| Landslides are major geological hazards that are second only to earthquakes,which can cause serious economic and environmental damage.The complex geologic environment and climatic condition have led to the frequent occurrence of geological disasters in the Three Gorges Reservoir area.Especially affected by the reservoir filling,lots of landslides occurred and caused huge economic losses and casualties.Among them,bedding rock landslide are one of the most widely developed and most harmful types.The long-term operation of the reservoir and more frequent heavy rain will undoubtedly threaten the long-term stable state of consequent rock bank slope widely distributed in the reservoir area.In recent years,landslides in the reservoir area have been characterized by miniaturization,concealment,and randomness.The causes and laws of these landslides are also quite different from those in the past,and they have become difficult problems in disaster prevention and reduction work.In this paper,the Shanshucao bedding rock landslide is taken as the research object,and a geomechanical model is established based on on-site geological survey and analysis.In addition,a great many of laboratory tests were conducted to study the formation and evolution rules and strength attenuation laws of slip soil under the influence of reservoir water and rainfall,and the residual strength characteristics of the slip zone soil under different test conditions and its microscopic mechanism.Finally,numerical simulation techniques were used to study the effects of rainfall and reservoir water level changes before landslides on the seepage field and stability of landslides,and to study the landslide deformation process based on the shear strength characteristics of sliding zone soils.The following conclusions and research results have been obtained:(1)Study on geomechanical model of Shanshucao landslide.Based on the investigation of the engineering geological conditions and the occurrence process of the Shanshucao landslide,the material composition and deformation characteristics of the landslide are analyzed.It is concluded that the Shanshucao landslide consists of the main rock sliding zone and the soil traction zone,the volume is 55.1×10~4m~3,with a maximum sliding distance of 160m,the direction of the main sliding zone shifts from107°to 78°during sliding,with an average sliding speed of 53mm/s and the front mass slides into the river of Luogudong.The geological factors and environmental factors that formed the landslide were analyzed,and the geological model of“unconfined pressure limit”that constructed landslide was established:the siltized intercalation was used as the bottom sliding surface,and two free face and two sets of separated structural planes as the boundaries.The mechanical model of“upper thrust and lower uplift”that induced landslide was constructed:rainfall infiltration resulted in weight increase of slip mass,increase of seepage force,decrease of shear strength of slip soil,formation of hydrostatic pressure in trailing edge fissures,and formation of uplift pressure at the rear.rapid rise of reservoir water level formed uplift pressure in the front.The Shanshucao landslide is a neogenic/thrust load caused/middle-scale/thick-layer/consequent/rock/fast/reservoir landslide.(2)Experimental study on formation and physical-mechanical properties evolution of slip soil.The results of microscopic tests on different rock masses in the landslide area showed that the clay mineral content in the siltized intercalation slip soil(48%)was higher than the residual slope accumulation layer slip soil(21%),and the particles are finer and more uniform.The disintegration and argillization tests of argillaceous soft rock under different dry and wet cycle conditions and water environment show that the disintegration and argillization rate of silty mudstone with higher clay mineral content is higher than that of shaly siltstone,and acidic water environment will accelerate the process of disintegration and argillization.The material basis,rock formation structure and dynamic conditions of the formation of the siltized intercalation slip zone were analyzed.It was shown that the formation of the slip zone experienced the complexity process of“interlayer argillaceous soft rock-shear fracture zone-siltized intercalation-slip zone”.The physical tests and shear tests under different conditions of water content,number of wet and dry cycles,and soaking time for two kinds of slip soils show that the shear strength decreases rapidly with the increase of water content.And with the increase in number of wet and dry cycles and soaking time,some of the detrital minerals evolved into clay minerals,and coarse particles converted into fine particles.At the same time,both the peak and residual shear strength exhibited significant attenuations at first and then slowly.(3)Experimental study on residual strength characteristics of slip soil.The ring shear tests under different test conditions for the two slip soil of Shanshucao landslide show that the higher the coarse particle content and water content,the rougher the shear surface.As the shear rate increases,the shear surface of the siltized intercalation slip soil becomes smoother,while the residual slope accumulation layer slip soil is opposite.The integrated residual internal friction angle of slip soil with the increase of coarse particle content and the decrease of water content.The response of the residual strength of the two slip soils to the shear rate is different.The integrated residual internal friction angle of siltized intercalation slip soil has a logarithmically negative correlation with the shear rate,while that of the residual slope accumulation layer slip soil is opposite.Compared the result of triaxial tests with pre-cut planes for saturated siltized intercalation slip soil and back analysis of parameters based on geological sections in main sliding zone with the ring shear tests and repeated direct shear test,concluded that the value of the integrated residual internal friction angle is reduced gradually according to the pre-cutting plane triaxial shearing,back analysis(limit equilibrium state),repeated direct shearing,and ring shear tests,and its integrated value in the state of limit equilibrium and overall instability are 21.5°and 20.5°respectively.(4)Study on microscopic mechanism of residual strength characteristics of slip soil.In the process of ring shear test,the phenomenon of coarse particle breaking and fine particle aggregation occur on the shear plane,resulting in delamination of the slip soil and uneven distribution of particles in the vertical and horizontal directions.By testing the particle size at different positions on the shear plane,it can be concluded that the change rate of the particle on the shear surface decreases linearly with the increase of coarse particle content and the decrease of water content,increases logarithmically with the increase of shear rate,and the particle change rate gradually decreases from the inside to the outside of the shear plane.Based on the microscopic observation and analysis of the shear plane,it is concluded that the increase in coarse particle content and the reduction of the water content will cause the attenuation of orientation,reduction of roundness and increase of porosity.The increase of shear rate will lead to the increase of orientation of the siltized intercalation slip soil and the weakening of the residual slope accumulation layer slip soil.These factors influence the strength of the sliding soil by changing the roughness of the shear plane.Based on the fractal study of the shear surface,it is shown that the shear surface soil particles have obvious fractal characteristics,and the residual strength has good quadratic function positive correlation with the fractal dimension.(5)Study on failure process and mechanism of Shanshucao landslide.Based on the impact analysis of the reservoir water level rise and precipitation process before the failure on the seepage field and stability of the landslide,it is concluded that the persistent heavy rainfall is the main precipitating factor,and the rapid rise of reservoir water level is not conducive to the stability of landslide,the coupling of the two factors leads to the overall instability of the Shanshucao landslide.At the same time,the presence of separation fracture at the trailing edge of the landslide strengthens the effect of rainfall.Based on the shear strength characteristics of slip soil,the failure and deformation process of Shanshucao landslide was analyzed.It was shown that the negative rate effect of residual strength of siltized intercalation slip soil was the reason for the rapid and large-displacement sliding of Shanshucao landslide after instability.The positive rate effect of the residual strength of residual slope accumulation layer slip soil will limit the deformation and slip of landslide. |
PDF Full Text Request