## 3 Results and Mechanism Analysis
### The Geological Origin of the Structural Differences between Undisturbed and Remolded Loess  
The structural differences between undisturbed and remolded loess are due to their different growth histories. Undisturbed loess has experienced a long geological history, in which countless drying/wetting cycles have occurred. This repeated history of increasing and decreasing moisture is bound to have an important impact on the distribution of clay particles, colloidal particles, and even soluble salts in loess. The microscopic mechanism of this process is presented in Figure 14. The \"adhesive bridge\" effect of cementation between loess skeleton particles is enhanced, and the surfaces of the mineral crystal particles of the skeleton, which forms the main part of the pore wall, are smoother, thereby weakening the viscous force of seepage through pores. For remolded loess samples, the distribution of clay particles returns to randomness (just like the random distribution of clay particles when loess is newly deposited) due to the screening, grinding, humidification, and other occurrences during the sample preparation process. Therefore, the distribution of clay particles around the skeleton minerals in remolded loess is not uniform, and the strength of the new \"adhesive bridge\" is relatively weak. On the other hand, due to the random distribution of the clay coat, the surfaces of the mineral crystals of the skeleton, which is the main component of the pore wall, are relatively rough, so they exert a strong viscous force on the seepage through the pores.  
Figure 13: SEM images of (**a**) undisturbed and (**b**) remolded loess and concept diagrams of particle contact.  
There are substantial differences between the structures of undisturbed and remolded loess samples. The reason for this is that undisturbed loess has experienced countless drying/wetting cycles during the process of soil formation after deposition. During these cycles, the redistribution of clay, colloidal particles, and soluble salt ions increases the effectiveness of the cementation between loess skeleton particles, thereby endowing the loess with a strong structure. However, remolded loess is characterized by the reduced effectiveness of cementation between skeleton particles, which causes its structure to be weaker than that of undisturbed loess.  
Clay migration also plays a very important role in the process of loess soil formation. To a certain extent, clay migration has two effects (Figure 15[50]). First, for particles with surface defects, the colloid clay component preferentially fills pores between the skeleton particles and the connected areas of finer particles via selective filling. Second, for flat and smooth particles, the colloidal components of clay tend to adsorb on the flat or concave surfaces of skeleton particles. The filling and adsorption effects can round the edges of loess skeleton particles, increase the particle size, slow the migration rate of clay particles, and improve the connection between skeleton particles and colloid clay components, thereby forming a more stable structure. These reasons explain why the cohesion and permeability coefficient of undisturbed loess are greater than those of remolded loess, which has been confirmed by many studies [51; 52]. The structural differences between undisturbed and remolded loess can also affect their permeability coefficients [53; 54], as shown in Figures 8-12.  
Figure 14: The conceptual model of the effect of drying/wetting cycles on the distribution of effective cementation between loess skeleton particles.