Abstract
Filling of Xiangjiaba Reservoir Lake in the Southwest China triggered and reactivated numerous landslides due to water fluctuation. In order to understand the relationship between reservoirs and slope instability, a typical reservoir landslide (Dasha landslide) at the right bank of Jinsha River was selected as a case study for in-depth investigations. The detailed field investigations were carried out to identify the landslide with respect to its surroundings and to find out the slip surface. Boreholes were drilled to find out the subsurface lithology and the depth of failure of Dasha landslide. The in situ geotechnical tests were performed, and the soil samples from exposed slip surface were retrieved for geotechnical laboratory analysis. Finally, stability analysis was done using the 3D strength reduction method under different conditions of reservoir water level fluctuations and rainfall conditions. The in-depth investigations show that the Dasha landslide is a bedding rockslide which was once activated in 1986. The topography of Dasha landslide is relatively flat, while the back scarp and local terrain is relatively steep. The total volume of landslides is about 580×104 m3 with an average thickness of 20 m. Bedrock in the landslide area is composed of Suining Formation of the Jurassic age. The main rock type is silty mudstone with sandstone, and the bedding orientation is 300~310° ∠ 7~22°. The factor of safety (FOS) of Dasha landslide obtained by 3D strength reduction cannot meet the minimum safety requirement under the working condition of reservoir level fluctuation as designed, with effect of rainfall and rapid drawdown.
Highlights
Landslides are common natural hazards which include a wide range of ground movement, such as rockfalls, deep failure of slopes, and shallow debris flows, which can occur in offshore, coastal, and onshore environments
Until 1963, the reservoir landslides were not studied in depth, and the first detailed studied landslide (Vaiont landslides in Italy) became the benchmark for reservoir slopes studies
It is revealed through drilling that the landslide slope is composed of clay, silt with some rock fragments, and the underlying bedrock orientation is 310°∠15°. This landslide was reactivated during the rainy season in 1986, resulting in structural damages to homes and the ground surface
Summary
Landslides are common natural hazards which include a wide range of ground movement, such as rockfalls, deep failure of slopes, and shallow debris flows, which can occur in offshore, coastal, and onshore environments. Gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Various studies have been done on reservoir slopes; there are two common factors which are always taken into consideration which include the effect of reservoir water on the slope and the corresponding effect of landslide on reservoir if slope fails. Particular considerations include the following: (i) reservoir slopes are subjected to load changes as a result of inundation when the reservoir is filled and subsequently as a result of variations in lake level during operations,. (ii) the reservoir impounding affects the slope material properties and generates more adverse groundwater conditions than previously existed within the slope, creating reservoir-specific changes that can lead to instability, and (iii) the reservoir can increase the probability and consequences of failure, which may include total or partial blockage of the reservoir, the possibility of damaging impulse waves which may have effects that extend beyond the reservoir [1].
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