Landslides located beside reservoirs tend to be unstable or are characterized by large deformation during the drawdown process. This has been accepted by many experts. In this paper, we use Qiaotou Landslide, which is located beside the Three Gorges Reservoir (TGR), as a typical case study to investigate and predict the deformation mechanism during the drawdown process of TGR in detail. According to field investigation, the landslide mass is mainly composed of thick, loose silt and clay mixed with fragments of rock. Bedrock is mainly composed of silty sandstone. Field and laboratory tests indicate that the landslide mass has a high permeability coefficient. If the water level declines fast, intense seepage force may result. Based on these data, we establish a three-dimensional geological model of Qiaotou Landslide by FLAC 3D and perform a numerical simulation using the saturatedunsaturated fluid-solid coupling theory. For the simulation, we assume that the drawdown from 175 to 145 m takes place with a speed of 25 cm/day, which is based on the extreme water level regulation program of TGR. The simulation shows that this causes a significant deformation in the landslide mass and that the maximum displacement within the landslide is 24.2 cm. During the drawdown process, the maximum displacement zone is shifting from the upper part of the landslide where bedrock surface is steeper and thickness of loose deposits is less to the middle part of the landslide where bedrock surface is less steep and thickness of loose deposits is higher. The deformation mechanism indicates that in the early stage of the drawdown the deformation of the landslide mass is mainly caused by seepage and in the later stage mainly by consolidation.

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As groundwater activity develops in a landslide system, the water-rock (soil) interaction increasingly influences the development of the landslide hazard. In this paper, Huangtupo landslide was chosen as the subject of our research, which is located beside the Three Gorges Reservoir and the deformation continued for a long time. First, based on a comprehensive field survey, ten types of bad geological elements which probably induce the deformation were collected from different parts of landslide. These include a soft rock layer, the sliding soil, a weak intercalation and the typical rock close to the slip zone. Then, to trace the internal relationships among these samples, the microstructure, chemical composition of minerals, the migration and evolution of clay minerals, also the particle size distribution characteristics of all the samples were analyzed separately. Comparison of the result shows, that the evolutionary paths are very obvious among these samples. With the activities of water-rock (soil) interaction, the formation and evolution of main bad geological elements can be summarized. Afterwards, the detailed mechanism of interaction is revealed by focusing on four samples which compose one of these three paths, using additional data from physical and mechanical tests, the study shows how water-rock (soil) interaction affects the microstructure and weakens the mechanical properties of rock and soil. The presented detailed research probes the water-rock (soil) interaction mechanism specifically for Huangtupo riverside landslide as a case study. Furthermore, the applied multi-sample dynamic tracing method is verified as a means to discover and illustrate the deformation mechanism of landslides.