Study On Evolution Mechanism And Deformation Prediction Of Chair Shaped Soil Landslide In The Three Gorges Reservoir Area

As one of the large-scale water control projects,a large number of landslides in the reservoir area of the three Gorges Dam induce deformation after the work of the dam.Based on a analysis of the distribution characteristics and profile shape of landslides in the reservoir area,it is found that there are similar landslides in the same basin in many aspects,such as the chair shaped soil landslides.The sliding surface of the landslide is slow in front and steep behind,the sliding bed is oblique and introverted bedrock,and the sliding surface is the overlying interface.After the impoundment of the reservoir,the deformation of this kind of landslide is large,and the development of cracks shows a trap trend.Only the macro deformation is a sign of imminent slide,which makes the stability of this kind of landslide difficult to warn.Once the landslide is destroyed,it will have a great impact on the roads,pedestrians and Yangtze river shipping.Therefore,it is necessary to conduct in-depth analysis and research on this kind of landslide in order to provide theoretical support for the deformation development trend of this kind of landslide and to provide guarantee for the normal and safe operation of the reservoir area.This paper takes the chair shaped soil landslide--Bazimen and Baijiabao landslides in the three Gorges Reservoir area as the main research objects,and carries on the research from the following three aspects.Firstly,by comparing with typical landslides in the reservoir area,the formation conditions are analyzed from three aspects: stratigraphic,topographic and geological structure,and the evolution process of this kind of landslides is inferred according to the formation conditions and the evolution of rivers and valleys.Secondly,based on the monitoring data,the deformation characteristics of this kind of landslides under the influence of reservoir water fluctuation and rainfall are analyzed,and the deformation mechanism of this kind of landslides is obtained.it is supplemented and verified by numerical simulation.Finally,based on automatic monitoring data,using the statistical method to this kind of landslide displacement step in the threshold of set,and integrated the landslide formation and evolution of the induced deformation characteristics,using qualitative analysis method to this kind of landslide deformation damage situation,the development trend of the future.The main conclusions are as follows:1)Three conditions should be met for the development of the chair shaped soil landslide.First,it should be developed in the canyon area.The flow velocity and volume of the leading edge of the river should not be too large.Secondly,the main slope is therock with poor weathering resistance.Finally,it is generally developed at the edge of synclinal tectonic units,and the tectonic units are uniformly uplifted without large deformation.The chair shaped soil landslide in Xiangxi river basin is formed gradually with the evolution of the river valley.Under the influence of the regional structure from east to west,the river gradually eroded and cut down to the left and the lower part of the river so that the leading edge of landslide has enough space to accumulate various materials,thus forming the chair shaped soil landslide.2)The whole starting of the landslide is mainly affected by the decline of the reservoir water level,and the rainfall has a significant effect on the deformation of the landslide.Under the influence of reservoir water level rise and fall,the whole deformation of the chair shaped soil landslide shows the characteristics of intermittent start.The overall step of the landslide is mainly caused by the seepage pressure and hydrostatic pressure pointing to the air direction when the reservoir water drops.3)The numerical simulation shows that the infiltration line inside the sliding body will gradually increase when the reservoir water rises,but the decreasing range of the infiltration line in the sliding body is not obvious when the reservoir water decreases.The decline of reservoir water level will cause the stability of this kind of landslide to decrease obviously,while the rise of reservoir water level will improve the stability.The displacement of the sliding body below the water level of this kind of landslide reservoir is affected by its wading height and physical properties.4)Through the analysis of the automatic monitoring data,it is found that the average minimum value of the starting threshold of this kind of landslide affected by the decline rate of reservoir water is 0.2m/d,indicating that the whole landslide will be significantly deformed after the continuous decrease of the reservoir water level.The influence of rainfall on this kind of landslide varies with the looseness of landslide slope.5)Under the action of reservoir water,the time of displacement step,the magnitude of step and the difference of internal water head are related.Rainfall infiltration will affect the duration of head difference and thus affect the displacement step.On the premise that the water level of the reservoir does not change,the deformation step of this kind of landslide starts with the decrease of the reservoir water.