# Similarity Model Test on Rainfall Scouring Mechanism of High-Speed Railway Subgrade Slope
## 1 Introduction  
The high-speed railway subgrade slope is often exposed to the natural environment for a long time, and is prone to slope landslides and slope erosion under rainfall scouring. There are methods that can be used to evaluate failures on unstable slopes, by considering slope failure mechanisms due to rainfall [1]. Furthermore, the effect of erosion on unstable slopes along with failure mechanisms has been studied [2]. For all of this type of research and its applications, at present, the research on slope landslides induced by rainfall infiltration is relatively mature [3, 4, 5], clearly stipulated in the technical specification. The Chinese railway code puts forward the calculation method for the overall instability of the slope based on different failure modes, such as circular slip surface, plane slip surface, or broken line slip surface [6]. Additionally, the Japanese code puts forward the calculation method for slope shallow stability based on the reduction in the soil's shear strength parameters [7]. There is little research on the mechanism instability of subgrade slopes induced by scouring erosion, in which qualitative analysis is common [8]. [PERSON] [9] used empirical formulas to calculate the incipient velocity of slope erosion, but this was not verified by model tests, the reliability of which needs to be improved. To study the mechanism and development process of slope erosion under rainfall, relevant scholars conducted rainfall scouring erosion model tests [10; 11; 12; 13; 14]. Based on the indoor model test, [PERSON] et al. [15] divided the slope shape and the sediment content of the runoff into four erosion processes with significant characteristics, proving that with the increase in slope length or rainfall intensity, the runoff rate and erosion modulus increased linearly. The model test conducted for loess and other special soil slopes [16] provides a comprehensive summary of the evolution process of slope scouring erosion failure. The process begins with splash erosion, followed by sheet erosion, fill erosion, gully erosion, and then collapse. The process displays traction failure characteristics until the slope is destroyed. Artificial rainfall is frequently utilized in model tests [17], with a focus on local rainfall patterns. However, the impact of the lateral infiltration of rainwater and slope deformation in non-rainfall areas on rainfall areas creates a boundary constraint effect. This effect is an essential factor that needs to be considered during the implementation of artificial rainfall models. [PERSON] [18] analyzed the scouring erosion process and the influencing factors of railway subgrade slopes under natural rainfall in the field test. The results indicate that the runoff generation form of the railway slope is generally super seepage runoff, including the stages from the rainfall beginning to the initial runoff generation, and then to the stable state. The above artificial or natural rainfall experiments have been used to analyze the occurrence and development process of roadbed slope erosion, mainly based on qualitative description, with less quantitative analysis. In addition, combining indicators such as flow velocity, incipient particle size and scouring erosion amount is beneficial for revealing the scouring erosion mechanism and quantitatively characterizing the scouring erosion law for railway roadbed slopes [19]. Model tests are commonly used to study slope instability [17; 20]. Due to the frequent use of artificial rainfall erosion in these experiments, which is often limited to the implementation of local rainfall patterns, there is a boundary constraint effect between the rainfall area and the non-rainfall area [21; 22; 23; 24], manifested as the lateral infiltration of rainwater and the lag of slope deformation in the non-rainfall area on the rainfall area. A deep-buried isolation trench test method has been proposed to address these boundary effects [25; 26; 27].