## 2 Sampling and Methodology
### Image Analysis of Loess Microstructure in Different Regions  
Figure 7 depicts the SEM images of loess samples from all four regions before and after immersion in water. The microstructure of the loess before and after soaking in the same area was compared. It was found that the number of macropores in the soil samples decreased sharply after immersion. The connection mode of the soil particles changes and the connection of particles after immersion gradually changes from point-point contact to point-surface contact or surface-surface contact. At the same time, the arrangement of the soil particles also changed. Before collapse, the soil samples were mainly overhead pores. After the occurrence of collapsibility, although the overhead pores exist in a small amount, the intergranular pores increase sharply. Comparative analysis of the microstructure of the loess in these regions reveals variations in the quantity of pores, pore distribution, and pore connectivity. Notably, the number of macropores and overhead pores in Malan loess diminishes gradually from west to east in western Henan, accompanied by a denser arrangement of loess particles in the eastern regions. Furthermore, the connection between loess skeleton particles in the Mianchi and Shanzhou areas primarily consists of point contact or point-surface contact, whereas surface contact predominates in the Gongyi and Xingyang loess. The morphology of particles within the microstructure of the loess varies across different regions. In the Shanzhou area, the microstructure reveals predominantly granular soil particles, interspersed with clusters formed by a small quantity of fine particles. Similarly, across western Henan, the particles of loess remain predominantly granular from west to east, albeit with a gradual emergence of agglomerated particles. For instance, in Xingyang loess (Figure 7g,h), the morphology of skeleton particles tends to exhibit a smoother profile, with fewer aggregates formed through aggregation, compared to loess in the western regions. Additionally, the number of intergranular pores in the Xingyang and Gongyi loess is notably lesser than that observed in the Mianchi and Shanzhou loess. In summary, the pore size has a great influence on the collapsibility of loess, which is basically consistent with the conclusion of loess collapsibility and pore size in the Jinzhong area studied by [PERSON] (2018).  
Based on the SEM image, it can be inferred that as the natural water content increases, water immersion induces a reduction in the connection strength among skeleton particles, resulting in the destabilization and disintegration of the overhead pore structure within the loess. At the same time, under the influence of upper soil pressure, soil compaction occurs, leading to an augmentation in structural strength and consequently, a diminishing trend in collapsibility across western Henan from west to east. The mechanism underlying the collapsibility change of Malan loess can be summarized as follows: In the Mianchi and Shanzhou areas, Malan loess primarily consists of granular particles, rendering it susceptible to damage. The presence of large pores and overhead pores facilitates collapsibility, while the connection mode of skeleton particles is significantly impacted by external factors. Conversely, in the Xingyang and Gongyi areas, the loess skeleton particles exhibit relatively denser characteristics, with fewer large pores and overhead pores. Moreover, the particle connection mode has been compromised by water immersion and is less susceptible to external factors.