## 2 Sampling and Methodology
### Characteristics of Loess Collapsibility  
#### 2.5.1 Characteristics of the Loess Collapsibility Curve in Various Regions  
Following the results of the indoor water-immersed compression tests, the wetting coefficients of the four groups under differing pressures were calculated individually (Figure 4). Through comparison and analysis of the collapsibility curve characteristics, it was observed that the variation trend in loess collapsibility with increasing overlying pressure can be broadly categorized into three stages: In the initial stage, the deformation  
Figure 3: Preservation of soil samples.  
of loess collapsibility undergoes significant changes and manifests rapid development with the increase of overburden pressure. Evidently, the loess undergoes compaction under the combined influence of external loading and moisture, leading to an escalation in damage deformation due to structural instability. During the second stage, a noticeable deceleration in the rate of collapsible deformation becomes apparent. This observation suggests that following the first stage, despite continued increments in external loading, the soil structure gradually densities and stabilizes. The third stage is exclusive to the Mianchi and Shanzhou areas, wherein the magnitude of collapsible deformation manifests a trend of continuous development. This phenomenon demonstrates that the loess in the Shanzhou and Mianchi regions is comparatively loose, offering ample space for structural disruption, whereas the loess in the Gongyi and Xingyang areas demonstrates greater density and stability. With the increase of overburden pressure, the wetting coefficient has a significant increase. The overburden pressure shows a good positive correlation with the wetting coefficient. This proves that the external load is an important factor affecting the soil subsidence, which means that the original macroporous structure of the soil is gradually destroyed under the action of load and the pore ratio decreases gradually.  
Specifically, the collapsibility coefficient in the Mianchi and Shanzhou areas exceeds 0.07 under a 200 kPa load, classifying it as a strongly collapsible loess. The initial collapse pressure of Malan loess in the four regions is different, that is, the minimum external pressure required for obvious collapse of loess is different. The initial collapse pressure of loess in Xingyang is greater than 75 kPa, which is 95 kPa and 126 kPa, respectively. The initial collapse pressure of loess in Mianchi area is 55 kPa, and the initial collapse pressure of loess in the Shanzhou area is less than 50 kPa. Notably, the collapsibility coefficient in the Shanzhou area reaches 0.15, indicating particularly high collapsibility. Conversely, the collapsibility coefficient of loess in the Gongyi and Xingyang areas hovers around 0.5, categorizing it as a moderately collapsible loess.  
The overall trend across the loess in the western part of the Henan Province indicates a gradual attenuation from northwest to southeast. To delve deeper into comprehending the patterns and mechanisms underlying the collapsibility of Malan loess in western Henan, it becomes imperative to investigate factors such as the initial water content, microstructural characteristics of the loess, and changes in microscopic quantitative parameters before and after soaking.  
Figure 4: The curve of collapsibility coefficient versus pressure in different areas.