# Experimental Study on the Efficacy of Water Infusion for Underground Mining of a Coal Seam
## 4 Results and Discussion
### Field Test Results and Discussion  
As mentioned earlier, in the case that the pressure, acting on the upper part of a coal seam, meets the maximum bearing capacity of the coal wall, the coal body starts failing and causes top spalling. Figure 9 shows two pictures, obtained from the field tests, of coalfaces before and after using water infusion techniques. The physical efficacy of the water infusion method for underground mining of the coal seam can be visibly observed first at the coalfaces. It can be explained by the fact that after water is injected into the seam, it leads to an increase in the moisture content of the coal seam. In this condition, the plasticity of the coal body can be increased and the brittleness can be decreased with the presence of injected water. Due to the modification of both mechanical properties of the coal seam, the stress concentration ahead of working coalfaces of a coal seam is re-distributed (i.e., with a considerable reduction of the acting pressure on the upper part of the coal seam) [11]. For this reason, less top spalling at the coalfaces could be expected due to the pressure acting on the top of the coal seam. In order to quantitatively evaluate the efficacy of water infusion for the stability of coalfaces in the field, the spalling depths were monitored successively at various coalfaces for both case studies (i.e., with and without water infusion). It should be noted that the depth of longwall face failure, the so-called spalling depth, is the key factor for assessing the stability of coalfaces in underground mining construction in Vietnam [15]. In total, 28 coalfaces (a case study of without water infusion) and 15 coalfaces (a case study of with water infusion) were measured in the field, from May 2016 to August 2016 and September 2016 to October 2016, respectively. Figure 10 illustrates the spalling depths of various coalfaces of the coal seam without (a) and with (b) using the water infusion method. As a result, the maximum and minimum spalling depths varied in ranges of 1.2-2.5 m and 0.8-1.8 m, respectively. As expected, these depths were reduced 2.8 times compared to those of coalfaces without using water infusion. All coalfaces had spalling depths of less than 0.9 m after us  
Figure 8: Effect of the water injection time on the strength properties of coal samples: (**a**) Effect on cohesion; (**b**) effect on the friction angle. method in the coal seam. In this sense, water infusion improved the stability of the coal seam in the aspect of spalling depth. More importantly, coalfaces with and without the use of water infusion were also subjected to field measurements in terms of the mining speed (Figure 11). As shown, the mining speed of the coal seam using water infusion was larger than that of the coal seam without using the proposed technique (i.e., water infusion), regardless of coalfaces. In particular, the mining speed of the coal seam without using water infusion was only in the range of 6.6-8.1 m/month. As expected, this speed could reach a maximum of 14.4 m/month (i.e., increasing 1.8 times) compared to that of coalfaces without using water infusion. The higher stability of the coal seam (i.e., less spalling depths) might be the basepoint leading to the higher mining speeds of the seam sections using water infusion. Moreover, a relationship between mining speed and spalling depth is also depicted in Figure 12. The data were collected from all average values of mining speeds and spalling depths each month from May 2016 to October 2016. This relationship aims to determine if the spalling depth can be used to predict the mining speed at any coalface of the coal seam. As a general trend, a faster mining speed is achieved with a smaller spalling depth. This finding can be explained by the procedure of mining construction in the field. A coalface with a smaller spalling depth can move much faster to the next mining advance than a coalface with a larger spalling depth, mainly due to the shorter time of support structure installation.