Study On The Progressive Failure Probability Of Baishuihe Landslide Under The Effect Of Rainfall And Reservoir Water Fluctuation

For the slopes which are impacted by the reservoir water in the Three Gorges Reservoir, the strength of the geomaterials would be weaken under the influence of the rainfall and reservoir water, further more, it will break the stability of the slopes. Studies on these slopes have focused on the change of the seepage field which were induced by the rainfall and reservoir water fluctuation, litter research has been done on the failure probability of the slope when considering the randomness of the geomaterials. It’s a long-term and slow process during the broken of the slope, so carrying out research on the progressive failure of these slopes which would be helpful to understand the evolution characteristics in the process of geological history.This dissertation presents the Baishuihe landslide as a typical research object, taking the rainfall and reservoir water effect into considering, and based on the information of the displacement field, the quantitative analysis has been done between the evolution time and the deformation, thus the main control factor can be identified during the period of evolution. The analytical solution of the phreatic line under the fluctuation of the reservoir water has also been derived, and combined with the Green-Ampt infiltration model, the formulation for calculating the depth of the wetting front is found, then the Safety Redundancy function for the slice could be developed. When considering the spatial random of the strength of sliding zone soils, the progressive failure law of the Baishuihe landslide is obtained at any reservoir level with any rainfall duration. The main contents and findings are as follows:(1) With the geotechnical investigation of the Baishuihe landslide, the fundation essential features and formation mechanism has been confirmed, which contains the stratum lithology, geological structure, geomorphology and other factors. The Easily-sliding stratum and listric feature of the geomorphology are beneficial to the sliding, so monitoring and forecast should be strengthened. And a new monitoring system has been estabilished on the Baishuihe landslide, including the distributed optical fiber which is perpendicular to the sliding direction lies in the anterior part of the landslide, the boreholes for the borehole clinometer along with the sliding direction and the GPS station, based on these monitoring ways the relationship between the deformation and environmental changes could be easily acquired. The results show that the deformation has close relationship with the environmental changes, and the deformation in the eastern is greater than the western, the deformation in the anterior part is bigger than that in the rear edge.(2) The grey correlation method is used to quantitative analysis the sensitivity of the deformation, and the monthly mean rainfall intensity, the monthly mean reservoir level and the monthly accumulative displacement are selected as the response factors, so the factor which play a major role in the deformation could be found during different geological history period. The calculation indicates the Baishuihe landslide demonstrates high sensitivity with the rainfall and the reservoir water fluctuation, the deformation is largely concentrated form june to july every year with the drawdown of reservoir water and abundant rainfall, and the deformation response lags behind the reservoir water one month. Overall, the sensitivity of the deformation can be describe in two stages divided by 2007, which showed high sensitivity with the reservoir water fluctuation before 2007 and high sensitivity with the rainfall after 2007.(3) To achieve the analytical solution of the phreatic line, the landslide is divided into two parts, including the front and the crest of the bank slope, and two formalas are derived form the empirical formula of the highest point of the hydraulic grade line and the Boussinesq’s seepage formula, which consider the influence of the inclind floor and the inclind surface, have the advantages of both high applicability and accuracy when compared with the numerical solution form the software Geo-Studio. The effect of reservoir water fluctuation velocity and hydraulic properties of soil include the permeability coefficient and the specific yield are also investigated through a series of calculations with different parameters based on the two formalas. The results show that all theses three variables have significant effect on the hysteresis effect of the phreatic line, the hysteresis effect would increase as the water fluctuation velocity and the specific yield are increased, whereas it decreases as the permeability coefficient is increased.(4) The process of rainfall infiltration is analyzed based on the Richard model and Green-Ampt model, then a new method according to the Green-Ampt model is proposed for the stability analysis of the slope under the rainfall, which discusses the influence to the safety factor of the slope whether the groundwater level exists. The evaluation index, the calculation of the safety factor should be carried out on the surface of the bedrock and the wetting front, and selecting the smaller value as the actual safety factor which deponds on the geometries and the mechanical parameters.(5) Based on the theory of reliability, the spatial random of the strength of sliding zone soils could be taken account to study the progressive failure law of the Baishuihe landslide at any reservoir level with any rainfall duration. The results show that the safety factor would decrease as the water drop from 175m to 145m, and it decrease as the rainfall duration increases; nevertheless, the progressive failure probability for the slice would increase by the increasing of the rainfall duration; in general, the progressive failure law of the Baishuihe landslide is divided into three stages, which starting from the crest and expand to the middle then stops, the evolution pattern performs as a thrust load caused landslide.The main innovations of this dissertation are as follows:(1) The analytical solution of the phreatic line is established vias the empirical formula of the highest point of the hydraulic grade line and the Boussinesq’s seepage formula, and combined with the Green-Ampt infiltration model, the Safety Redundancy function for the slice is developed which could be used to evaluate the stability of the slope.(2) Based on the theory of reliability, and combined with the Safety Redundancy function for the slice, the spatial random of the strength of sliding zone soils could be taken account to study the progressive failure law of the Baishuihe landslide at any reservoir level with any rainfall duration.