## 1 Introduction  
China harbors the most extensive loess deposits globally, spanning geographically from west to east across a northwestern dry inland basin, a central loess plateau, and an eastern region characterized by hills and plains. The loess deposits in western Henan Province, situated within the upper and lower reaches of the Yellow River, are notable for their widespread distribution, significant outcrop thickness, and a well-defined evolutionary sequence of loess-paleosol strata. This area not only holds important meaning in studying the formation and evolution of the Yellow River, but also serves as a focus for investigating paleoclimate changes since the Quaternary [1; 2]. During the period spanning 17-23 July 2021, Henan Province experienced an unprecedented historic heavy rainstorm. This event precipitated urban waterlogging, river floods, and landslides in various cities, leading to substantial casualties and property damage. Notably, locations such as Luoyang witnessed numerous incidents of road collapses and structural damage to houses due to the collapsible nature of loess, significantly endangering lives and property [3]. After the loess is soaked in water under a certain pressure, the structure of the loess is destroyed rapidly and additional subsidence is produced, which is called collapsibility. Loess shows robust stability under low water content conditions. However, the suction and bond strengths of the loess in question are subject to alteration upon encountering moisture and external loading, which in turn results in an increase of collapsibility [4; 5; 6; 7; 8; 9; 10]. Concurrently, the collapsibility of loess often triggers a cascade of geological hazards, including collapses and landslides, posing severe threats to human lives and property safety. Therefore, the investigation of loess collapsibility has remained a focal point for researchers in the field of geotechnical engineering [11]. Since the 1960s, the continual advancement and refinement of observation techniques, such as scanning electron microscopy (SEM) and computer-based research methodologies, have significantly improved researchers' understanding of the microstructural intricacies of loess. In particular, insights into the pore distribution and soil particle connectivity within loess have been deepened [12; 13; 14]. For example, [PERSON] conducted a study utilizing SEM imaging to investigate the variation in pore area during loess collapsibility, successfully establishing a logarithmic relationship between pore area and collapsibility coefficient [15]. [PERSON] examined changes in particle morphology, pore size, and pore area throughout the process of loess collapsibility, proposing a comprehensive mechanism for loess collapsibility that integrates relevant parameters and material composition [16; 17; 18]. Several researchers have studied the internal mechanisms underlying loess collapsibility by developing quantitative models of its three-dimensional microstructure [19; 20; 21; 22; 23]. [PERSON] and [PERSON] investigated the microstructural changes in loess before and after collapsibility through SEM tests, revealing a linear relationship between pore area and collapsibility [24; 25]. [PERSON] employed SEM testing to compare the microstructure of loess in its natural and remolded states, highlighting alterations in the connectivity mode between soil particles in remolded loess [26]. The microstructure of loess evolves during its deposition and subsequent soil formation processes. This evolution is influenced by various geological environmental conditions and the geological history of the region. Consequently, the microstructural characteristics of loess exhibit considerable variation across different layers and regions. Previous studies have extensively employed scanning electron microscopy and image processing technology to examine undisturbed loess samples. For example, [PERSON] et al. utilized serial sectioning methods to investigate the pore size distribution of loess across distinct regions of the Loess Plateau [27].