# Development Characteristics and Mechanism of Crack in Expansive Soil under Wet-Dry Cycling
## 1 Introduction  
As a distinctive type of soil predominantly composed of clay minerals, expansive soil exhibits pronounced expansibility and strong hydrophilicity. Its volume deformation is highly sensitive to changes in moisture content [1; 2; 3]. Under the influence of wet-dry cycling, the moisture content in different depth zones of an expansive soil slope becomes unevenly distributed, leading to varying degrees of volume deformation and the development of cracks in shallow regions [4]. Subsequent rainfall further deepens these cracks, ultimately forming a complex network of cracks [5]. This adversely affects the slope's infiltration capacity, compromises soil structure integrity, and ultimately diminishes slope stability [6; 7]. Geological survey data indicate variations in initial moisture content, compaction degree, and thickness across different regions of the same slope surface due to the influence of slope structures and geometric shapes [5]. Consequently, even under identical rainfall conditions, significant differences in crack development can be expected among different regions of the expansive soil slope, leading to localized instability and deformation.  
Current scholarly discourse on the mechanisms governing the evolution of cracks in expansive soils is predominantly centered on the frequency of wet-dry cycles [8; 9; 10; 11; 12]. Empirical evidence suggests a correlation between an increased number of these cycles and a more pronounced progression of soil cracking. Nonetheless, the existing body of research is characterized by a dearth of quantitative analyses pertaining to the developmental traits of these cracks. Specifically, there is a notable absence of detailed, quantitative assessments examining how intrinsic soil properties [13; 14; 15; 16]--such as initial moisture levels, compaction, thickness [17], and soil density--affect the measurable progression of cracking. The majority of existing studies have isolated single factors for investigation, overlooking the fact that the evolution of soil cracks in expansive soils is a multifaceted phenomenon influenced by a confluence of factors. The current state of research, particularly in the context of wet-dry cycling, is superficial with respect to the multifactorial influences on crack evolution, and is further hampered by a paucity of theoretical underpinning. Consequently, a nuanced exploration of how cracks in expansive soils develop under the influence of wet-dry cycles is imperative. Such an inquiry should aim to elucidate the multifactorial dynamics of crack evolution and uncover the underlying mechanisms [18; 19; 20; 21; 22; 23; 24; 25].  
The study focuses on the reshaping of expansive soil in the Guangxi Zhuang Autonomous Region. Indoor basic physical property tests assist us in obtaining the soil's basic performance indicators and conducting quantitative descriptions of crack development characteristics through digital image analysis processing techniques. Secondly, the evolutionary laws of crack development characteristics in expansive soil under various conditions, including different initial moisture contents, compaction degrees, and thickness, are analyzed based on indoor wet-dry cycle test results. Subsequently, soil mechanical property tests were conducted, focusing on the variation in shear strength indicators of expansive soil concerning the number of wet-dry cycles and crack ratio. Finally, the evolutionary mechanism of cracks in expansive soil under wet-dry cycle conditions is elucidated from the perspective of crack development characteristics.