Response of landslide deformation to rainfall based on multi-index monitoring: a case of the Tanjiawan landslide in the Three Gorges Reservoir

Landslide deformation in the Three Gorges Reservoir Area (TGRA) is mainly caused by water level fluctuations; however, it is also influenced by rainfall. The Tanjiawan landslide is one of the most notable landslides affected by rainfall in the TGRA. This study investigates the landslide's deformation mechanism by analysing reconnaissance data, which are surface deformation survey data, and GPS displacement data for the last 16 years along with several types of monitoring data from equipment installed in 2020. The landslide has undergone significant deformation since September 2014. Subsequent rainfall events have caused severe deformation, and tension cracks have appeared in many parts of the landslide. Owing to the slope structure and rainfall events, a deformation feature has developed, in which sliding masses no. 1 and no. 2 are the active and secondary deformation zones, respectively. The surface displacement of the landslide is clearly step-like, and its episodic deformation is controlled by rainfall. Landslide deformation leads to the development of cracks in the slope, producing dominant seepage channels. As rainwater enters belowground along the cracks and with the influence of the bedrock morphology, the groundwater level in the middle and rear parts, and hence, the hydrodynamic pressure, increase, thereby activating landslide deformation. Both continuous rainfall and heavy rain are responsible for this phenomenon; sliding mass no. 1 shows an overall downward motion. Finally, the seismic data recorded by microcore piles and the change rate in soil moisture content after rainfalls can currently be used for reliable early warnings of landslide instability.