# Characterization of the Migration of Soil Particles in Lateritic Soils under the Effect of Rainfall
## 4 Particle Migration within the Soil
### Spatial Structure Evolution of Soil Particles  
In this section, the spatial structure evolution of the soil parameter \(\mu\)-value under different rainfall intensities is established by sampling the soil after different rainfall calendar times. In order to clarify the spatial changes of the soil body after rainfall, the slope body corresponded to the sampling locations (i.e., U\({}_{1}\)\(\sim\)M\({}_{9}\)) of Figure 6 one by one in establishing the spatial structure evolution of soil particles. And the soil parameter \(\mu\) = 0.0328\(\sim\)0.0377 under the initial conditions was used as the basis for the division when the value of \(\mu\) less than 0.0328 was considered as the loss of fine particles and the value of \(\mu\) greater than 0.0377 was considered as the accumulation of fine particles. Such a division helps to further study the transport of fine particles in the soil body and their effects on the soil's physical properties. However, considering the different ranges of fine particle characterization parameters under different rainfall intensities, the division of fine particle loss and accumulation is slightly different, and the specific division is based on the color scale card under the spatial structure evolution map of soil parameter \(\mu\) value. Under rainfall infiltration conditions, the migration and aggregation of fine particles within a slope are influenced by the randomness and inhomogeneity of the pore distribution of the soil. Therefore, the aggregation and loss of fine particles within the slope occur randomly during the structural evolution of the soil particles. It is because of the existence of this randomness that the migration of fine particles within the slope is always difficult to capture. Therefore, this paper establishes the spatial structure evolution of the soil parameter \(\mu\) value under different rainfall intensities via the soil scale distribution parameter, and the spatial structure evolution of the soil particles of this lateritic soil is shown in Figures 8-10. Through the observation of different spatial structure evolution, it was found that obvious fine particle aggregation and loss phenomena were observed at the position of the toe of the slope in the spatial structure distribution maps of the slope body at the moments of 10 min, 20 min and 30 min in Figure 8; 10 min and 20 min in Figure 9; and 20 min and 30 min in Figure 10. This indicates that under rainfall infiltration, fine particles always accumulate at the toe of the slope and are lost there. In addition, the effects of rainfall duration and rainfall intensity on the migration and accumulation characteristics of fine particles are both controlling factors [27], where it is feasible to understand the early stage of rainfall as a comprehensive consideration of low rainfall intensity or short rainfall duration. At the early stage of rainfall, the aggregation of fine particles inside the slope in Figure 8 for 20 min and 40 min and in Figure 9 for 20 min is always in the form of banded downslope aggregation. With the increase in rainfall intensity or the extension of rainfall calendar time, the fine particles of the slope are mostly aggregated in the form of clusters at the toe of the slope or in the slope body. This suggests that in the early stage of rainfall, the fine particles first dominate vertical downward migration, leading to the aggregation of fine particles to deeper depths, forming a strip-like distribution downslope; this strip-like aggregation of fine particles is similar to a layer of water film with good lubricity found by previous scholars in their experiments and that the slope body will likely undergo shallow damage along this layer [9; 28]. Subsequently, the migration direction will have both vertical downward and downward along the slope due to the hydraulic gradient, after which the fine particles will gradually converge or be lost towards the toe of the slope. Figure 8: Evolution of spatial structure of soil particles (50 mm/h).