## 5 In Situ Coring Test
### SEM Observations  
A JSM-5900 scanning electron microscope was adopted in this study. The microstructure observations were carried out via SEM at different magnification times to evaluate the stabilization effect of silty sands stabilized with HPCG, RPCG and MKCG.  
Figure 10a shows the spatial network structure of the high-polymer additive; a series of chemical reactions occurred during the curing process of the polymer grouting material to form water-insoluble polymer compounds. The cluster blocks were connected by a dense network, the number of voids was greatly reduced, and the particle units and flocs were closely arranged, which improved the water-stopping performance of HPCG.  
In Figure 10b, on the surface of the large particles and in the middle of the pores, there was an obvious rod-like or needle-like structure, which is a significant feature of the existence of ettrinigite. The structures in the system gradually connected to form a dense large block; therefore, the overhead pores could be easily observed. The positively charged Ca\({}^{2+}\) provided in the RPCG, on the one hand, promoted the formation of ettrinigite and other hydration products; on the other hand, the positively charged Ca\({}^{2+}\) and negatively charged soil particles combined to form colloidal condensation; however, there were voids in the internal structure of the soil, and the flocculant only attached to the granular units.  
In Figure 10c, the MKCG cementation material transformed the contact connection between the flaky sand particles into a cementation connection. Due to the existence of different degrees of aluminosilicate polymers on the surface of soil particles and among the particles, previously tiny individual particles, stacks and flocs in the silica-aluminate polymer formed large bulk agglomerates that attached to the granular units, with voids between the granular units and the flocs.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Case ID** & \begin{tabular}{c} **Seepage** \\ **Pressure/(kPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Permeability** \\ **Coefficient/(cm/s)** \\ \end{tabular} &
\begin{tabular}{c} **Average Permeability** \\ **Coefficient/(cm/s)** \\ \end{tabular} \\ \hline RPCG-1 & 600 & \(4.23\times 10^{-6}\) & \\ RPCG-2 & 1000 & \(1.20\times 10^{-6}\) & \\ RPCG-3 & 500 & \(1.49\times 10^{-6}\) & 2.7 \(\times 10^{-6}\) \\ RPCG-4 & 500 & \(2.08\times 10^{-6}\) & \\ RPCG-5 & 500 & \(1.48\times 10^{-6}\) & \\ \hline \hline \end{tabular}
\end{table}
Table 7: Permeability coefficients of RPCG.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Case ID** & \begin{tabular}{c} **Seepage** \\ **Pressure/(kPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Permeability** \\ **Coefficient/(cm/s)** \\ \end{tabular} &
\begin{tabular}{c} **Average Permeability** \\ **Coefficient/(cm/s)** \\ \end{tabular} \\ \hline MKCG-1 & 200 & \(3.09\times 10^{-6}\) & \\ MKCG-2 & 200 & \(3.08\times 10^{-6}\) & \\ MKCG-3 & 1000 & \(2.42\times 10^{-6}\) & 3.31 \(\times 10^{-6}\) \\ MKCG-4 & 500 & \(3.68\times 10^{-6}\) & \\ MKCG-5 & 500 & \(4.28\times 10^{-6}\) & \\ \hline \hline \end{tabular}
\end{table}
Table 8: Permeability coefficients of MKCG.