## 5 Test Results
### Displacement and Soil Retention  
Normal compressive stress can cause a thickness reduction in the soil layer and downward displacement in the clamped geotextile specimen, resulting in a denser state of the tested specimen. To evaluate the compression of the soil layer under normal stress, the soil-geotextile combination settlement and geotextile downward displacement should be measured simultaneously. Before the pre-test, the geotextile downward displacement corresponding to different normal stresses was calibrated separately by ASTM D5199-12 [33]. The new gradient ratio filtration test apparatus can measure the soil-geotextile combination settlement by protruding the probe of an LVDT through the porous plate.  
Figure 7 illustrates the downward displacement of geotextiles and the soil layer settlement of soil-geotextile combinations (C/M-GT) under various normal stress conditions. It was found that the soil layer settlement increased continuously with the increase of normal stress, while geotextile downward displacement increased initially and then decreased when the applied normal stress exceeded 10 kPa. This was possibly due to the partial clogging of the geotextile, which reduced its compressibility. In addition, the normal stress applied on the top of a 100 mm thick layer was unable to be fully transmitted to the geotextile [34]. Thus, soil layer settlement showed a sensible significant response to normal stress when compared to geotextile displacement. Furthermore, soil layer settlement or geotextile displacement was slightly greater in the soil-GT4 combinations (C/M-GT4) than in the soil-GT2 combinations (C/M-GT2). This was attributed to the thickness of tested geotextiles  
Figure 6: Filtration test results for silt with four geotextiles: (**a**) M-GT1 combination; (**b**) M-GT2 combination; (**c**) M-GT3 combination; (**d**) M-GT4 combination.  
as well as soil type. Hence, a certain normal stress could cause in-plane compression on the clamped geotextile specimen, and the compression was affected by the initial thickness and fine particle retention of the geotextile.  
The fine particles retained (\(\eta\)) of the four soil-geotextile combinations at different normal stresses are shown in Figure 8. The \(\eta\) values were closely related to the soil type and geotextile thickness. The fine particles retained (\(\eta\)) of the four soil-geotextile combinations increased with an increase in normal stress. When the normal stress was 0 kPa, the \(\eta\) values of C-GT2, M-GT2, C-GT4, and M-GT4 were 271 g/m\({}^{2}\), 465 g/m\({}^{2}\), 605 g/m\({}^{2}\), and 610 g/m\({}^{2}\), respectively. When the normal stress was increased to 25 kPa, the \(\eta\) values of C-GT2, M-GT2, C-GT4, and M-GT4 increased by 96%, 53.8%, 39.8%, and 53.9%, compared with  
Figure 7: Downward displacements for soil–geotextile combinations subjected to normal stresses: (a) C-GT2/GT4 combinations; (b) M-GT2/GT4 combinations.  
those free of normal stress. This was mainly because, when the normal stress increased, soil layer settlement increased, and the structure of fibers became denser, resulting in an increase in the \(\eta\) values. Comparing the results from GT2 and GT4 combinations, it was observed that for geotextile with larger thickness, more fine particles tended to be gathered. The mass of fine particles retained from filtered silt (internal instability) was clearly greater than that of silty clay in terms of geotextile combinations. These observations were consistent with the above results, i.e., the thicker geotextile had a larger \(GR\) and the \(k_{\text{sg}}\) of silt-geotextile combinations decreased significantly, especially at a normal stress of 25 kPa.