Deformation characteristics and thresholds of the Tanjiawan landslide in the Three Gorges Reservoir Area, China

Since the first impoundment of the Three Gorges Reservoir (TGR) in China in 2003, more than 5000 landslides including potential landslides were identified. In this paper, a deep-seated active landslide in TGR area was analyzed. Fourteen years' monitoring data and field investigations from 2006 to 2020 were used to analyze the deformation characteristics, influencing factors, and meteohydrological thresholds. The landslide showed a none-overall periodic movement pattern featuring acceleration during long-duration rainfall and rapid transition to constant creep after rainfall events. Two secondary sliding masses, No. 1 and No. 2, were defined via field investigation. The reservoir has no impact on the deformation whereas long-duration-low-intensity rainfall is the main factor. At present, the cumulative displacements of the main sliding mass range from 0.9 to 3.2 m, and the deformation during the rainy season is gradually increasing. The boundary of this landslide was formed, and the boundary of No. 2 sliding mass became obvious. The probability of the failure of sliding mass No. 2 is very high under the conditions of continuous rainfall. The 15-day antecedent rainfall combined with 4-day cumulative rainfall could be the rainfall threshold which could be associated with the groundwater level S1 of 294 m above sea level for forecasting large deformation of Tanjiawan landslide.