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<2 Materials and methodology
2.1 Apparatus
The experiment was conducted in an improved GR apparatus, as shown in Figure 1. The apparatus is divided into three parts: upper, middle, and lower, which are connected by flanges and bolts, and the flanges are sealed with rubber pads. The apparatus can accommodate soil samples with a height of 100 mm and a diameter of 100 mm, and the soil sample in the apparatus is used to simulate the foundation soil at the location of BEP water outlet. The flow direction of the water is vertically upward, and the opening in the upper part of the apparatus are used to simulate the BEP outlet. To protect the sand sample from being eroded by the inflow water, a perforated plate and a layer of nylon mesh (with a pore size of 0.12 mm) are placed at the bottom of the sand sample. The geotextile is placed on the top of the sand sample to simulate the process of filtration of geotextile preventing BEP in engineering sites, and the edges of the geotextile are fixed with flange plates. There are five pressure measurement ports distributed on the side of the apparatus to measure the water head at different positions of the sample. The distance between the center of port #5 and the geotextile is 3 mm, and this port is more sensitive to the changes at the soil-geotextile filter interface6,14. The port #5 can capture the interaction between the protective soil and the geotextile filter. The water inlet is connected to the overflow system, which can provide a constant water head for the test. The average hydraulic gradient of the system can be adjusted by changing the water head of the water inlet.

2.2 Materials
Punched nonwoven geotextiles with short fiber and woven geotextiles were selected in this study(Figure 2), which are commonly used in engineering. The detailed parameters are shown in Table 1, and the equivalent opening size (O95) of the geotextile was measured by the bubble point method. The material of nonwoven geotextiles is polyester synthetic fiber. The unit area masses of the three specifications of nonwoven geotextiles are 200 g/m2, 300 g/m2, and 400 g/m2, respectively, and the corresponding O95 are 0.106 mm, 0.051 mm, and 0.047 mm, respectively. The woven geotextile is a flat wire woven geotextile, with material of polypropylene fiber and unit area mass of 200 g/m2, and the O95 is 0.078 mm.

The sand used in the experiment is medium sand, which is a typical sand for foundation of the Yangtze River dike. The specific gravity of the sand is 2.52, and the grading curve is shown in Figure 3. According to the interior stability criterion of Kenney and Lau15, the sand was classified as internally stable. The characteristic particle size of the sand sample is shown in Table 2, with a maximum dry density of 1.82 g/cm3, a minimum dry density of 1.57 g/cm3. In the test, the relative density of the sand was 26.8%, which was in a loose state to simulating the actual situation. The corresponding permeability coefficient at the fill density is 2.33×10-3 cm/s.

2.3 Experimental design
Five sets of experiments were designed to study the influence of geotextile specifications and weaving types on the filtration characteristics of geotextile filters in emergency rescue of dike BEP. Table 3 provides detailed information about the experimental design, and three different specifications of nonwoven geotextiles and one woven geotextile were selected. Test-1 is a control group without geotextile laid at the sand sample. The upstream water head was gradually raised in Test-1 to determine the critical hydraulic gradient of BEP and this hydraulic gradient would be applied in subsequent tests. In Test-2 to Test-5, geotextile filters were placed above the sand sample to study the filtration characteristics of geotextiles. Test-2, Test-3, and Test-4 aim to investigate the influence of nonwoven geotextile specifications on their filtration characteristics, while Test-2 and Test-5 aim to study the influence of geotextile types on their filtration characteristics. The average hydraulic gradient in Test-2 to Test-5 was set to 2.0, and the water head was raised to the design height at one time. In order to improve the reliability of the experimental results, parallel tests were conducted from Test-2 to Test-4.>