## 4 Microstructure and Stress Transfer Mechanism of Compressed Soil
### Analysis of Stress Transfer Mechanism  
The soil skeleton is mainly composed of soil particles. Soil particles are products of rock weathering, and they mainly comprise quartz, feldspar, and other mineral particles, hardness, and rigidity. In the process of soil failure, due to the substantial rigidity of soil particles, the deformation and breakage degree of soil particles under stress are generally small. [PERSON] (2018) provided statistics on the size of soil particles before and after the failure of the sample, and their study observed that only 2% to 10% of soil particles were broken during the failure process of the sample. For the clay soil with small soil particles, the number of broken soil particles amounted to less than 5%. Therefore, it can be considered that soil particles themselves do not deform in the mechanical failure process of soil, and the deformation and failure of soil are results of the change in soil skeleton caused by the migration of soil particles under effective stress.  
In the process of soil compression, the height of the sample actually decreases continuously. Therefore, after soil particles come into contact with one another, the upper and lower soil particles inevitably invade the middle layer vertically. After the soil particles in the middle layer are invaded, they can only be extruded laterally to make room for invading soil particles. The movement of soil particles is shown in the figure below.  
Figure 16: Rose grain profile of each sampling point: (**a**) sample1-2; (**b**) sample2-2; (**c**) sample3-1; (**d**) sample4-1.  
It can be observed from Figure 17 that particle 1 at the top is subjected to pressure \(F_{N}\) under external loads. Under the action of \(F_{N}\), soil particles 1 and 2 in the top layer invade the soil of the second layer. During the invasion process, cementing force \(F_{t}\) and friction force tan \(\varphi\) between soil particles exhibit resistance to the invasion movement. As compaction continues and \(F_{N}\) increases, the following two motions occur between soil particles.  
1. If the mechanical property of the soil is relatively plastic and cementation force Ft still has a certain residual strength after a certain distance between the particles, the upper and lower soil particles will completely invade the middle soil particles via insertion such that the middle soil particles are extruded and retain their structure, resulting in local swelling (tension) failure within this area.
2. If the mechanical properties of the soil are brittle, when cementation force Ft reaches its tensile strength, the cementation of the soil particles will be completely broken, and the connection between 1-2, 3-4, and 2-5 will be broken. The upper soil particles invade the middle soil particles in the form of shear failure, resulting in local shear failure within this area.  
Therefore, when the soil is under compression deformation, the soil particles in the upper layer of the internal soil particles will invade the next soil layer in the form of insertion or shear failure, and the particles in the next soil layer will be squeezed out, resulting in local tensile failure or local shear failure.