The following is the first draft of an academic paper. Please help me polish the language.
<2 Materials and methodology
2.2 Experimental materials
The sand used in this paper is fine sand, which is a typical foundation soil for the Yangtze River dike. The specific gravity of the sand is 2.52, and the particle grading curve is shown in Figure 2. The sand is an internally stable soil based on Kenney and Lau's internal stability evaluation criteria [19]. The characteristics of the sand sample is shown in Table 1, with a maximum dry density of 1.82 g/cm3 and a minimum dry density of 1.57 g/cm3.
Table 1 Parameters of sand samples
2.3 Experimental scheme
To study the 3D geometric characteristics of the pipe, three evolution experiments of BEP were conducted. Table 2 provides a detailed experimental information. Test-1 and Test-2 aim to investigate the influence of the roughness of cover plate on the 3D geometry of the pipe. In Test-1, the sand layer is in contact with a smooth acrylic plate which is common test configurations. While in Test-2, the sand layer is in contact with a soft silicone plate, and an acrylic plate is covered on top of a silicone plate. Silicone plates can simulate the contact between the sand layer and the cohesive blanket layer more realistically. The purpose of Test-2 and Test-3 is to investigate the effect of relative density (RD) of sand layer on the geometry of the pipe. The RD of the sand layer in Test-2 and Test-3 tests was 59.6% and 74.9%, respectively, in a medium and dense state. The permeability coefficients of sand in Test-2 and Test-3 are 2.33×10-3 cm/s and 2.38×10-2 cm/s, respectively.
2.4 Experimental procedure
(1) preparation  of the sand sample
The model box was placed horizontally, and sand samples were filled in layers underwater to ensure saturation of the samples. The sand sample was filled in three layers, with the first two layers each having a thickness of 5 cm, and the last layer having a thickness of 5.5 cm. After the filling process, a transparent silicone plate or acrylic cover plate were placed on the top of the sand sample to simulate cohesive blanket, and the cover plate is fixed by bolts. Afterwards video recording equipment was installed above the model box. Water inlet was opened and the upstream water level was slowly raise. After the water level in the model box reached the outlet height, the water inlet was closed and the sand sample was is kept stationary for a period of time. After the pore pressure measured by the piezometers at different positions of the model box is consistent, the test was started.
(2) Test procedure
The upstream water head was gradually increased to conduct the test. The water head was raised by 2 cm each time before the sand boils, and the height of the water head was raised by 1 cm each step after the sand boils. After raising the water head, the phenomenon of BEP was observed. When the seepage deformation stabilized, various parameters of the system was measured and the upstream water head was further increased. The criterion for the stability of seepage deformation is that the pore water pressure at different positions of the model box was stable and the pipe no longer developed. The flow rate was measured by a measuring cylinder within a given time period (usually 1 minute, depending on the flow rate). If a pipe was formed, the maximum length and width of the pipe were measured at every stable stage. After the measurement of different parameters, the upstream water head was increased to next level, and the pipe gradually moved towards the upstream direction.
After the pipe reached to the inlet chamber, the test was immediately stopped by closing the water inlet. Then the water inside the model box was discharged by the water inlet, and the gypsum in flowing state was poured into the pipe through the outlet hole in the cover plate. After the gypsum solidified, a 3D gypsum model of the pipe was obtained and was subjected to 3D laser scanning. The model of the 3D scanner is EinScan Pro 2X, with a scanning accuracy of 0.1mm. The scanning speed is 30 frames per second, and the depth of field is ± 100mm. Due to the embedding of some sand particles on the surface of gypsum model, the size of the pipe was 1-2 particle sizes larger than the actual size.
(3) Analysis of 3D morphological characteristics of the pipe
As the pipe did not develop along a straight line but rather advanced in meandering way, the cumulative length of the medial axis of the pipe was taken as the length of the pipe, and the center of the outlet hole was taken as the starting point for calculating the pipe length. The cross-section of perpendicular to the pipe axis was selected as the calculation section, and the spacing between two calculated sections was 10 mm. The average width and average depth of the pipe cross-section was calculated. For each cross-section of the pipe, 8-10 calculation points with the same spacing were selected along the horizontal direction, and the average depth of all calculation points was taken as the average depth of the cross-section. Three calculation points with the same spacing were selected along the vertical direction of the section, and the average pipe width at the three calculation points was taken as the average width of the section.>