## 5 Discussions
### PV Sensitivity of Permeability  
The permeability of the collapsed rock mass is mainly determined by the voids and the void structure. The low porosity of the mudstone layer in the lithology combination sample is the main reason for the low permeability of the combination sample. To analyze the permeability variation characteristics of compacted rock mass, the PV sensitivity parameter of permeability change (\(D_{\text{g}}\)) is adopted.  
\[D_{\text{g}}=\frac{k_{\text{s}}-k_{\text{e}}}{\varphi_{\text{s}}-\varphi_{ \text{e}}} \tag{5}\]  
where \(k_{\text{s}}\) is initial permeability, \(k_{\text{e}}\) is compacted permeabilities, \(\varphi_{\text{s}}\) is initial PV, and \(\varphi_{\text{e}}\) is compacted PV.  
In Table 8, the \(D_{\text{g}}\) of the combined samples is not more than 20. The \(D_{\text{g}}\) of the sample containing mudstone combination is significantly smaller than that of the C-3. The \(\varphi_{\text{s}}\) and \(\varphi_{\text{e}}\) of the C-3 samples are the largest. In addition, the combination of sandstone and coal lacks the plugging effect of mud, which together leads to the C-3 exhibiting significantly higher \(D_{\text{g}}\) values compared to other samples.  
The low \(k_{\text{s}}\) and \(D_{\text{g}}\) jointly indicate that the mudstone and sandstone combination has the best water-proof recovery ability, followed by the sandstone + mudstone + coal combination and the mudstone + coal. To reduce the loss of water resources in overlying strata and eliminate the risk of water inrush in lower coal seam mining, the caving coal seam in goaf should be reinforced and anti-seepage treated to ensure that the goaf of overlying strata has a water-proof function. The value of \(k_{\text{s}}\) indicates that the permeability of the combined samples containing mudstone is significantly lower under the uncompacted state, and \(k_{\text{s}}\) is related to \(\varphi_{\text{s}}\), showing a consistent order of magnitude. The combination of different lithology shows that, under the condition of compacted, that is, under the condition of self-weight stress recovery, the permeability of the combined samples containing mudstone is similar and the water resistance performance is better. The permeability and porosity of the composite samples containing coal are larger. Therefore, the existence of residual coal in goaf is not conducive to the recovery of compaction and water insulation performance in goaf.  
The results of Figures 7 and 8 show that one of the important reasons for the fastest recovery of sandstone and mudstone combined compaction is that the initial porosity of the compaction process is the lowest and the porosity under different loading levels is always the smallest. The minimum \(D_{\text{g}}\) of C-1 sample shows that the sandstone and mudstone combination sample has the characteristics of low permeability and small reduction in permeability during compaction.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & \(k_{\text{s}}\)**/D** & \(k_{\text{e}}\)**/D** & \(\varphi_{\text{s}}\) & \(\varphi_{\text{e}}\) & \(D_{\text{g}}\)**/D** \\ \hline C-1 & 0.18 & 0.02 & 32.02 & 18.37 & 1.17 \\ C-2 & 0.53 & 0.03 & 38.35 & 22.00 & 3.06 \\ C-3 & 3.84 & 0.77 & 39.17 & 22.96 & 18.94 \\ C-4 & 0.29 & 0.02 & 36.51 & 19.81 & 1.62 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Permeability void sensitivity analysis results of compaction process.