## 5 Test Results  
### Filtration Behavior of Silty Clay  
Twenty filtration tests were performed using silty clay (internally stable soil) in combination with four different geotextiles (GT1-GT4) to assess their filtration behavior under normal stresses. Figure 5 depicts the \(GR\) and the permeability coefficient values for the silty clay-geotextile combinations. As a reference, the filtration test results for the same soil-geotextile combination free of normal stress were used. As shown in Figure 5, the \(GR\) values decreased slowly with the elapsed time but increased with the increase of normal stress, and the coefficient of permeability decreased with the elapsed time and the increase of normal stress. These findings suggested that the filtration performance of soil-geotextile combinations was significantly affected by normal stress. Additionally, a little variation of \(GR\) was observed after an elapsed time of 48 h at each system, indicating a steady state flow condition. The permeability coefficient \(k_{sg}\) of geotextile samples with silty clay exhibited some fluctuation during the initial 25 h. Following that, the \(k_{sg}\) values tended to decrease as normal stress increased. There were no significant differences in \(k_{sg}\) between the geotextiles in GT1 and GT2. When the applied normal stress exceeded 10 kPa, the differences in permeability coefficient \(k_{sg}\) for thick geotextiles (GT3 or GT4) were more pronounced than for thin geotextiles, i.e., the \(k_{sg}\) of thick geotextiles decreased from \(k_{sg}\times 10^{-9}\) m/s to \(k_{sg}\times 10^{-10}\) m/s, while thin geotextiles changed in the order of \(k_{sg}\times 10^{-9}\) m/s. The \(k_{s}\) values were marginally higher than \(k_{sg}\) values for the most of silty clay-geotextile combinations (Table 7), indicating the migration of fines from the soil towards the geotextiles where clogging or blinding may occur. Therefore, a decrease in \(k_{sg}\) values of the soil-geotextile composite under larger normal stress could result in an increase in \(GR\) values. Figure 5: _Cont._Figure 5: _Cont._  
Table 7 summarizes the results of the filtration test for combinations composed of each geotextile and the silty clay under five different magnitudes of normal stress. As shown in Table 7, an increase in \(GR\) values was noticed as the normal stress increased from 0 kPa to 25 kPa. For the test cases using thin and medium thick geotextiles (GT1, GT2), the increment in \(GR\) values increased around 28% and 22%. For the thicker geotextiles (GT3 or GT4), however, the \(GR\) values increased significantly with normal stress, increasing by about 198% and 269%, respectively, particularly when the normal stress was greater than 10 kPa. It is generally acknowledged that the specific surface area (\(Sa\)) of the nonwoven geotextile responds significantly to fine particles retained on the surface of the geotextile filter. The greater the \(Sa\) is, the more prone the geotextile filter is to be deposited or adsorbed by fine particles. Furthermore, the curved and continuous pores in the nonwoven geotextile form its filtration channel, and thickness is an important structural parameter influencing the size of the geotextile filtration channel. The greater the thickness is, the smaller the geotextile filtration channel is, resulting in more fine particles retained and larger \(GR\) values [31, 32]. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Soil** & **Geotextile** & **Normal Stress (kPa)** & \(k_{\text{sg}}\) **(m/s)** & \(k_{\text{g}}\) **(m/s)** & \(k_{\text{R}}\) & \(GR\) \\ \hline \multirow{5}{*}{Silty clay (C)} & & 0 & \(3.17\times 10^{-9}\) & \(3.44\times 10^{-9}\) & 1.08 & 1.08 \\  & & 5 & \(2.45\times 10^{-9}\) & \(2.69\times 10^{-9}\) & 1.10 & 1.10 \\  & GT1 & 10 & \(1.47\times 10^{-9}\) & \(1.71\times 10^{-9}\) & 1.16 & 1.16 \\  & & 15 & \(1.36\times 10^{-9}\) & \(1.63\times 10^{-9}\) & 1.20 & 1.20 \\  & & 25 & \(1.09\times 10^{-9}\) & \(1.51\times 10^{-9}\) & 1.38 & 1.38 \\ \hline \end{tabular}
\end{table}