## 2 Engineering Background  
The Zhuquedong landslide is located in Luxi County, Hunan Province, China (Figure 1a), and belongs to the hilly area of tectonic denudation, with an elevation of 282 m at the top of the slope and about 125 m at the foot of the slope, and a slope gradient of 10\({}^{\circ}\)-20\({}^{\circ}\). The slope body is basically a bedding slope, forming a stepped slope topography with orange groves, paddy fields and vegetable fields. The dip angle of the slope ranges from 14\({}^{\circ}\) to 30\({}^{\circ}\), which shows the trend that the dip of the rock layer from the trailing edge to the front edge gradually changes from steep to gentle. The perimeter of the landslide is in the shape of a circle chair (Figure 1b,c), which is mainly controlled by the structure. The length of the landslide is 448 m along the direction perpendicular to Changji Road, the width is 425 m along Changji Road, and the width of the front edge at the foot of the slope is about 500 m.  
The landslide is located in the red layer area where a large number of geological hazards occur. Mudstone, siltstone and other soft rocks with weak permeability and strong hydrophilicity are more abundant in the red layer, which have the characteristics of easy softening with water, easy disintegration with water loss and low strength. The soft interlayer formed by water softening of mudstone and the interlayer misalignment zone in the red layer rock body largely controls the deformation and damage mode of the rock body [10]. Therefore, red-layered mudstone slope instability is characterized by strong suddenness and difficulty in early identification. The field survey results show that the upper part of the Zhuquedong slope is covered by the Quaternary System, and the surface layer exposed is mainly fill soil, planting soil distributed in farmland, sub-clay and block gravel, etc. The underlying rocks are mainly silty mudstone and calcareous sandstone, and decomposed rocks and soft interlayer are revealed during the survey. The silty mudstone of the strongly weathered layer is relatively broken, the core is gravelly and fragmented, and the lithology is relatively soft. Weakly weathered silty mudstone is soft and relatively intact, which is enormously easy to be softened by water to form soft interlayer (Figure 1e), with an interlayer misalignment zone existing locally. Slightly weathered calcareous sandstone is hard and relatively intact. Sliding zone soils are mainly the soft interlayer and mudification interlayer formed by silty mudstone and the interstratigraphic misalignment zone after water softening, which is about 0.5-2 m thick. The landslide toe is located at the foot of the slope, and part of the rock-soil body washes into the Danqing River. The sliding surface (belt) attitude of the slope body is controlled by the attitude of bedrock. The dip angle of the slide surface (belt) is steep in the trailing and slow in the front, with the dip angle of the trailing edge about 21-30\({}^{\circ}\) and the dip angle of the front edge about 14-17\({}^{\circ}\). The shape of the sliding surface is controlled by the rock strata, and also by the influence of steeply dipping joints, so the sliding surface is locally stepped. The slide surface develops landslide scrapes, and its slip direction is consistent with the main slide direction. The lower slide bed lithology of the sliding surface is mainly relatively intact weakly and slightly weathered rocks. The landslide mass is mainly composed of residual gravelly soil--strongly and weakly weathered rocks, with a slide thickness of about 0-20 m and a sliding volume of about 2.6 million m\({}^{3}\).