Abstract: Since the implementation of professional monitoring in 2006, the Tanjiawan landslide in the Three Gorges reservoir area has seen a continuously increasing displacement. Especially since 2015, the deformation trend has intensified, posing a great threat to the lives and property safety of villagers in the reservoir area. According to long-term field geological survey, macroscopic inspection, automatic monitoring data of landslide surface crack displacement and artificial GPS monitoring data for over 12 years, we examined the basic deformation characteristics and deformation mechanism of the landslide under heavy rainfall and reservoir water level fluctuation. Results suggest that the Tanjiawan landslide is a rainfall-triggered retrogressive landslide with medium-thick layer chair-shaped groove shape. Heavy rainfall and continuous rainfall are major external factors affecting landslide deformation, while water level fluctuation has little effect. Heavy rainfall and continuous rainfall led to evident response: when daily rainfall exceeds 90 mm or the accumulated rainfall of previous three days reaches 50 mm while rainfall on the same day and the previous day exceeds 15 mm, the surface crack width-time curve and the cumulative displacement-time curve will witness an apparent step upward. At present, the deformation of Tanjiawan landslide is increasing year by year, with boundary cracks basically formed. It is highly probable that sliding will occur under extreme conditions such as heavy rainfall. Automatic monitoring equipment and detail inspection must be strengthened.

Key words: rainfall-induced landslide, deformation characteristics, step curve, landslide formation mechanism, Tanjiawan landslide