Rapid water absorption stage, slow water absorption stage, and stabilization water absorption stage. In the first 8 h, the moisture content of the coal specimens increased rapidly. Subsequently, the water absorption rate of  
\begin{table}
\begin{tabular}{c c} \hline
**Test Case** & **Water Soaking (Height Ratio)/h = 100 mm** \\ \hline \#1 & 0 h \\ \#2 & 0.25 h \\ \#3 & 0.5 h \\ \#4 & 0.75 h \\ \#5 & 1 h \\ \hline \end{tabular}
\end{table}
Table 2: Test design.  
Figure 4: Different soaking heights coal specimens.  
the coal specimens decreased rapidly. With the increase of the water-soaking time, the water contents of the coal specimens increased slowly until 104 h. The moisture contents of the coal specimens treated by each group reached a relatively stable value. After 104 h, the coal specimens' contained water rate hardly changed, and it can be considered that the saturation state was reached under the conditions of the respective soaked height.  
After 104 h, the moisture contents of all coal specimens no longer changed, when it could be considered that the water content reached a stable state.  
Due to different soaking heights in the water, the degree of contact between each group of coal specimens and water was different, then the final moisture content was different. It shows that the coal specimens of the nonsoaking Group absorbed moisture in the air slowly, the final moisture content was the lowest among all the testing groups, and the average water content was 1.01%.  
As the soaking height increased, the final moisture content of the coal specimen increased. The coal specimen with 25 mm water-soaking height had a final moisture content of 15.13%. The coal specimen with 50 mm water-soaking height had a final moisture content of 16.68%. For the coal specimens with 75 mm water-soaking height, the final average moisture content was 18.18%. The coal specimens with 100 mm water-soaking height had the final moisture content of 19.11%.  
The water content data of coal specimens with water-soaking heights of 25 mm, 50 mm, 75 mm, and 100 mm were linearly fitted (Figure 6). It was found that there is a good linear fitting relationship between the final moisture content of the coal specimen and the soaking height. The fitting relationship is expressed in Equation (2):  
\[\mathrm{y}=13.91234+0.0537x\hskip 28.452756 ptR_{2}=0.98 \tag{2}\]