## 3 The Occurrence Mechanism of SCWID
### Verification by Physical Modelling
The coal seam continued to be mined to the left. When the mining width reached 67 cm, KS1 and KS2 were still not damaged, and there were no cracks in KS1 and its upper strata. The coal seam continued to be mined to the left. When the mining width reached 70 cm, KS1 and KS2 suddenly broke synchronously under the action of upper pressure. At this time, a large number of fractures were generated between KS1 and KS2, and between KS2 and UCA, and these fractures directly connected the coal seam and the aquifer. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No**} & \multirow{2}{*}{**Stratum**} & \multirow{2}{*}{**Thickness**} & \multirow{2}{*}{**Unit Weight**} & \multirow{2}{*}{\begin{tabular}{c} **Elastic** \\ **(cm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Cohesion** \\ **(kN\(\cdot\)m\({}^{-3}\))** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Internal** \\ **(MPa)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} } \\ \hline \multirow{2}{*}{1} & Unconsolidated & \multirow{2}{*}{50} & \multirow{2}{*}{16.0} & \multirow{2}{*}{0.064} & \multirow{2}{*}{0.016} & \multirow{2}{*}{25} & \multirow{2}{*}{0.4} \\  & formation & & & & & & \\
2 & UCA & 20 & - & - & - & - & - \\
3 & Zone of & 3 & 16.0 & 0.064 & 0.032 & 25 & 0.35 \\
4 & Soft strata & 16 & 17.6 & 0.120 & 0.032 & 33 & 0.33 \\
5 & KS2 & 6 & 18.5 & 0.240 & 0.064 & 40 & 0.25 \\
6 & Soft strata & 12 & 17.6 & 0.120 & 0.032 & 33 & 0.33 \\
7 & KS1 & 5 & 18.5 & 0.240 & 0.064 & 40 & 0.25 \\
8 & Roof & 12 & 16.0 & 0.080 & 0.032 & 28 & 0.33 \\
9 & Coal seam & 4 & 12.0 & 0.032 & 0.012 & 23 & 0.33 \\
10 & Floor & 5 & 18.5 & 0.240 & 0.064 & 40 & 0.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanics parameters of the overlying strata. Figure 5: Strata breakage and movement between the scenarios without UCA: (**a**) advance 60 cm; (**b**) KS1 broken; (**c**) KS2 broken. From the comparison of the two tests, it can be seen that the mining width was different when KS1 broke: 73 cm when there was no UCA, and 70 cm (slightly smaller than no UCA case) when there was UCA. This suggests that KS1 is subject to greater load when there is aquifer, thus shortening the breaking distance. To further analyze the difference of the upper load on the broken strata between the cases with and without aquifer, the pressure data of the same position in the same height from the coal seam are now analyzed. From the simulated results in Figure 7, before the working face passed through the pressure monitoring point, the stress basically remained unchanged with UCA. The pressure has an obvious stable period, which shows that the load has been acting on the key layer, but it showed a continuous upward trend without UCA after stable stage (due to the transfer of the abutment pressure), which shows that when there is UCA, the load is continuously acting on the lower strata (at this stage, the abutment pressure is not transferred to the mining boundary as in the case of no UCA). When the working face passed through the monitoring point, in the scenario of UCA, before KS1 and KS2 broke, the stress still remained unchanged, but when it continued to advance to 70 cm or passed through the monitoring point 10 cm, KS1 and KS2 broke synchronously, and the stress value decreased quickly. In the scenario of no UCA, the stress of the measuring point decreased immediately after passing through the monitoring point. When the working face advanced about 13 cm through the measuring point and the cumulative advance distance was about 73 cm, KS1 broke first, and KS2 broke when the working face advanced about 19 cm through the measuring point and the cumulative advance distance was about 79 cm. The breaking distances of KS1 and KS2 were greater than those in the situation of UCA, suggesting that the larger the load, the shorter the breaking distance of KS1 and KS2.