## 1 Introduction  
Geotextiles have been widely used as a substitute for the granular filter to provide filtration and drainage functions in underground structure drainage systems due to their ease of availability and convenient construction [1; 2; 3; 4; 5; 6]. A geotextile filter should meet not only the soil retention and permeability criteria but also the anti-clogging criterion, because a filtration system may become clogged by fine particles trapped inside geotextile pores during its service life. Geotextiles placed horizontally between the base soil and the gravel layer can perform the filtration function in filtration engineering. That is, it effectively prevents soil particles from undue migration or loss, as well as excessive seepage deformation or even soil destruction. Simultaneously, the water in the soil is allowed to discharge smoothly to avoid soil instability caused by the increase in pore pressure. Geotextiles can also perform the filtration function when wrapping the surface of a blind ditch filled with gravel to form an edge drainage system. In general, the gravel blind ditch covered by geotextile is difficult to be repaired or replaced during the service period of active anti-floating engineering of underground structures. As a result, the primary concern for a geotextile used as a filter layer is its long-term performance in terms of permeability and clogging prevention. Early geotextile filter research primarily focused on the geotextile filtering granular soil [7; 8; 9; 10; 11]. The formation mechanism of the \"self-filter layer\" of geotextile filtering granular soil, also known as a natural filter layer, has been proposed based on experimental research and numerical simulation analysis [12; 13]. The mechanism is as follows: while the geotextile performs its filtration function, fine particles close to the filter layer are allowed to pass through the geotextile, forming a thin layer of arch composed of coarse-grained soil forms on the surface of the geotextile, i.e., the natural filter layer. However, such a \"self-filter layer\" in fine-grained soil with particle size less than 0.075 mm barely exists due to the cohesive connection between soil aggregates during the filtration process. [PERSON] et al. [14] investigated the filtration behavior of clayey sludge with nonwoven geotextiles and discovered that the working principle of geotextiles for filtering clayey sludge or fine particle suspensions was more complex than that for filtering granular soil suspension. In this case, the filtration mechanism of geotextile filtering granular soil may not be applicable to fine-grained soil. At the present, the gradient ratio (\(GR\)) filtration test is mainly adopted for the evaluation of the filtration characteristics of geotextiles [6; 9; 15; 16; 17; 18]. The gradient ratio filtration test calculates the hydraulic gradient of the corresponding area by measuring the water head difference between different positions of the sample under no stress action to determine the \(GR\) of the soil-geotextile system. \(GR\) value is the ratio of water head loss per unit seepage path of the soil-geotextile combination (25 mm thick soil layer plus geotextile thickness) to the water head loss per unit seepage path of the soil layer (50 mm thick). [PERSON] also stated that \(GR\) values can be used to assess the filtration compatibility of the soil-geotextile interface. When \(GR=1\), the hydraulic head loss through the soil sample and filter is uniform. \(GR<1\) indicates that soil particles are passing through the geotextile forming piping, whereas \(GR>1\) indicates that there is potential clogging in the geotextile's upstream or adjacent surface [19]. Therefore, the US Army Corps of Engineers [20] proposed that the \(GR\) of the soil-geotextile system should be less than 3 as the critical value for no clogging, as well as \(GR\leq 3\) was included in the filter criteria for geotextiles as a clogging resistance criterion.