# Characterization of the Migration of Soil Particles in Lateritic Soils under the Effect of Rainfall
## 4 Particle Migration within the Soil
### Fine Particle Migration Mechanism  
Under the action of rainfall, fine particles within the soil are transported between the large granular skeleton via percolation. This transport consists of two aspects: one is the transport of fine particles between the large granular skeleton, and the other is the loss that occurs with water. Inside the slope, fine particles tend to accumulate at the smallest pore size, leading to changes in pore water pressure and water content, which, in turn, affects the stability of the slope.  
In the rainfall process, part of the fine particles in the early stage of rainfall are lost in the form of slope runoff, and the other part is infiltrated with water. This part of the infiltration of fine particles in the early part of the vertical infiltration will form a layer of fine particles in the shallow layer of the slope body aggregation band. The fine particle aggregation band of low shear strength may cause shallow damage to the slope body. For continued rainfall, i.e., fine particles in the slope body continue to migrate, at this time, the migration of fine particles via the unevenness of the pore space in the slope body and the randomness of the influence of the fine particles has become complex. But, in general, the fine particles migrate toward the foot of the slope. This also leads to an increase in the content of fine particles at the foot of the slope; the greater the content of fine particles, the larger the specific surface area of soil particles. Therefore, the water content is greater. The increase in the content of fine particles at the foot of the slope leads to changes in the foot of the slope spatial structure and pore size, reduced soil permeability, and impeded water seepage, resulting in pore water pressure increases. Therefore, the accumulation of fine particles to form a water film with good lubrication and the increase in the pore water pressure at the foot of the slope may lead to the destruction of the slope from the foot of the slope.  
Inside the slope, the fine particles of the soil are transported via seepage through the pore space formed by the coarse skeleton, which is controlled by the connectivity and heterogeneity of the pore space. When the fine particles are transported to the smallest aperture and gathered here, but as the content of fine particles increases, the hydrodynamic strengthening of the smallest aperture will be opened, and the aggregated seepage carries the fine particles to continue to migrate as a \"dam bursting\" type, resulting in a precipitous reduction in the water content of the place. The expansion of the channel or the loss of fine particles will inevitably lead to the pore size and then the seepage to fill the pore so that the  
Figure 14: The variation of internal seepage sand transport rate with rainfall time.  
pore water pressure gradually increases. It should be noted that the increase in this type of pore water pressure has a certain lag because seepage to fill the pore needs some time. In addition, such temporarily opened seepage channels are unstable and may collapse during sudden changes in water content, which will lead to a more complex change in pore water pressure, but the overall growth trend is maintained because the loss of fine particles inevitably leads to an increase in pore size. The migration process of fine particles within the slope is shown in Figure 15.  
Figure 15: Schematic diagram of fine particle migration: (**a**) in the early stage of rainfall, fine particles migrate vertically; (**b**) during the middle period of rainfall, fine particles randomly aggregate within the slope, resulting in the phenomenon of “coarsening” of the slope surface; (**c**) in the later stage of rainfall, fine particles accumulate at the foot of the slope and are lost in large quantities.