## 2 Study Area
### Study Area Setting
#### 2.1.1 Geological Structure  
To investigate the geological conditions of the landslide, data from 22 new boreholes were utilized. The main geological profile of the landslide (Figure 4) was derived based on the stratigraphic conditions revealed by boreholes (Some typical geological conditions are shown in Figure 5) and the boundary. The Quaternary (Q) cover layer is widely distributed in Cheyiping village and its vicinity, underlain by the Middle Jurassic Huaikai Left Formation (J\({}^{\text{2h}}\)). The bedrock primarily consists of purple-red laminated mudstone. The rock layers exposed at the surface exhibit significant tilting towards the open face (Lancang River, 2# Gully, 3# Gully and the major gully downstream of Cheyiping). The pre-tilting attitude of the exposed rock layers at the surface is N20-30\({}^{\circ}\) W, NE <20\({}^{\circ}\)-35\({}^{\circ}\), while boreholes reveal a dip angle of 5-10\({}^{\circ}\) for the deeper rock layers. The slate rock mass exhibits a high degree of weathering, with completely weathered rock cores appearing as a mixture of soil and debris, while strongly weathered rock cores appear as fragments or debris.  
The Quaternary deposits are categorized into three types based on their genesis: colluvial deposits (Q\({}^{\text{dl}}\)), debris deposits (Q\({}^{\text{dl+col}}\)), and landslide deposits (Q\({}^{\text{del}}\)). The Q\({}^{\text{dl}}\) deposits are remnants of the fourth-level terrace of the Lancang River, characterized by a layer of gray-yellow sand and gravel with a thickness ranging from 0.5 m to 1 m, primarily distributed near the lower part of the village road platform. The Q\({}^{\text{dl+col}}\) deposits consist of gray-yellow and brown-yellow gravelly loam, sandy loam with embedded angular stones, and isolated stones. These deposits exhibit a moderate density, typically ranging from 5 m to 20 m in thickness. The stones are present in a skeletal form, with gaps filled with gravelly or gravelly loamy soil, and are widely distributed on most landslides in the entire study area. The gray-yellow and brown gravelly soil is primarily composed of gravel, angular stones, and sandy clay. The content of angular stones ranges from 20% to 40%, with diameters typically between 1 cm and 2 cm, with a few exceeding 5 cm. The main component is sandstone, with the soil's density ranging from loose to slightly dense. The soil's uniformity is poor, with thicknesses ranging from 23.8 m to 63.98 m, representing the deposits formed after the movement of the ancient landslide.  
Figure 3: Cheyiping landslide in Huangdeng Reservoir.  
The layer of soil in the sliding zone (Q\({}^{\text{del}}\)) presents a predominant color palette of purple-red, gray-black, and brown-yellow. It is primarily composed of clay containing gravel interspersed with occasional fragments and gravel stones, the latter of which are sub-rounded. Notably, some sections of the sliding zone soil exhibit distinct mirror-like shear phenomena. The soil demonstrates a range of plasticity from malleable to rigid, with thicknesses typically ranging from 10 cm to 30 cm, although occasionally exceeding 50 cm. Multiple layers of this soil are distributed within the landslide body, indicating a history of multi-layered and multi-phase sliding deformations. Analysis of borehole data near the road and its upper section reveals different phases of sliding deformation occurring at depths around 60 m, 48 m, 40 m, 36 m, and 20 m. Boreholes located in the lower section of the road generally exhibit signs of sliding deformation at depths of 60 m, 50 m, 45 m, and 25 m.