## 4 Discussion
### Grouting Recommendations for Different Sandstone Subcategories  
The principles of the mercury injection and grouting tests are very similar. Thus, this paper evaluates grouting difficulty and proposes grouting suggestions by evaluating the mercury injection test parameters of the different subcategories of sandstone in Table 10.  
As shown in Table 10, among the results of all the subcategories, the primary intergranular pore content of subcategory I\({}_{1}\) sandstone is more than 7%, the pore structure is well connected, the threshold pressure and the pressure at 50% mercury saturation are the smallest, and the average pore throat radius is the largest. Considering that the minimum average radius of ultrafine cement particles on the market is less than 1 \(\upmu\)m and the average pore throat radius of subcategory I\({}_{1}\) sandstone is greater than 3 \(\upmu\)m (making it more than three times the average radius of microfine cement particles), according to practical experience of related grouting engineering work [50; 51], a microfine cement slurry could penetrate through the pore throats of subcategory I\({}_{1}\) sandstone. Therefore, the suspension slurry approach is recommended for grouting in subcategory I\({}_{1}\) sandstone. The average pore throat radii of subcategory I\({}_{2}\) sandstone are 0.738-3.623 \(\upmu\)m, namely, generally less than three times the radii of microfine cement particles. Consequently, suspension slurry grouting is quite difficult, so chemical grouting is generally recommended for subcategory I\({}_{2}\) sandstone. However, if the primary intergranular pores are quite well developed locally in subcategory I\({}_{2}\) sandstone and the average pore throat radius is greater than 3 \(\upmu\)m, it is possible to use suspension slurry grouting for subcategory I\({}_{2}\) sandstone. Subcategory II\({}_{1}\) sandstone has a small intergranular pore content, poor pore connectivity, a high threshold pressure and pressure at 50% mercury saturation, and an average pore throat radius that is smaller than the minimum radius of suspension slurry particles; thus, chemical grouting is recommended. Moreover, if the average pore throat radius of subcategory II\({}_{1}\) sandstone is \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Parameters and Recommendations**} & \multicolumn{2}{c}{**Category I**} & \multicolumn{2}{c}{**Category II**} \\ \cline{2-5}  & **Subcategory I\({}_{1}\)** & **Subcategory I\({}_{2}\)** & **Subcategory II\({}_{1}\)** & **Subcategory II\({}_{2}\)** \\ \hline P\({}_{\text{d}}\) (MPa) & 0.089–0.112 & 0.089–0.312 & 0.207–0.770 & 0.780–2.550 \\ R\({}_{\text{max}}\) (\(\upmu\)m) & 6.273–8.258 & 2.356–8.258 & 1.097–3.551 & 0.580–2.356 \\ P\({}_{\text{B}}\) (MPa) & 0.110–0.174 & 0.177–0.687 & 1.646–5.764 & 5.843–63.111 \\ R\({}_{\text{80}}\) (\(\upmu\)m) & 4.67–6.68 & 1.070–4.155 & 0.121–0.446 & 0.012–0.111 \\ R\({}_{\text{avg}}\) (\(\upmu\)m) & 3.79–4.93 & 0.738–3.623 & 0.139–0.622 & 0.048–0.137 \\ PIP (\%) & \(>\)7 & 2–7 & 0.5–2 & \textless{}0.5 \\ \hline \hline \multirow{2}{*}{Grouting recommendations} & \multirow{2}{*}{\begin{tabular}{c} Supersion \\ (microfine \\ cement) \\ \end{tabular} } & Chemical & \multirow{2}{*}{\
\begin{tabular}{c} Chemical grouting \\ grouting or local \\ suspension \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} Unable to grout \\ needed locally \\ \end{tabular} } \\ \cline{1-1} \cline{4-4}  & & & & \\ \hline \hline \end{tabular} Note: PIP—the content of primary intergranular pore (\(\%\)).  
\end{table} 
Table 10: Parameters and recommendations for different sandstone subcategories. too small locally, it is necessary to expand the pore cavity before chemical grouting. Subcategory II\({}_{2}\) sandstone contains few intergranular pores and mainly contains micropores and various intragranular pores, and it has the highest threshold pressure and the smallest average pore throat radius (generally smaller than 0.1 \(\upmu\)m).