## 1 Introduction
Numerical simulations have been used to combine deformation analysis and infiltration analysis [18; 19; 20; 21], whereby the performance of an expansive soil slope under rainfall could be assessed. Reduced-scale model tests have been carried out in a 1 g condition [6; 22] or in a centrifuge environment [23] where artificial rainfall was used to trigger a landslide. Digital image correlation (DIC) technology was used to quantitatively obtain horizontal and vertical deformation data of physical experiment. DIC technology was first proposed by [PERSON] [24] and [PERSON] [25] in the early 1980s. This method is a non-contact measurement method that compares the digital image differences in samples before and after deformation, so as to calculate the strain field. DIC technology is widely used in the analysis of strain field distribution, deformation mode, progressive failure process and the sliding shear zone characteristics of rock or soil masses [26; 27; 28; 29; 30]. Great attention should be paid to avoiding the development of surface fissures and minimizing the amount of water infiltration in expansive soil slopes. Many studies have demonstrated that chemical stabilizers, such as fly ash, lime and cement, are widely used to modify and improve the hydro-mechanical characteristics of expansive soils [31]. The effect of coupling pond ash and polypropylene fiber has been investigated to control the strength and durability of expansive soil against tensile cracks [5; 32]. Different capillary barrier systems have been proposed, such as the use of recycled crushed concrete and geosynthetics [5; 33], recycled asphalt pavement wastes [34], waste tire textile fibers as reinforcement materials [35], residual soils as cover system [36] and soil bags [37; 38]. Although extensive studies on the stability and improvement of expansive soil slopes have been carried out, the mechanism of the influence of cracks on the stability of an expansive soil slope during loading and irrigation is not yet fully understood. Field testing can be used to study the failure process of an expansive soil slope subjected to artificial rainfall. However, this kind of experiment is costly, takes a long time and the experimental data are highly discrete. The physical model test method is an alternative to research the stability of soil slopes in the laboratory, although the correct stress field of a soil slope cannot be reproduced. Loose granular materials can normally be used in most of the model tests due to the ease of triggering slide or flow [6]. However, the effect of cracks on slope stability cannot be considered in this method. In this study, a series of physical model tests with different types of cracks were conducted, in which the fissured expansive soil slopes were subjected to different loadings (1.6, 3.2, 4.8, 6.4, 16 MPa) and irrigated at a flow rate of 25 mL/min. The VIC-2d software, which utilizes the DIC principle, was used to quantitatively obtain the horizontal and vertical strain data of the slope model. The closure and opening of cracks, and the slope state after loading and irrigation were monitored by strain data analysis using VIC-2d software. The aim of this study was to investigate the relationship between the crack strain state and the deformation trend of a fissured expansive soil slope. The results of this research are of great importance in analyzing strain field distribution, deformation rule, progressive surface- or shallow-layer sliding mechanisms and key reinforcement areas for fissured expansive soil slopes.