# Progressive Formation of Retrogressive Landslide and the Lateral Length of Instability
## 1 Introduction  
Deformation types of landslides generally include retrogressive landslide, load-caused landslide, and complex landslide [1, 2]. Among them, retrogressive landslide is a process of backward progressive failure [3, 4, 5]. Due to slope erosion, or artificial slope cutting, the slope surface is deformed and unstable, and the lower rock mass first slides, and then gradually forms cracks from bottom to top [6, 7]. With the development of deformation, the rock mass on the upper part of the slope loses support and slips [8]. Retrogressive landslides mostly occur in sloped areas composed of sedimentary layers, fault zones, weathering zones, and weak interlayers of rock and soil. The main sliding bodies are unconsolidated layers, weak interlayers, or ancient landslides, creep bodies, etc. [9, 10, 11]. When building expressways in mountainous areas, many sections of the road are constructed by removing the foot of the slope. Due to the man-made destruction of the foot of the slope, retrogressive landslides are easy to occur under the action of long-term natural forces (gravity forces). As a geological hazard, retrogressive landslide has the characteristics of wide distribution, high frequency, and great harm, which poses a major threat to engineering construction and people's property safety [12, 13, 14]. Many scholars have done a lot of research work on retrogressive landslide [15, 16, 17]. [PERSON] et al. [18] proposed a simplified numerical method for the prediction of rainfall-induced retrogressive landslides, which can effectively reduce the harm caused by retrogressive landslides caused by rainfall. [PERSON] et al. [19] investigated the Audru landslide in detail and studied the influence of various modeling parameters on the overall slope stability. [PERSON] et al. [20] carried out slope model experiments by fillingan experimental flume with unsaturated fine sand and an artificial rainfall simulator and summarized the mechanism of rainfall-induced retrogressive landslides. At present, the research on retrogressive landslide generally uses indoor experiments or engineering cases to carry out qualitative analysis on its failure mode [21; 22; 23]. In order to control the retrogressive landslide and treat it [24; 25; 26], it is necessary to determine the instability range of the retrogressive landslide. As the retrogressive landslide is sliding from top to bottom, the slope will slip and crack to form numerous irregular slides. The reasons for the formation of these sliders and the range of the number of sliders need to be further studied. The length of the first sliding block formed by the retrogressive landslide is called the lateral length of traction instability, which is the distance between the bottom instability slip surface from the excavation surface to the trailing edge crack. In this paper, relying on the actual highway retrogressive landslide, the particle flow discrete element is used to analyze its failure mode, and the lateral length of traction instability is calculated through the stability theory of the sliding pull-crack failure slope, and the traction instability range of the retrogressive landslide is determined, and the influence of various rock parameters on the lateral length of traction instability is analyzed. The technical flow chart is shown in Figure 1.