# Formation and Failure Mechanism of the Xinfangzi Landslide in Chongqing City (China)
## 5 Factors Influencing Landslide Stability  
After the occurrence of the Xinfangzi landslide, the current factors affecting its stability involve the following five aspects.  
TopographyThe slopes of the study area tend to be north (342\({}^{\circ}\)) and east (55\({}^{\circ}\)). The height difference between the northern and southern parts of the landslide is about 80.2-88.50 m, and the difference between the eastern and western parts is about 35.4-48.3 m. The height difference between the front and rear edges of the landslide is large, and the northeast side of the landslide is a steep slope with an average slope angle of 65\({}^{\circ}\) that is undercut by the Songkan River. This topography provides space for sliding.  
Stratum lithologyThe middle and upper parts of the sliding mass are composed of silty clay intercalated with crushed stones and rocky soil. The lower part of the sliding mass is composed of landslide debris and fragmented rocks, in which cracks are developed, leading to good water retention. The slip bed is a relatively complete mudstone and acts as a water barrier. Rainfall infiltration causes the landslide soil to become saturated and infiltrated for a long time, and the soil layer easily slips along the sliding surface of the ancient landslide.  
GroundwaterDuring the dry season, springs leak from the right boundary of the landslide and the front edge of the shear outlet. According to the drilling results, the soil in the middle and lower parts of the sliding mass is saturated. The groundwater level in the sliding mass is relatively high, and runoff and drainage channels are developed. The groundwater mainly flows northward along the sliding surface from the middle and rear parts of the landslide, and it is discharged at the front of the landslide. A high fissure water pressure and sliding surface uplift pressure are formed by the groundwater stored in the unloading and expanding fissures in the middle and rear parts of the landslide. Due to the runoff and the seepage of the groundwater, the sliding mass will become saturated and softened for a long time, the fine-grained material will be eroded, and the shear strength of the sliding surface will continue to decrease, forming a potential weak sliding zone. These are the main factors leading to the instability of the Xinfangzi landslide.  
Heavy rainfallAccording to the field surveys, the water catchment area at the cliff on the west side of the landslide is large, and this area contains stepped terraces with developed lateral drainage systems. This causes a large amount of the catchment water to flow back into the landslide along the cliff fissures. The middle and upper parts of the landslide are mainly composed of silty clay mixed with crushed stone and rocky soil, which is conducive to accelerating rainfall infiltration to form groundwater. In addition, based on the deformation history of the landslide, the two large-scale slips that occurred in 1998 and 2020 were both induced by heavy rainfall during the flood season. Therefore, it is concluded that heavy rainfall is the main factor affecting the instability of the landslide.  
Figure 6: Surface water on the landslide area (8 July 2020).  
Human engineering activitiesLong-term farming has resulted in the better water permeability of the soil. These human activities contribute to the infiltration of surface water into the landslide. Due to the construction of rural roads, a slope with a height of about 3 to 4 m has been formed on the front and left sides of the landslide, providing space for local deformation of the landslide.