Study On Characteristics Of Slip Zone Soil And Failure Mechanism Of Huangtupo Landslide In Three Gorges Reservoir Area

Landslide is one of the significant geological disasters affecting human life and property safety,especially in China's Three Gorges Reservoir Area.Therefore,it is of great practical significance to further study the mechanism of landslide failure and deformation.As an essential part of landslide,slip zone soil is one of the most critical control factors for landslide formation,development,and evolution.Strength parameters of slip zone soil are essential parameters for landslide stability evaluation and anti-sliding engineering design.The study on residual strength characteristics and water-force interaction of slip zone soil has important practical significance and guiding value for landslide disaster prevention and control.The method of correctly analyzing landslide failure and failure process based on residual strength characteristics of landslide soil and water-force interaction is a common concern of academic and engineering circles and a subject worthy of in-depth study.The selection of the topic in this thesis is based on the National Natural Science Foundation of China Major Program(Basic Research on Major Landslide Prediction and Forecast)and the National Key Research and Development Program of China(Key Technologies and Standardization System of Landslide Prevention and Control Based on Evolution Process).Further and complete research on Riverside Slump#1,the most unstable part of Huangtupo landslide,was conducted based on the in-situ investigation tunnel.The relative study resources,such as soil samples and in-site experiments,are conducted in the comprehensive field test site of the Huangtupo landslide to study the Riverside Slump#1.The mechanism of landslide failure based on residual strength characteristics of slip zone soil and water-force interaction is studied comprehensively utilizing laboratory tests,theoretical derivation,and numerical simulation.Ring shear test,microstructure test,soil-water characteristic test,and unsaturated direct shear test were carried out in the laboratory.Established the seepage field model of Huangtupo landslide based on the experimental results and theoretical analysis.The failure process and final state of Huangtupo landslide are systematically studied by numerical analysis method.The main research contents and achievements are as follows:(1)Study on residual strength characteristics of slip zone soil.Aimed at the scene of the slip zone soil specimen,the system was carried out by ring shear test to reveal the slip zone soil under the condition of large shear displacement of residual strength characteristics,focus on the coarse particle content,moisture content,and shear rate on the influence of soil shear properties.The results show that the slip zone soil exhibits strain-softening shear behavior under large displacement shear.The equivalent internal friction angle increases with the increase of coarse particle content and the decrease of water content and is negatively correlated with shear rate.The law of strain-softening is different from that of common clay.Also,the vertical settlement of slip zone soil during shearing is related to normal stress,coarse particle content,water content,and shear rate.Based on the test,the shear strength parameters of slip zone soil are calculated by the inversion method according to the actual section of the Huangtupo landslide and compared with the test results.It is considered that the ring shear test is an ideal test method for obtaining the residual strength index of slip zone soil.(2)Research on micro-characteristics of slip zone soil.Through micro-observation and analysis on the shear surface before and after the ring shear test,it is concluded that the increase of coarse particle content and decrease of water content will cause the decrease of grain orientation,roundness,and porosity of shear surface soil.The increase of shear rate will cause the increase of directional degree in the clay interlayer of slip zone soil and the decrease of the directional degree in the residual slope layer of slip zone soil.These factors change the shear surface.The roughness affects the strength of the slip zone soil.The exertion mechanism of soil strength in the slip zone is revealed from a micro perspective.Established a microstructure model of residual strength of slip zone soil based on fractal theory.The test shows that the shear plane soil particles have obvious fractal characteristics.The residual strength of the slip zone soil has a good positive quadratic function relationship with the fractal dimension of the shear plane particles.(3)Soil-water characteristics of slip zone soil.With the rise and periodic change of reservoir water level,some areas of the reservoir wading landslide body are saturated and unsaturated wet and dry circulation.Various soil-water characteristics tests were carried out on the remolded slip zone soli with different pore structure characteristics in the full suction range.The test results show that the soil-water characteristic curves in the full suction range can be well measured by combining pressure plate,filter paper,and steam balance methods.The soil-water characteristic curve with roughly the same initial pore ratio has obvious hysteresis phenomena in the low suction range's dehumidification and moisture absorption curves.The regularity of the pore ratio with the change of suction is affected by the size of suction and the suction history and suction path.Because of the different suction paths,the relationship between the pore ratio and the saturation is not one-to-one;the dehumidification.It presents an obvious hysteresis effect in the hygroscopic path,which tends to be consistent near saturation.Use the vapor balance method to measure the saturation within the suction range.Under different suction conditions,the saturation increases with the increase of the pore ratio.The relationship between the pore ratio and saturation is approximately linear.(4)Research on unsaturated strength characteristics of slip zone soil.The relationship between unsaturated shear strength and saturation of slip zone soil is studied,and the formula of shear strength of unsaturated slip zone soil is put forward.The comparison between the predicted strength results with the actual measurements shows that the unsaturated shear strength of slip zone soli increases first and then decreases with saturation under the same positive stress.And there is a peak value.The internal friction angle has little change with the saturation and can be regarded as a constant.It would roughly reflect the trend of the shear strength of unsaturated slip zone soil in the full suction range,while suction stress?_s is represented by(u_a-u_w)S_rs and(u_a-u_w)(S _r-S _rf)/(1-S _rf)in the equation of unsaturated soli-shear strength.The prediction result of the unsaturated soli-shear strength equation is affected by?,when suction stress?_s is represented by(u_a-u_w)S_rs.When suction stress?_s is represented by(u_a-u_w)(S _r-S _rf)/(1-S _rf),the prediction result of the unsaturated soli-shear strength equation is not affected?.(5)Prediction method based on dilatancy equation and peak state.Analogy in saturated soil Nor-sand model framework,obtained under different moisture content is suitable for the unsaturated compacted slip zone soil constitutive model of shear deformation characteristics.Through comparison with test results,verify the suggested model can better simulate the unsaturated compacted slip zone soil under different moisture content of the peak strength and dilatancy properties.(6)Research on failure deformation process and mechanism of the Huangtupo landslide.According to geological field survey,utilizing comprehensive research combined with field monitoring and numerical simulation.The influence of reservoir water fluctuation process on seepage field and stability of landslide shows that the result of comprehensive mechanical mechanism action caused by reservoir water level change is dominated by hydrostatic pressure.Since the stability coefficient of the slip zone will periodically drop below one during the period of reservoir water level change,the sliding mass intermittent slippage persists.