The following is the first draft of an academic paper. Please help me polish the language.
<3 Results
3.1 Critical hydraulic condition of BEP
In Test-1, no geotextile was installed to study the critical hydraulic gradient of BEP in selected sand, and this test was taken as a control group. The upstream water level was raised step by step, sand boiling began to occur when the average hydraulic gradient reached 0.84, with a sand boiling diameter of 2 cm. When the average hydraulic gradient increased to 1.04, the muddy water containing a large amount of sand particles rapidly gushed at the BEP outlet, resulting in a failure of the sand specimen (see Figure 4). In Test-2 to Test-5, geotextile filters were installed above the sand to study the filtration characteristics of geotextile in preventing BEP. To ensure that BEP can occur when there is no geotextile filter in Test-2 to Test-5, the average hydraulic gradient was set to 2.0. Table 4 provides the experimental results for Test-2 to Test-5, and the GR25 and flow rate were the final stable values.
The geotextile filter needs to satisfy the requirement of soil retention during usage process. Soil retention means that the pore size of the filter should match the particle size of the protected soil to prevent seepage deformation of the protected soil caused by the loss of skeleton particles. After the test, the sand particles that passed through the geotextile filter were collected, dried and weighed. The sand mass that passed through the geotextile in all tests was close to zero, indicating that all geotextile filter met the soil retention requirements. Based on GR value, flow rate of the filter, and distribution pattern of clogging materials, the clogging and permeability characteristics of geotextile filters during preventing BEP were analyzed in following part.
 
3.2 Influence of geotextile specifications on its filtration characteristics
3.2.1 Clogging analysis of geotextile filter.
Figure 6 shows the variation of GR value over time for different specifications nonwoven geotextile. The 300 g/m2 nonwoven geotextile system had the highest gradient ratio and changed little over time. Throughout the entire experiment, the GR25 of the 300 g/m2 system was consistently greater than 3.0, indicating significant clogging of the geotextile. The GR values of the 200 g/m2 and 400 g/m2 nonwoven geotextile systems gradually decreased over time, and they were similar at the end of the experiment, with GR25 both less than 3.0. This indicates that there were no severe clogging of these two geotextiles.
For Test-2 and Test-3, there was a temporary decrease in GR value during the initial stage of the experiment, mainly due to non-steady flow. In the later stage of the experiment, the GR value of the 300 g/m2 nonwoven geotextile system tended to stabilize, with a GR25 of 3.98 at 720 minute. The main reason for clogging of 300 g/m2 nonwoven geotextile was the small pore size of the filter, causing fine-grained sand to easily block on the surface of the geotextile or clog inside the geotextile. For the 200 g/m2 nonwoven geotextile, there was little variation in the GR value within 30 to 240 minutes, after which the GR value gradually decreased over time. This indicates the formation of a stable natural filter layer on the surface of the geotextile. The GR25 of 300 g/m2 nonwoven geotextile decreases to 2.60 at the end of the experiment, indicating no severe clogging compared to the 300 g/m2 nonwoven geotextile. This is mainly due to the larger pore size of the 200 g/m2 geotextile (O95 is 0.106 mm), which brings stronger anti-clogging ability.
For the 400 g/m2 nonwoven geotextile, there was a temporary increase in the GR value in the first 10 minutes, followed by a relatively constant GR value from 10 to 120 minutes. From 120 to 720 minutes, the GR value gradually decreased, with a GR25 of 2.38 at 720 minutes, lower than that of the 300 g/m2 nonwoven geotextile. Although the pore size of the 400 g/m2 geotextile was smaller than that of the 300 g/m2 geotextile, its thickness was greater. The thicker the geotextile, the more seepage paths it has. When a part of filtration paths of thick geotextile are blocked by soil particles, there are still abundant filtration paths for drainage. As a result, thicker geotextiles have stronger resistance to physical clogging17. The results show that larger pore size and greater thickness within a certain range contribute to a stronger resistance to the physical clogging of nonwoven geotextile.
 
3.2.2 Flow rate
To ensure the smooth discharge of seepage water, the geotextile filter needs to meet the water permeability during usage. Figure 7 shows the variation of flow rate over time for different specifications of nonwoven geotextile in the experiment. It is observed that the 400 g/m2 nonwoven geotextile had the lowest flow rate, and it was mainly due to its smallest equivalent pore size (0.047 mm) and permeability coefficient. Hence the pore size greatly affects the water permeability of the geotextile filter. In the early stage of the experiment, the flow rates of the 200 g/m2 and 300 g/m2 geotextiles were similar. After that, the flow rate of the 200 g/m2 geotextile was higher than that of the 300 g/m2 geotextile. 
The flow rates of the 200 g/m2 and 400 g/m2 nonwoven geotextiles changed little in the early stage of the experiment (0-120 minutes). After 120 minutes, both decreased gradually over time. The speculated reason is that in the initial stage of the experiment, the migration distance of fine sand particles under the action of seepage force was limited, resulting in a little impact on the permeability of the entire sand sample. As the experiment progressed, fine sand particles continued to migrate, causing localized compaction in the sand sample. At the same time, part of the fine-grained sand was blocked on the surface of the geotextile or was trapped inside the geotextile under the action of seepage force. All of these factors led to the reduction of permeability of the sand-geotextile filter system over time. At the end of the experiment, the flow rates for the 200 g/m2, 300 g/m2, and 400 g/m2 geotextile filter were 260 mL/min, 206 mL/min, and 192 mL/min, respectively. The flow rate of the 200 g/m2 geotextile was 35.4% higher than that of the 400 g/m2 geotextile. The larger the equivalent pore size of the nonwoven geotextile, the higher its drainage capacity. Compared to the 300 g/m2 and 400 g/m2 geotextiles, the filtration performance of the 200 g/m2 geotextile was better in preventing BEP.
 
Figure 7. Variation of flow rate of nonwoven geotextile filter over time>