# Experimental Study on the Efficacy of Water Infusion for Underground Mining of a Coal Seam
## 4 Results and Discussion  
### Laboratory Test Results and Discussion  
#### 4.1.1 Effect of Moisture Content on the Strength Properties of Coal  
The strength properties (e.g., cohesion and friction angle) are presented with respect to the moisture content in Figure 7. As shown in Figure 7a, when the moisture content of the coal increased from 7.07% to 22.58%, the value of cohesion also increased up to the maximum value of 15.4 kPa. This is due to a result of the cementation of water (i.e., water may act as \"cement\") [2] or the bonding coal particles [4], with an appropriate amount of moisture. This value decreased with a gradual increase in moisture content (22.58-29.5%), due to a reduction of the bonding effect (i.e., a coal particle is easy to detach with a high moisture content). A similar effect of moisture content on the value of cohesion has been observed in other materials, such as, soil examined by [PERSON] et al. (2012) [17], mudstone investigated by [PERSON] and [PERSON] (2012) [18], and coal explored by [PERSON] et al. (2016) [19], remodeled coal studied by [PERSON] et al. (2017) [20]. In the study by [PERSON] et al. (2012) [17], the value of cohesion increased as the moisture content varied in the range of 3.0-12.0% and then dropped rapidly after the peak due to an increase in moisture content (12.0-27.5%). As the authors' explained in their paper, the enhancing cohesion effect might be a result of the cementation of water (i.e., water may act as \"cement\" with an appropriate amount of moisture content and it may shift to acting as a \"lubricant\" after a threshold value of moisture content). [PERSON] and [PERSON] (2012) [18] found that the peak value of the cohesion of mudstone in terms of the moisture was 13.0% (i.e., increased as the moisture content varied in the range of 10.0-13.0% and decreased as the moisture content ranged from 13.0% to 20.0%). A similar trend for coal was pointed out by [PERSON] et al. (2016) [19], who demonstrated that the cohesion of coal was strengthened due to the bonding coal particles after adding water. However, the cohesion of coal being reduced after the threshold moisture content was explained by the reduction of friction between adjacent coal particles. In addition to the similar trend, the ranges of cohesive strength and moisture content in this study are very close to those in the study conducted by [PERSON] et al. (2017) [20], interestingly.  
Meanwhile, the effect of moisture content on the friction angle of the coal specimen was also studied (Figure 7b). A very small change of the friction angle (i.e., a very slight decrease) can be observed for a considerable increase in the moisture content. The effect of moisture content on the friction angle was much less pronounced than that on cohesion, interestingly. This finding agreed well with that of [PERSON] et al. (2017) [20], as they concluded that the internal friction angle was almost independent of the moisture content. In conclusion, the moisture content of 22.58% was recommended as the optimum moisture content for further tasks (e.g., water injection time and field water injection).  
Figure 7: Effect of moisture content on the strength properties of coal samples: (**a**) Effect on cohesion; (**b**) effect on the friction angle.