## 6 Discussion
### Influence of the Normal Compressive Stresses on Filtration Performance
#### 6.2.3 Gradient Ratio and Geotextile Head Loss  
Figure 11 depicts the gradient ratio (\(GR\)) and geotextile head loss (\(GHL\)) versus relative pore volume data obtained from the tests described in the previous section. \(GHL\) represents the change in water level difference between manometer tubes 2 and 3 over time. As shown in Figure 11, the \(GR\) and \(GHL\) showed the same trend with the relative pore volume. This was because the \(GR\) represented an indicator of the erodibility of the fines in the soil matrix, and \(GHL\) was a measure of the clogging state of the geotextile openings. Under free normal stress, the higher the \(GHL\), the finer particles were transported from the soil into the geotextile, and the higher the \(GR\) of the geotextile filter. A significant increase in \(GR\) and \(GHL\) was observed for both of these two natural soil types with normal stress, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Natural Soils** & \multicolumn{3}{c}{**Silty-Clay (C)**} & \multicolumn{3}{c}{**Silt (M)**} \\ \hline
**GT Type** & **Thin GT2** & **Thick GT4** & **Thin GT2** & **Thick GT4** \\ \hline Normal stress (kPa) & 0 & 25 & 0 & 25 & 0 & 25 \\ Total flow volume (cm\({}^{3}\)) & 10.05 & 6.52 & 8.87 & 5.1 & 894.2 & 374.1 & 865.7 & 360.7 \\ Average flow rate (cm\({}^{3}\)/h) & 0.21 & 0.14 & 0.18 & 0.11 & 18.63 & 7.79 & 18.04 & 7.51 \\ Final soil void ratio & 1.02 & 0.99 & 1.02 & 0.98 & 0.89 & 0.85 & 0.89 & 0.86 \\ \(K_{\text{init.}}\) (\(10^{-8}\) m/s) & 0.41 & 1.03 & 1.06 & 0.59 & 173 & 134 & 170 & 132 \\ \(K_{\text{final}}\) (\(10^{-8}\) m/s) & 0.31 & 0.18 & 0.24 & 0.09 & 39.50 & 8.90 & 40.20 & 8.51 \\ Final \(K_{rel}\) & 0.77 & 0.17 & 0.22 & 0.15 & 0.23 & _0.07_ & 0.24 & _0.06_ \\ Final (\(RPV\)) & 0.0002 & 0.0001 & 0.00017 & 0.00007 & 0.034 & 0.0074 & 0.032 & 0.0068 \\ Fine particles retained (g/m\({}^{2}\)) & 271 & 531 & 605 & 846 & 465 & 715 & 610 & 939 \\ Filtration result & clog & clog & clog & clog & Blind + clog & clog & Blind + clog \\ \hline \hline \end{tabular}  
* Notes: \(K_{\text{init}}\) and \(K_{\text{final}}\) are the initial and final permeability coefficients of the soil-geotextile system, respectively; clog = clogging, blind = blinding. Bold italic indicates that the geotextile filter occurs blinding due to the \(K_{rel}\) < 0.1.  
\end{table} 
Table 9: Summary of the operational data and gradient ratio filtration test results at 48 h.  
particularly when normal stress of 25 kPa was applied in combination with the thick, geotextile, indicating more fine particles migration occurred over the duration of the test. Figure 11: Cont. Figure 11a,b shows that the _GR_ and _GHL_ are significantly higher when the M-GT2/GT4 combination is subjected to \"blinding + clogging\" (Table 9) under normal stress of 25 kPa than when subjected to 0 kPa action. For the combination of M-GT2, _GR_ and _GHL_ increased by 63% and 26%, respectively. The M-GT4 combination, _GR_ and _GHL_ increased by 58% and 39%, respectively. This was because, under normal stress, the planar compression of GT4 was greater than that of GT2 (Figure 7), and the filtration opening size of GT4 was smaller than that of GT2 (Figure 9), reducing the movement of fine particles and causing them to retain or accumulate on the soil-geotextile interface, resulting in blinding (Table 9). As a result, the head loss of GT4 was approximately 13% greater than that of GT2. As shown in Figure 11c,d, the _GR_ of the normal stress of 25 kPa acting on the C-GT4 is significantly higher than that of the M-GT4 combination. This was because when the protected soil is silty clay (C), the single grain was small but had a certain adhesiveness and hydrophilicity and often appeared as an aggregate particle.