## 6 Discussion
### Influence of the Normal Compressive Stresses on Filtration Performance
#### 6.2.2 Gradient Ratio and Relative Permeability Coefficient  
Figure 10 presents the relationship between \(K_{rel}\) or \(GR\) and relative pore volume (\(RPV\)) for two natural soils filtered by GT2 and GT4. As shown in Figure 10, the relative permeability coefficient (\(K_{rel}\)) decreased with the decrease of the relative pore volume (\(RPV\)) and presented a linear change, indicating that the \(K_{rel}\) constantly decreased over time. The relative pore volume (\(RPV\)) decreased while the \(GR\) increased as the normal stress increased, and the effect of relative pore volume on the \(GR\) was insignificant. When the protected soil was silty clay as shown in Figure 10a,b, the relative permeability coefficient was \(K_{rel}>0.1\) and the change of relative pore volume was small within 0.0016. This indicated that the clogging occurred when migrating fine particles penetrated the filter fabric and encountered fiber constrictions that were too narrow for them to travel further. Fines could then accumulate within the geotextile and obstruct its drainage channels. This was also the reason why \(k_{gg}<k_{s}\), i.e., the head loss through the geotextile filter was greater than the base soil of the adjacent layer (Table 7). When the normal stress was 25 kPa, the \(GR\) changed dramatically, with the medium thickness GT2 reaching 1.60 and the thick GT4 exceeding the critical value of 3.0. This indicated that the high-stress level had a pronounced influence on \(GR\) for the natural soil-geotextile system. Figure 10: _Cont._  
Figure 10: The \(K_{rd}\) and \(GR\) versus relative pore volume (_RPV_) for two natural soils filtered by GT2 and GT4: (**a**) C-GT2 combination; (**b**) C-GT4 combination; (**c**) M-GT2 combination; (**d**) M-GT4 combination. When the protected soil was silt as shown in Figure 10c,d, the relative permeability coefficient \(K_{rel}\) < 0.1 appeared as the normal stress was 25 kPa under the combination of M-GT2/GT4, indicating the occurrence of blinding. The change in relative pore volume was within 0.16, which was two orders of magnitude greater than the above silty clay-geotextile combination. This was because the protected silt was more permeable than silty clay (Table 2). The \(GR\) varied similarly to silty clay, i.e., the high-stress level had a significant influence on the \(GR\), but the \(GR\) was less than the critical value of 3.0. This was most likely due to the protected silt's higher permeability and internal instability, which caused blinding and local clogging in the geotextile filter. This phenomenon would be explained further in subsequent discussions (Table 9). Table 9 summarizes the key findings from two typical tests performed under normal stress conditions of 0 kPa and 25 kPa. The average flow rate values are useful indicators of the filter system's hydraulic performance. The higher these values, the better the filter system. As shown in Table 9, the measured values of the average flow rate varied greatly from 0 kPa to 25 kPa. In particular, with the M-GT2/GT4 combinations, the average flow rate decreased by approximately 58%. Clogging was the most common filtration result observed in the tests conducted, except for the combination of silt and geotextile under high normal stress conditions. This was due to the fact that when higher normal stress was applied to the soil-geotextile combination, the geotextile produced plane compression and the structure became dense, resulting in a decrease in the average flow rate. The internal instability of the protected silt caused more fine particles migrating from a distance to be retained and accumulated in the interface zone near the geotextile, resulting in blinding, i.e., M-GT2/GT4 combinations. The seepage channel was filled with fine particles as a result of the decrease in porosity in the interface zone near the geotextile, resulting in a local increase in permeability in the zone from where the fine particles originated, but the permeability in the interface zone near the geotextile decreased.