## 1 Introduction  
In Northeast China, sandstone-mudstone interbedding is a typical stratigraphic distribution. Both sandstone and mudstone are sedimentary rocks. Among them, sandstone has great weathering resistance and good permeability, and surface water can easily permeate it. Mudstone, on the other hand, has low strength, weak permeability and is quickly absorbed and softened by water [1]. It is a slippery stratum in engineering construction. In recent years, the trend of summer rainfall in Northeast China increased [2]. Due to the rain erosion and temperature change, the exposed mudstone interlayer will disintegrate and soften quickly. This weakens the layer and reduces its strength, which causes a significant drop in the slope safety factor and even destabilization [3, 4]. Therefore, the sandstone-mudstone interlayer's slope stability problem has increasingly caught the attention of academics working in the area.  
The geotechnical structure and influencing factors are the two main areas of attention for studies on the stability of sandstone and mudstone slopes at the moment. According to [PERSON]'s [5] analysis of the stability of mudstone and muddy siltstone landslides, impermeable siltstone is a necessary element for landslide formation, and surface water infiltration and rainfall both contribute to landslide deformation. However, there is no in-depth analysis of the rainfall factor. By examining the structural characteristics of mudstone rock, [PERSON] [6] made the observation that the rate of rainfall infiltration into the rock affects the rate of expansive argillization, but neglected the intensity of the rainfall infiltration and duration of rainfall infiltration. Based on the instability of siltstone slopes, [PERSON] et al. [7] hypothesized that slope deformation can be divided into a slow creep period caused by excavation and an accelerated sliding period caused by rainfall. Consideringthe three factors of rainfall intensity, duration and slope gradient, [PERSON] et al. [8] found that gradient had a bigger impact on slope safety coefficient than rainfall factor. Instead, [PERSON] [9] simulated infiltration characteristics and deformation damage characteristics of landslide under different rainfall conditions by artificial rainfall testbed. However, none of them took into account the impact of stratigraphic lithology on slope stability. Using indoor simulated rainfall experiments, [PERSON] et al. [10] investigated the effects of slope angle, rainfall intensity, and vegetation coverage on sandstone slopes, and concluded that rainfall intensity has the greatest impact on slopes. Researchers both domestically and internationally agree that, according to analysis of the above research, rainfall causes sandstone and mudstone slopes to become unstable. However, there are few studies on landslides caused by rainfall infiltration in sandstone-covered mudstone formations. Determining the crucial range of landslides in intricate rock under different rainfall conditions is thus necessary.  
In order to further deepen the understanding of mudstone softening caused by rainfall infiltration, this paper takes the typical landslide along the Heishan Village to Jixi section of Dan-A Highway Expansion Project as an example. Based on extensive field work and laboratory tests, the landslide mechanism and instability state under different rainfall conditions are studied by numerical analysis. Additionally, the influence of different rainfall intensity and duration on the stability of sandstone-covered mudstone slopes is analyzed. Therefore, this case is used to explore the causes of landslides in such slopes so as to better prevent landslide disasters and provide reference for highway construction in the world's sandstone-mudstone distribution areas.