## 1 Introduction
Soil microstructure is defined as the spatial arrangement of soil particles and pores [11]. [PERSON] et al. [12] used scanning electron microscopy (SEM) images to observe the microstructure and reported that the particle skeleton became progressively looser with \(N\) increasing. They also quantified pore characteristics from the images, revealing a greater development of pores in both number and diameter. In addition, [PERSON] et al. [13], [PERSON] et al. [14], [PERSON] et al. [15], and [PERSON] et al. [16] employed comparable methods and reached similar conclusions. These studies validate the feasibility of using SEM for qualitative observation of microstructure. However, in fact, the cleavage effect of wet-dry cycling at the micro level is not only confined to the microstructure but also extends to the mineral composition. Surprisingly, the impact on the mineral composition has rarely been investigated under wet-dry cycles in this context. Wet-dry cycles not only induce changes in the soil microstructure, but also lead to the formation of numerous mesoscale cracks that are visible to the naked eye. The initiation and propagation of these cracks progressively compromise the overall structural integrity of the soil. Investigating the formation and evolution of soil cracks is therefore essential. Currently, numerous studies have also investigated the crack propagation mechanisms under cyclic wetting and drying conditions. [PERSON] et al. [17] used digital photography and image processing to extract surface crack parameters under varying \(N\) and analyzed the patterns of crack development. The results indicate that cyclic wetting and drying induce the formation and propagation of cracks, with the mid-term stage of the cycle exerting a greater influence on crack propagation. In addition, [PERSON] et al. [18] and [PERSON] [19] utilized computed tomography (CT) to extract crack parameters and provided a new method for crack analysis. Currently, the mainstream approach for extracting crack characteristics involves acquiring crack images using digital photography, followed by the extraction of characteristic parameters through image processing [20; 21]. At the macroscale, shear strength indices represent critical parameters in slope stability analysis. [PERSON] et al. [22] and [PERSON] et al. [23] employed different testing methods to investigate the mechanical properties of engineering materials, and the results demonstrated that mechanical strength deteriorates progressively with an increasing \(N\). In addition, numerous scholars have discovered that wet-dry cycles exert an influence on compressive strength [24], fracture toughness [25], and permeability [26]. In summary, the following methods have been used to study the deterioration characteristics of soils under wet-dry cycles: (i) damage assessment at the expansion and contraction properties, based on axial strain; (ii) damage assessment at the microscale, based on microstructural characteristics and pore properties; (iii) damage assessment at the mesoscale, based on the analysis of crack parameters; (iv) damage assessment at the macroscale, based on experimentally derived physical parameters. On the one hand, numerous scholars tend to concentrate exclusively on one or two aspects in their research, resulting in a limited research perspective, a dearth of multi-scale investigations, and inadequate attention to the degradation mechanisms influenced by alterations in mineral composition. On the other hand, weak correlations exist among the test results acquired at different scales, and a direct linkage mechanism to bridge test results across scales is absent. Although the above studies have enhanced and advanced methods for evaluating the deterioration characteristics of engineering materials, most studies continue to focus on common clay specimens, and the research on the deterioration characteristics and mechanisms of sandy red clay under wet-dry cycle conditions remains limited and lacks comprehensiveness.