## 1 Introduction  
Expansive soils are characterized by repeated swelling and shrinkage due to the layered crystal structure of clay minerals, e.g., smectite, illite, etc., and the adsorbed cations [1]. As a result, a large number of expansion and shrinkage cracks are produced in expansive soil. Expansive soils cause great damage to slope stability because of their expansiveness, over-consolidation, and propensity to crack. Thus, expansive soil is called \"calamitous soil\". Expansive soils are widely distributed throughout the world, including the United States, Canada, Australia, India, and many other countries [2]. In China, expansive soils occur in >20 provinces, covering an area of 600,000 km\({}^{2}\)[3]. Damage caused by expansive soil can be observed around the world. An economic loss of approximately USD 7-9 billion per year has been reported in the USA alone [4,5]. Recent studies have shown that the existence and the development of cracks in expansive soil have an important effect on the stability of expansive soil slopes. The presence of fissures or cracks near the ground surface is critical to the soil suction and water interaction [6; 7]. The cracks tend to heal through horizontal growth due to preferential water movement, and reopen upon lateral contraction due to a weak connection with the clay matrix [8]. The shrinkage of expansive soil can cause the development of tensile stress and variations in water content along the fissures [9; 10]. When tensile stress exceeds tensile strength, desiccation cracks will appear on the slope surface of expansive soil regardless of slope inclination, and especially on the crest of the slope. Desiccation fissures can initiate and propagate in expansive soil subjected to drying-wetting cycles. The fissures provide pathways for rainwater infiltration and evaporation to further reduce the shear strength [10; 11]. The relationships between the geometric parameters of these cracks and the water content/drying time were presented to further quantify the three stages of crack development by a digital imaging method, i.e., the initial stage, the primary stage and the steady-state stage [11]. A digital imaging method was recently used to obtain images and the geometric parameters (i.e., crack porosity, crack aperture, and crack density) of clay-rich soils with desiccation cracks at the ground surface [11]. The relationship between the crack strain change and deformation trend of fissured expansive soil slopes has not yet been investigated. Three different irrigation methods, including spray irrigation, drip irrigation and surface irrigation, were used to study crack variation and flow rules in expansive soil by dye experiments. The results show that the cracks closed rapidly under surface irrigation but partially closed under spray irrigation, and the degree of crack closure decreased with the distance from the drip center under drip irrigation [12]. During freezing on the slope surface, the migration of water and the growth of ice crystals result in frost heave, the formation of cracks and alterations in the structure of expansive soil [13; 14]. During thawing, the water content of expansive soil increases due to the inflow of snowmelt water, causing a decrease in the stiffness, strength and thaw settlement of expansive soil [14; 15]. Methods such as sketching superficial cracks, drilling holes and non-destructive detection for verification can reflect the real distribution characteristics of interior cracks to a certain extent [16]. It is important to understand the distribution characteristics of cracks in expansive soil to study the mechanisms of expansive soil landslides. Surface- or shallow-layer failure can easily occur in expansive soil slopes [6; 17]. Different techniques can be used to research the mechanisms of slope failure during loading, rainfall or irrigation. A field test was carried out to study the influence of slope inclination and rainfall intensity on evaporation, infiltration and water content on both vegetated and bare slopes of expansive soil [10].