## 5. Revival Mechanism  
### Causes of Ancient Landslide Revival  
The influence of water on the stability of landslides is notably significant [24; 25]. This influence primarily manifests in two aspects. Firstly, water has a softening and liquefying effect on soft rocks, extremely soft rocks, weak interlayers, and fine-grained materials (particularly clay and silt particles), leading to a significant reduction in the strength of rock and soil masses as well as structural surfaces [26]. Secondly, water creates dynamic and static pressures. Infiltration of rainwater and reservoir impoundment elevate the groundwater table, increasing pore water pressure and reducing the landslide's resistance to sliding, thereby causing landslide deformation and failure. Additionally, the discharge of groundwater from the landslide generates dynamic water pressure, intensifying the sliding force along the direction of groundwater flow, which is detrimental to landslide stability [14]. (1) The Influence of Reservoir Water Level Fluctuations  
According to our investigation, the primary cause of ancient landslide reactivation in this project is the frequent fluctuation of water levels during reservoir operation. A substantial increase in reservoir water level results in significant water infiltration into the landslide surface, altering its moisture content and simultaneously raising the groundwater level within the landslide. In permeable soil layers, the rate of groundwater level rise often fails to match that of the reservoir, leading to unstable transient seepage within the landslide [27]. The combined effects of transient seepage and water softening result in a significant reduction in the mechanical properties of the rock and soil mass within the landslide. Moreover, the rise in reservoir water level significantly increases water pressure acting on the landslide's outer surface. Determining the dominant factor between these two competing influences requires computational analysis [28]. The surface soil layer on the landslide of this project exhibits a moderate permeability. The rate of groundwater level rise within the slopes is capable of adapting well to the rate of reservoir water level rise. Consequently, the rapid rise of the reservoir water level results in minimal transient seepage effects within the slopes, as the dominant influence is the water head pressure acting on the outside of the slopes. As a result, the slopes ultimately stabilize. Numerical computation results also indicate a significant increase in the slope stability coefficient with the rise of the reservoir water level. Despite the rainy season coinciding with the reservoir impoundment period, a substantial amount of rainfall did not induce slope instability, indicating the limited influence of rainfall in this case. It is noteworthy that the rise in reservoir water levels does not always have a positive effect, as cases of slope instability caused by reservoir impoundment, such as those at the Three Gorges Dam and the Jinping I Hydropower Station, are also common [29; 30]. The sudden reduction in reservoir water levels can significantly jeopardize landslide stability. In June 2019, when the reservoir water level began to drop sharply, the ancient landslide at Cheyiping was reactivated. During such events, the inability of the landslide's internal pore water pressure to dissipate promptly leads to the generation of seepage pressure along the direction of sliding. This heightened pressure increases the traction force along the sliding surface, inducing instability and landslide failure. This study also reveals a direct relationship between the safety factor of the landslide and the fluctuationsin reservoir water levels, with a notable decrease in the safety factor as the reservoir water level decreases. This observation suggests that the project falls under the classification of seepage-induced landslides [31].