Study On Unsaturated Shear Strength Of Sliding-zone Soil Of Majiagou Landslide Based On Dehumidification

Majiagou landslide is a typical landslide in the Three Gorges reservoir area.It has been deformed since the Three Gorges reservoir began to store water in 2003.The Three Gorges Reservoir is located in the area with abundant rainfall,and the water level fluctuates periodically from 145 m to 175 m all the year round,which causes the rock and soil mass in the water-level-fluctuating zone to undergo drying-wetting(hereinafter referred to as D-W)cycle change continuously.This long-term groundwater effect makes the strength of the rock and soil mass in the bank slope of the reservoir gradually reduce,thus weakening the stability of the bank slope of the reservoir.Once the landslide occurs,it will be a threat to the life and property of the people around.Part of the deep sliding-zone of Majiagou landslide is located in the water-level-fluctuating zone,and the soil in the sliding-zone is in the state of saturated-unsaturated cycle for a long time.The saturated strength of the sliding-zone soil has been widely studied,but the unsaturated strength of the sliding-zone soil under D-W cycle needs further study.In view of the importance of research on unsaturated shear strength of sliding-zone soil under D-W cycle,the sliding-zone soil of Majiagou landslide is taken as the research object in this thesis.Through the unsaturated direct shear test,D-W cycle test and soil-water characteristic curve test,the variation law and meso-mechanism of moisture content of sliding-zone soil under D-W cycle are revealed.Based on the test results,Fredlund’s bivariate theory and suction stress theory,a formula for predicting the unsaturated strength of Majiagou sliding-zone soil under D-W cycles is proposed.The evolution law of displacement field and stability of Majiagou landslide under the fluctuation of reservoir water level is analyzed by numerical simulation method.The main research contents and conclusions are as follows:(1)Based on the D-W cycle test,the change law and meso-mechanism of moisture content are revealed.The change law of moisture content with cycle number is exponential attenuation,and the moisture content is negatively correlated with time in the process of dehumidification.The equation of moisture content change s is obtained by data fitting,which can predict the moisture content under any cycle number and time.The quantitative analysis of fracture development shows that the fracture formation is mainly characterized by the development of new micro-fractures in the first cycle,the expansion of micro-fractures to small fractures in the second cycle,and the expansion of original fractures in the third cycle.At this time,the fracture rate continues to increase and the average fracture width decreases gradually.After 5 cycles,the original fracture expands to a certain extent and gradually connects,the fracture network forms,and the fracture development tends to be stable.The meso-mechanism of moisture content change under D-W cycle is: The fine particles and clay particles are gradually lost,and the original fissures are developed and accompanied by new fissures,which leads to the decrease of water holding capacity of soil,which is manifested as the decrease of water content.(2)Based on the soil-water characteristic curve test and unsaturated direct shear test under different water content,the unsaturated shear strength characteristics of sliding-zone soil are studied.The VG model is used to fit the soil-water characteristic curve.The air intake value of sliding-zone soil increases with the increase of normal stress,which shows that the increase of normal stress makes the soil compacted and the drainage more difficult.The parameters related to pore size distribution decrease with the increase of normal stress,and the pore size distribution becomes more and more inhomogeneous.Stress-strain curves of different moisture content sliding-zone soil show hardening type.With the increase of moisture content,shear strength and shear strength index decrease continuously with the increase of moisture content,and the decrease rate is faster when the moisture content is 14～18%.The curve of matric suction-water content was drawn by measuring matric suction of shear section after direct shear,and the unsaturated strength was measured.establish a relationship chain of shear strength-matric suction-water content,deducing a prediction formula of that unsaturated strength of the sliding-zone soil based on the Mohr-Coulomb criterion and the suction stress theory,converting the change of the strength into the change of the wat content,it lays a foundation for the analysis of landslide stability.With the increase of water content,the shear surface becomes smoother and smoother,and the scratch arrangement becomes more and more regular.When the water content is small,there are obvious cracks on the shear surface,the shear surface is rough,and the scratch width is larger.With the increase of normal stress,the shear surface becomes more and more smooth,and the width of scratch decreases gradually.When the normal stress is small,the shear surface produces uneven cracking,the crack is obvious,the opening is large,and the crack development direction is oblique to the shear direction.(3)Deformation and stability analysis of Majiagou landslide considering sliding-zone soil unsaturated strength attenuation with the increase of D-W cycles caused by reservoir water fluctuation.The total displacement of landslide increases with the increase of the times of D-W.Without considering the strength attenuation,the displacement of landslide increases more in the first 4 cycles,and the maximum displacement of landslide tends to be stable after 5 cycles.It shows that the strength deterioration of sliding zone soil will accelerate landslide deformation under the fluctuation of reservoir water level.With the increase of D-W cycle times,the stability coefficient of landslide body has a significant decline trend,which is consistent with the law of soil strength degradation.The stability coefficient decreased by 0.25 after 5 cycles without considering strength reduction,and finally reached stability around 1.13,while the stability coefficient decreased after 3 cycles without considering strength reduction.Under the action of long-term fluctuation of reservoir water level,the deterioration of strength of sliding zone soil will further deteriorate the slope stability.