## 4 Test Program
### Test Parameters  
The clogging behavior of a given soil-geotextile system can be assessed by its gradient ratio values [18]. To calculate the hydraulic gradients, relative measurements of differential water heads and seepage path lengths at different pressure ports are taken at the same time intervals. The behavior of the soil-geotextile composite (the combination of soil length 25 mm and geotextile specimen thickness), which was also assumed to be that of a geotextile filter, is characterized by the ratio:  
\[GR=\frac{i_{sg}}{i_{s}}=\left(\
icefrac{{\Delta h_{sg}}}{{l_{sg}}}\right)/ \left(\
icefrac{{\Delta h_{s}}}{{l_{s}}}\right)=l_{s}\Delta h_{sg}/l_{sg} \Delta h_{s}=\frac{2l_{34}l_{31}}{(25+t_{GT})(h_{34}+h_{56})} \tag{1}\]  
where \(i_{sg}\) is the hydraulic gradient across the filter (the combination of 25 mm soil and geotextile thickness), and \(i_{s}\) is the reference hydraulic gradient in the soil measured in a region away from the geotextile filter (i.e., 50 mm soil layer). \(\Delta h_{sg}\) is the hydraulic head loss due to the geotextile filter, which is defined as the difference between the average hydraulic head values of No.3 and No.5 piezometers and the hydraulic head value of No.1 piezometers. \(\Delta h_{s}\) is the hydraulic head loss through the soil away from the filter layer, which is calculated by the average hydraulic values of the head difference between the No.3 and No.4 piezometers and those of No.5 and No.6 piezometers. \(l_{sg}\) and \(l_{s}\) are the seepage path corresponding to the hydraulic head loss, respectively.  
As suggested by [PERSON] et al. [3], [PERSON] and [PERSON] [28], and [PERSON] et al. [29], the permeability coefficient ratio \(k_{R}\) was also preferred to define the clogging conditions, shown as:  
\[k_{R}=k_{s}/k_{sg} \tag{2}\]  
Using the continuity of flow, the permeability of the soil \(k_{s}\) between ports 3 and 4 is related to the permeability of the soil-geotextile composite \(k_{sg}\):  
\[k_{s}i_{s}=k_{sg}i_{sg} \tag{3}\]  
Thus, the gradient ratio can also be expressed:  
\[GR=i_{sg}/i_{s}=k_{s}/k_{sg} \tag{4}\]The flow behavior of any given soil-geotextile system can be assessed using its flow curve, which depicts the hydraulic conductivity versus time. [PERSON]'s law is used to calculate the hydraulic conductivity \(k\) of the entire soil-geotextile system, as shown in Equation (5):  
\[k=\frac{Ql}{At\Delta h} \tag{5}\]  
where \(Q\) is the seepage flow quantity passing through the filter during time \(t\) (m\({}^{3}\)), \(A\) is the section area of the drainage surface (\(A\) = 7.85 \(\times\) 10\({}^{-5}\) m\({}^{2}\)) and \(\Delta h\) is the total hydraulic head loss through the whole specimen (m). \(l\) is the height of the soil sample causing the hydraulic head loss, which is 100 mm. \(t\) is the seepage time (\(t\) = 3600 s).  
The mass of fine particles retained inside geotextile fibers after the filtration tests for any given system was measured as the weight of fine particles per unit (retained) area (g/m\({}^{2}\)), which was calculated in a formula (6). For each system tested, the amount of fine particles retained inside the geotextile fibers is given in Table 5.  
\[\eta=\frac{m_{1}-m_{0}}{A} \tag{6}\]  
where \(\eta\) is the mass of entrapped soil particles inside geotextiles (g/m\({}^{2}\)), \(m_{0}\), \(m_{1}\) are the mass of geotextiles before and after the test (g).