## 4 Discussion  
The result of numerical simulation indicates that the risk of aquifer rock failure, based on the safety factor by strain, is extremely low. The most hazardous zone, according to the simulation results, is the longwall edge zone (to the right of the number 817 conveyor roadway). At the same time, the safety factor by stress indicates conditionally a stable state (SF = 1.1-1.3) [32] of the rock mass near the aquifer area. Unfortunately, at this present moment in time, there are no generally accepted laws linking safety factors and probabilities of rock mass failure. Figure 15 shows the relationship between safety factors and probabilities of rock failure based on some current geotechnical engineering studies. Although it is accepted that they may only be applied within the constraints of the studies undertaken, it is proposed to use their results to assess the probability of failure in this study. The probabilities of rock failure near the aquifer were assessed in accordance with the graphs shown in Figure 15. The safety factor of 1.15 corresponds to probabilities of failure of 0.12-0.25 according to the studies of [PERSON] et al. and [PERSON] [32]. Thus, it is proposed to use probability of rock failure based on safety factors as criteria for the risk of water inrush. Figure 14: Vertical (**a**) and horizontal (**b**) strains along the monitoring lines: 1—monitoring line 1; 2—monitoring line 2; and 3—monitoring line 3. Based on statistical data, [PERSON] et al. [37] proposed to implement a four-score system to estimate inrush risk in terms of the levels: \"low\" level (16.0-36.9); \"average\" level (37.0-57.9); \"high\" level (58.0-78.9); and \"critical\" one (79.0-100.0) (Table 5). According to this classification, the risk of water inrush in the number 817 longwall panel is assessed as being \"low\" (16.0-36.9). The integration of simulation results with the geological condition analysis indicates that the flooding incident in the 815 longwall occurred within the influence zone of the 815F16 geological fault, with a steep inclination (78 degrees) and a large displacement (7.5 m). This geological fault led to the rock failure, creating a water migration channel from the aquifer. Thus, the highest risk of future water inrush from the aquifer can be expected in zones influenced by faults with steep dip angles. Specifically, in the 817 longwall panel, two faults have been identified, with inclinations of less than 50 degrees and displacement of about one meter. Therefore, available geological data do not allow to predict the occurrence of any additional risk of water inrush in the 817 longwall panel. Based on the conducted research, the following algorithm for determining the risk of any water inflow from aquifer is proposed:  
1. Construction of a numerical model based on geological exploration data. 2. Calibration of the model and verification of its adequacy. 3. Calculation of the stress-strain state of the rock mass. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Inrush Risk Level** & **Probability of Failure, \%** & **Probability of Failure Level** & **Accident Risk Caused by Rock Failure** \\ \hline Low & 16.0–36.9 & Very low & Acceptable \\ \hline Average & 37.0–57.9 & Possible & Acceptable in case of supervision and repetitive monitoring \\ \hline High & 58.0–78.9 & Expectable & Nonacceptable without regular control measures \\ \hline Critical & 79.0–100.0 & Very expectable & Nonacceptable \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Qualification of the inrush risk levels. Figure 15: Relationship between safety factors and probabilities of rock failure [32]. Analysis of the stresses and strains at the lower boundary of the aquifer. * Calculation of safety factor (Stress) and safety factor (Strain) by mathematical expressions (4), (5). * Determination of the probabilities of rock mass failure (Figure 15). * Estimating intrush risk (Table 4). * Additional assessment of the influence of faults with steep dip angles and large displacement.