## 3 Test Preparation
### Apparatus  
The current study developed a new gradient ratio filtration test apparatus to evaluate the clogging potential of a fine-grained soil-geotextile system that could apply normal compressive stress to the soil layer through an overlying porous plate. A schematic diagram and an image of the experimental apparatus are presented in Figures 3 and 4. It mainly consists of a pneumatic pressure device, a permeameter chamber and a water supply overflow system. The permeameter chamber is made up of two 100 mm internal diameter and 120 mm outer diameter acrylic cylinder sections, as well as a clamp specimen, mounted between two cylinder sections. A clamp made of two stainless steel flanges with an internal diameter of 100 mm is employed to secure the geotextile specimen. On the permeameter chamber's sidewall, there are seven nanometer ports. Each port is fitted with a screen to prevent fine particles from clogging it. In addition, manometer ports connected by glass tubes are used to monitor the water head distribution at different locations. Ports 3 and 5, as well as ports 4 and 6, are symmetrically distributed to ensure sample homogeneity and improve measurement accuracy. Ports 1 and 7 are each 25 mm beneath the geotextile and 25 mm above the top soil layer. Port 2 is 3 mm from the top surface of the geotextile, and the clogging potential of the geotextile could be determined by observing the water head change of the glass tubes 1 and 2.  
The pneumatic pressure device includes a cylinder, loading rod, air compressor, pressure-regulating valve and pressure gauge. The pressure-regulating valve is adjusted to simulate the normal compressive stress applied to the soil-geotextile system. The pressure sensor and the cylinder output end then display the stress in real-time. The linear variable differential transformer (LVDT) is mounted on the top cover of the permeameter chamber and is linked to the loading rod. The vertical displacement change of the soil-geotextile system is recorded when the applied normal compressive stress is stable. Upper and lower acrylic cylinders in the permeameter chamber allow the clamped geotextile specimen to be inserted between a 100 mm soil layer and a layer of stainless steel beads. The steel beads, 16 mm in diameter, are placed above the perforated plate and beneath the geotextile specimen to simulate a drainage layer and to support the soil-geotextile system. The lower acrylic cylinder section, measuring 100 mm in height, is fixed on the base through the flange. The stainless steel beads and the perforated plate are housed in this section tosupport the test geotextile and drain seepage water. The upper acrylic cylinder section, 200 mm in height, contains the test soil as well as a perforated plate placed on top of the soil to distribute the applied normal compressive stresses. A top plate is attached to the top of the acrylic cylinder section, leaving holes for water inlets, a vent valve, and a loading piston that enters the permeameter chamber. The entire assembly is held together by six steel rods: the base, two acrylic cylinder sections, clamped geotextile, and the top plate. To seal off the water flow, O-rings are placed in the flange groove connecting the upper and lower acrylic cylinders, as well as between the upper acrylic cylinder and the top plate. To ensure that the opening area for water flow is the same for each test, the stainless steel beads are arranged in a specific pattern. Therefore, in this test series, the contact area between the geotextile and drainage layer will not be a variable for seepage flow [10].  
Figure 3: Schematic of a new gradient ratio device for applying normal stress: (**a**) diagram of normal stress application; (**b**) details of gradient ratio permeameter.