# Characterization of the Migration of Soil Particles in Lateritic Soils under the Effect of Rainfall
## 4 Particle Migration within the Soil
### Soil Pore Water Pressure and Water Content
#### 4.2.3 Dynamic Change Mechanism Analysis  
In the process of rainfall infiltration, the migration and aggregation of fine particles in the soil body will change the particle size distribution of the soil body, thus causing changes in the pore water pressure and volumetric water content of the soil body. Therefore, the changes in pore water pressure and water content in the slope body can be analyzed from the aggregation and loss of fine particles. The spatial structure evolution of soil particles provides information on the migration and aggregation of fine particles in the slope body. Figure 12 provides the changes in pore water pressure and water content. In order to better link the migration of fine particles with the changes in pore water pressure and water content, the locations of sudden changes in volumetric water content in the soil body were selected for the analysis of the mechanism of dynamic changes in pore water pressure and water content, i.e., the location of sensor No. 3 of the slope body at a rainfall intensity of 100 mm/h and the location of sensor No. 2 of the slope body at a rainfall intensity of 150 mm/h. In Figure 9, the spatial structure map of the soil after 10 min of rainfall shows fine particle loss at the location of sensor No. 3 and aggregation at 20 min. The volumetric water content at this location suddenly changed, decreased at 7 min, and increased again at 11 min, while the pore water pressure fluctuated after 2-3 min of volumetric water content change. Therefore, combining the changes in the fine particle content, volumetric water content, and pore water pressure in position 3, when the rainfall intensity is 100.32 mm/h, the slope is in the infiltration stage from 0-7 min, so the volumetric water content and pore water pressure rise in position 3, but the pore water pressure rises slowly. At 7 min, the volumetric water content decreases suddenly, which indicates that there is a sudden loss of fine particles at this point in time in position 3, and the sudden loss of fine particles. The sudden loss of fine particles inevitably led to the increase in the pore space of the soil body here. Although the pore space increased, it took time for the pore space to be filled with water, so the pore water pressure rose again only after 2-3 min. As the rainfall continues, the seepage within the slope does not stop, so the pores within the slope will be supplemented by fine particles, and even the expansion of the pore space leads to the collapse of large particles, so the volumetric water content decreases to a certain extent and will rise again, and pore water pressure fluctuates due to the sudden change in the pore space. Therefore, the sudden loss of fine particles at 7 min and the slow accumulation after 12 min make position No. 3 of the fine particle content distribution map after 10 min of rainfall show the loss of fine particles and the aggregation of fine particles at 20 min. When the rainfall intensity was 150 mm/h, the volumetric water content at the No. 2 sensor location of the slope body suddenly decreased at 12 min, followed by a sharp increase, while the pore water pressure went through a process of sudden increase and then gradual decrease. As shown in Figure 10, the soil spatial structure map of the soil body after 10 min rainfall of the No. 2 sensor location shows the aggregation of fine particles, and 20 min shows the loss. It indicates the loss of fine particles in 10-20 min at the No. 2 sensor location. And the sudden decrease of water content after 12 min indicates that the sudden loss of fine particles occurred after 12 min of rainfall, which led to the decrease in water content and the increase in pore space in the soil. With the continuous infiltration of rainfall, the pore space was gradually filled with water, resulting in an increase in pore water pressure.