## 6 Discussion
### Implications for Design Practice  
Regardless of the geotextile filter chosen for drainage applications, it must meet two competing requirements (i.e., retention and permeability) to ensure optimum performance. Soil retention necessitates a pore structure fine enough to retain erodible soils while avoiding clogging in the geotextile filter. The permeability requires the geotextile filter to maintain adequate permeability so that seepage from the protected soil is prevented. According to the results of gradient ratio filtration tests, nonwoven geotextiles are more prone to clogging when filtering fine-grained soil, although they do not meet the requirements of soil retention ratio (\(O_{95}/d_{85}\), \(O_{95}/d_{50}\)) in the existing filtration criteria [15; 19; 38; 41]. This was because most fine particles would inevitably be trapped on the upstream surface and within the geotextile during the process of migration with water flow, resulting in a decrease in geotextile permeability and an increase in soil retention. The degree of geotextile clogging increased with continuous filtration until it reached a stable state. Therefore, for this protected soil, whether the clogged geotextile filter can meet the requirements, i.e., the value determination standard of \(GR\) and \(k_{sg}\), is a challenge for filter design. The hydraulic gradient through the soil and soil-geotextile composite is determined by the type of soil and any interactions it has with the geotextile. Using the above typical experiment data with the silty clay and thicker geotextile combinations, Figure 12 schematically illustrates the relationship between water head distribution and gradient ratio without taking blinding into account. Based on the interrelationship between gradient ratio and relative permeability as shown in Equation (4), three zones are defined in Figure 12. The first boundary selects the water head distribution of the GT4 combination at different positions above the geotextile at 0 kPa. The first zone is defined as \(GR\) less than one and the permeability of the soil-geotextile composite exceeds that of the soil. The second boundary selects the water head distribution of the GT3 combination at different positions above the geotextile at 5 kPa. The second zone is a distribution in which the \(GR\) is greater than one but less than 3.0, and the permeability of the soil-geotextile composite is (0.35-0.98) times that of the soil, with a small value when the normal stress is high. The third boundary selects the water head distribution of the GT4 combination at different positions above the geotextile at 25 kPa. The third zone is defined as \(GR\) > 3.0, and the criterion of \(GR\)\(\leq\) 3.0 is proposed for the clogging resistance. It is generally recommended that the permeability of the soil-geotextile composite increases along the flow direction (i.e., \(k_{sg}\) > \(k_{s}\)) to avoid unacceptable energy losses in filter design. Figure 12 shows that the water head distribution of zone 1 satisfied this condition, while zones 2 and 3 did not. The water head distribution in zones 1 and 2, however, was acceptable according to the relative hydraulic gradient defined by \(GR\) < 3.0. Note that the permeability \(k_{sg}\) of a thick 25 mm soil and geotextile composite layer is \(k_{31}\). A significant inconsistency occurred in zone 2, i.e., the \(k_{31}\) could be less than 35% of the soil. The design criterion for a graded granular medium ensures the filter is approximately 20 times more permeable than the protected soils when the flow is perpendicular to the filter and into a permeable layer that meets the discharge requirements [42]. Since the geotextile filter criterion evolved from the graded sand filter criterion, similarly, the criterion for geotextile filters can be expressed as:  
\[k_{sg}>\lambda k_{s} \tag{10}\]  
where \(\lambda\) is a factor governed by the water head loss that occurs across the soil-geotextile composite during seepage flow and the normal stress imposed on the soil-geotextile system.