Fifteen-year professional monitoring and deformation mechanism analysis of a large ancient landslide in the Three Gorges Reservoir Area, China

The activation of large ancient rock landslides caused by reservoir operation is a primary threat to the TGR. Understanding the deformation characteristics and failure mechanisms of these landslides can be helpful for stability assessment and long-term landslide prediction. The primary aim of this study is to research the motion characteristics of ancient rock landslides and their relationships with engineering activities and natural factors. The Tanjiahe landslide, a large reactivated landslide in the TGR region, is used as an example. Long-term professional monitoring data collected since 2006 show that the landslide is experiencing creep deformation. By combining 15 years of meteorological, hydrological and manual GNSS monitoring information, 5 years of displacement measurements from open fractures and deep boreholes, automatic GNSS measurements, and macroscopic inspection results, the deformation mechanisms of landslides are studied in detail. The study shows that landslide deformation is affected by the reservoir water level and that rainfall promotes landslide deformation. Specifically, from November to May, during the high reservoir water level stage and subsequent declining water level stage, the landslide exhibits a very distinct movement pattern with high deformation, lasting nearly 7 months. As the water level rises, the deformation subsides until almost no deformation exists. Landslide displacement continues to increase each year, indicating likely slope instability. However, the deformation rate of the Tanjiahe landslide is gradually decreasing, and the probability of large-scale sliding is low; still, the monitoring system must be strengthened, and additional research must be performed on the corresponding deformation mechanism.