# Study on Disintegration and Infiltration Characteristics of Nanjing Jiangxinzhou Embankment Soil
## 4 Discussion  
### Soil Disintegration  
The test microstructure under different dry density degrees was observed by electron microscope scanning. The results are shown in Figure 12. With the increase in dry density degree, the number of soil particles per unit space increased, the number of pores decreased, and the structure was more compact. Therefore, after the same amount of time of water immersion, the position of the internal wetting peak of the sample with significant dry density was farther from the center of the sample. Second, increased capillary forces and substrate suction due to increased dry density, increasing the internal potential energy difference, will impede the water infiltration rate. The sample's resistance to water infiltration and the pace of disintegration increased with the magnitude of the potential energy differential. As can be seen from the mass time curve, the lower the test moisture content, the greater the percentage mass increment. The reason is that due to the existence of matrix suction, the lower the water content, the higher the matrix suction, the more significant the potential energy difference that exists, and the greater the suction of the sample's interior to external moisture. As the water content increased, the sample matrix suction decreased, weakening the water absorption capacity. With the increase in disintegration time, the matrix suction continued to decrease. When a critical value was reached, the \"restraining\" force of the potential energy difference on the sample almost disappeared, and the sample underwent a continuous flaking phenomenon. Second, the sample changed from its solid form to its plastic condition after being submerged in water. The sample remained plastic for a while at the beginning of water immersion. With the increase in water immersion time, the samples softened continuously. When the water content of the test reached a particular value, there was almost no cementing ability between the soil particles in the sample. The softened sample wedged into the metal mesh under gravity, causing cracking and sudden disintegration. The disintegration pattern of the samples under different water content conditions is shown in Figure 13.  
Figure 12: SEM results of the test at different dry density levels. (a) 1.4 g/cm\({}^{3}\); (b) 1.5 g/cm\({}^{3}\); (c) 1.6 g/cm\({}^{3}\). According to the plasticity index, a water membrane has been adsorbed on the surface of the clay particles. This water membrane is free, weakly bound, and firmly bonded water. Clay particles, therefore, have high hydrophilicity. The clay particles adsorb the nearby water molecules during the sample disintegration, expanding their surfaces. The volume increases, the gravitational force weakens, and the cohesive strength decreases so that the clay structure is dispersed and the clay disintegrates. [PERSON] [27] and [PERSON] [28] proposed the diffusive bilayer theory in the early 20 th century. It divides the hydrated ions into two layers: the adsorption layer (very thin, usually consisting of several layers of water molecules, also known as the firmly bound water layer) and the diffuse layer (thicker, with variable thickness, also known as the weakly bound water layer), as shown in Figure 14.  
Two critical swelling stages comprise the entire disintegration process. The surface hydration swelling stage came first. It involved the close-range interaction of clay particles and water. At this point, water adsorbed onto clay particles, the water film was thicker, and the clay interlayer was thicker. The forces currently at work were the van der Waals force, the electrostatic gravitational force between the positive and negative charges of the layers, and the hydration energy between the water molecules and the layers. The next stage was the infiltration hydration swelling stage. Surface adsorption was no longer significant as the distance between clay layers increased. The double electric layer repulsion and osmotic pressure were the dominating forces.