Complex rock slope deformation at Laxiwa Hydropower Station, China: background, characterization, and mechanism

An unstable rock slope called the Guobu landslide, observed in granite with a maximum cumulative displacement of 45 m, located on the upper stream of the dam of the Laxiwa Hydropower Station, China, and directly threatening the safety of the dam and the people living downstream, was investigated. Detailed field surveys, geological structure investigations, remote sensing image analysis, and GPS displacement monitoring were carried out to investigate the deformation characteristics of the slope after reservoir impoundment, historical deformation, ancient landslide reactivation, the influence factors of the large deformation of the slope, and the mechanism. Based on the results of the above analyses, it was seen that there is a clear geological structural dependence of the failure mechanisms in the study area, and that the complex failure mechanisms include toppling, subsiding wedge failure, rockfalls, and tension cracking in deep-seated rock masses. Moreover, analysis of the remote sensing images indicated that the ancient landslide had been reactivated between 2005 and 2008. Detailed field displacement monitoring data showed that the whole slope was deforming significantly and continuously with the increase of the water level in the reservoir. The displacement rate had a positive correlation with the variation of the reservoir water level, increasing and decreasing as the the latter rose and fell, respectively. The vulnerable geological conditions, water infiltration, and the reservoir impoundment were the main factors causing the large deformation of the Guobu landslide. Water infiltration was the driving force resulting in the reactivation of the ancient landslide, and the reservoir impoundment accelerated the slope deformation. The mechanism of the large deformation of the rock slope was a combination of the upper pushing deformation induced by wedging and toppling, due to the ancient landslide reactivation, and the lower traction deformation due to the reservoir impoundment.