15. Mass Rock Creep and Landsliding on the Huangtupo Slope in the Reservoir Area of the Three Gorges Project, Yangtze River, China

The Huangtupo slope is one of the most noted large-scale slopes with geologic hazards that cause problems related to the residential safety of immigrants in the reservoir area of the Three Gorges Project, Yangtze River, China. The gravitational process, blended with tectonic deformation, large-scale covering of loose debris, and long-term surficial mass movement, complicates the Huangtupo slope and gives rise to a lack of consensus on the slope nature and stability. Characterization of the structural geometry of the slope deformation and reconstruction of its development history are believed to be pivotal in understanding what has happened and what will happen to the slope. Based on a thorough field investigation combined with an electrical resistivity survey, a three-stage model involving mass rock creep–primary landsliding–partial reactivation is proposed. The first stage follows the incision of the Yangtze River along the axes of the Guandukou syncline, when Huangtupo experienced long-term gravitational deformation referred to as ‘mass rock creep’. Mass rock creep in the Huangtupo slope can be classified as two basic processes: toppling and deep-seated creep. Toppling mainly occurs on the exterior part of the slope and is characterized by inclination, sliding, and segmentation of brittle deformation of cleavage rocks. Deep-seated creep occurs predominantly in the interior part of the slope and brittle–ductile flow folding accompanying low-angle shearing is its representative deformation. A primary, large-scale landslide occurred in the second stage as a subsequence of previous mass rock creep. The landslide was ca. 4×107 m³ in volume with elevations of 640 m a.s.l. at the head and of 80 m a.s.l. at the foot. Significant evidence for the landslide is the variation of attitudes of structural foliation and lineation among outcrops in the slope. The last stage began subsequent to the original sliding; surficial and partial reactivation on the Huangtupo landslide plays a leading role in this stage. Two sliding events in this area in 1995 were ascribed to the partial reactivation mainly due to rainfall, water level fluctuation of the Yangtze River, as well as human activity. It was suggested that analogous failures in 1995 would continue on the Huangtupo landslide and become even more frequent under the combined effect of human activity and reservoir filling.