## 5 Mechanism and Discussion  
When saturated soil is freezing, the phase variation of pore water would cause volume expansion which generates wedge force to increase the distance of soil particles [18]. Due to the capillary force and van der Waals force, the capillary water and film water in soil are less likely to be frozen than gravity water. The smaller the pores, the greater the proportion of capillary water and film water in these pores and the less likely they are to be frozen [19]. This induces the large pores near the freezing front to freeze first. With the continuous expansion of the freezing front, the medium pores and small pores nearby are frozen successively [20].  
The soil of silt not only contains a large amount of silt particles (5-75 \(\upmu\)m), but also contains some clay particles (<5 \(\upmu\)m) and sand particles (>75 \(\upmu\)m) (Figure 22a). Clay particles are smaller in size, larger in specific heat capacity, more spherical in shape, and the diameters of their nearby pores are small. Due to the small particle size, small diameter of nearby pores, and large specific heat capacity, pore water surrounding clay particles are less likely to be frozen under the same conditions than silt particles and sand particles. During the freezing process, the wedge force formed by the growth of ice crystals first acts on sand particles and silt particles, causing more frost heave displacements for large and medium particles, and increasing the diameter of nearby pores [21; 22]. This exacerbates the possibility of mutual attraction and aggregation for clay particles (Figure 22b). Because of their small diameter and spherical shape of clay particles, the wedge force of ice crystal growth does not easily impact the clay particles themselves, so the shape variations of clay particles are not obvious. Due to the large particle size and irregular shape of silt particles and sand particles, the wedge force of ice crystal growth is more likely to make their structure looser and induces irregular boundary collapse of particles. Hence, it produces the phenomenon of silt particles and sand particles becoming broken, the total particle arrangement becoming messy, and the total particle shape becoming spherical (Figure 22c). Lowering the freezing temperature could cause the wedge force for ice crystal growth  
Figure 21: The effect of fine content on the total particle shape.  
to become greater. Increasing the effective confining pressure has the same impact as lowering the freezing temperature. The higher the compaction degree, the greater the friction between soil particles, and soil particles are more likely to break rather than slip. Therefore, greater effective confining pressure, lower freezing temperature, and greater compactness could all exacerbate the impact of freeze-thaw action on total particle size and total particle shape to a certain extent. The result would be that freeze-thaw action has greater effects on total particles characteristics of silt with greater effective confining pressure, lower freezing temperature, and greater compaction degree.  
The content of clay particles in silty sand is very small, and total particle size is much larger than that of silt. Because of the small content of clay particles, SEM could not clearly observe the flocculent structure in silty sand after freeze-thaw, which only shows the characteristics of particle rearrangement and particle fragmentation (Figure 22e,f). The effects of freeze-thaw action on silt particles and sand particles in silty sand is similar to that in silt. Because the total particle size of silty sand is larger, the ability of individual particles to resist crushing deformation is stronger. Therefore, the effects of freeze-thaw action on silty sand is similar to that of silt, but the influence degree is smaller. The main distinction is the role of fine aggregate, especially for clay particles.