A complex rock topple-rock slide failure of an anaclinal rock slope in the Wu Gorge, Yangtze River, China

This paper presents the results of an investigation of the Gongjiafang landslide with the volume of about 500,000 m3 in the Three Gorges Reservoir region (TGR), occurred on 23 November, 2008. In this study, detailed geological and geomorphological surveys, in-situ and laboratory tests, and numerical simulation are carried out to reveal the failure mechanism of this landslide. The anaclinal rock slope is characterized mainly by a set of outward dipping and non-persistent joints, a hard stratum (limestone) in the middle portion and an underlying soft base (argillaceous limestone interbedded with shale). Because of the long-term intensive river erosion caused by periodic fluctuation of the reservoir water level, the soft rock masses at the toe of the slope were softened, or even washed away, inducing the first sliding-toppling failure at the toe. The collapse at the toe leaves the hard strata in the middle unsupported, like a “beam”. Thus, a conceptual “cantilever beam” model is proposed to explain the failure mechanism of the landslide. The DEM simulation further highlights the failure process with a complex rock topple-rock slide failure mode, i.e., first composite sliding and toppling occurred at the toe, a composite shear and flexure failure of the middle “cantilever beam” and a simultaneous overall sliding failure of the rock masses in the upper part of the slope. The results imply that preventing water erosion at the toe could be an effective way in landslide prevention and treatment in the study area.