## 1 Introduction
[4] found through the unloading test that the cohesion of the unloading normal stress direct shear test decreased significantly, while the internal friction angle increased slightly, the shear displacement increased linearly at the initial stage of the unloading normal stress, and then rapidly increased nonlinearly before failure. [PERSON] et al. [16] analyzed and compared the deformation characteristics and failure mechanisms of homogeneous loess slopes and double paleosol interlayer loess slopes under different excavation slope angles and believed that the response of lateral horizontal earth pressure in the excavation area was closely related to the failure of the slope. In addition to the landslide caused by artificial disturbances, such as excavation and blasting, rainfall infiltration is also one of the main factors inducing slope instability [6]. Soil in nature is a loose porous medium [17]. Under rainfall infiltration, the stress and seepage field of the soil slope interact with each other [18], and this interaction between fluid and solid is called fluid-solid coupling. The rainfall-type landslide is mainly affected by stress and seepage fields. The infiltration of rainwater into the soil produces a seepage field. With the continuous infiltration of rainfall, the seepage field changes continuously and then has seepage pressure [19]. Seepage pressure acts on the soil for a long time, which changes the stress field of the ground [20]. The change in the stress field will shift the pore volume of the soil and cause the evolution of the seepage field and the permeability coefficient of the ground. The stress field and seepage field of slope soil are closely related to each other. However, the analysis of slope stability under rainfall infiltration mainly focuses on a single field or indirect coupling analysis. The production of rainfall-type landslides results directly from multi-field coupling [21]. Therefore, it is necessary to study the coupling of the seepage field and stress field of the slope under rainfall conditions. Numerical simulation, especially the fully coupled water force large deformation model, is helpful for correctly simulating the complex failure and post-failure mechanisms of rainfall-induced landslides [22]. By analyzing the measured data, [PERSON] et al. [1] concluded that rainfall has the most significant impact on vertical displacement, accounting for 56.76% of total vertical displacement. However, it is still an elastic displacement, which usually causes minor damage to the slope. [PERSON] et al. [6] conducted mechanical tests on rock samples with different water contents, studied the change law of rock mechanical behavior under the action of water, and established the function of rock water-induced weakening. [PERSON] et al. [23] found that the stability of rainfall-induced dams is relatively poor based on the collected 1578 dam case studies; with the increase of dam height and storage capacity,the stability first decreases and then increases; with the increase of dam volume and dam material D50, the stability increases gradually; with the increase of catchment area, the stability decreases gradually. [PERSON] et al. [24] analyzed slope cracks' development and failure modes and studied the early warning strategy for rainfall-induced fill slope failure. The results show that rainfall-induced loess landslides have hysteresis, and saturation and matrix suction responses are earlier than pore water pressure. The slope is damaged when the saturation and matrix suction are at their maximum and minimum values, respectively. Due to the pore water in the unsaturated soil slopes, the seepage field and stress field cannot be separated in a direct manner. [PERSON] et al. [25; 26; 27] suggest that the seepage-stress separation method can simulate the slope excavation-rainfall coupling process by analyzing the excavation stress field first and then imposing rainfall on the formed boundary.