Research On The Response And Failing Process Of Unsaturated Residual Soil Slope Under Rainfall Infiltration

Rainfall-induced landslide often caused catastrophic disasters,and such disasters regularly bring about significant casualties and property losses.For this reason,the prevention or mitigation of landslide hazards through the scientific research is paid more and more attention by academics and engineers.Hence,it is important and practical to conduct a comprehensive and systematic research on the response and failing process of Southeast Fujian unsaturated residual soil slope under rainfall infiltration,which are based on the methods of laboratory test on unsaturated residual soil as well as physical model test and numerical simulation,then discuss the selection of slope instability early warning factor and the tactics for reducing disasters.The main research work and creative results in this dissertation are summarized asfollows:Firstly,the Stress-dependent pressure plate extractor system was used to study the residual soils SWCC curve of different vertical stress,drying and wetting cycles and different types of soils.The laws of SWCCs hysteresis loop area with and without volume change and AEV and AExV of residual soils are analyzed and compared,and then point out that not only the ability of soil to resist deformation will be overestimated by the traditional method of SWCC curve,but also the underlyingcause of SWCC curve changing is due to soil internal structure change constantly.Meanwhile,according the simplicity of model parameters and the goodness-of-fit analyze results of the 3 kinds of improved SWCC models,the improved VG model has been found to be the most suitable model for all SWCCs of the natural residual soil in southeast of Fujian.Secondly,the triaxial test of unsaturated soil was used to study the shear characteristics and shear strength of unsaturated soil.The knowledge of the soil structure,effective stress principle,mineral composition and hydraulic hysteresis characteristics are used to reveal the change laws of unsaturated soil mechanics characteristics under different drying and wetting cycles.And the change of soil saturation and the irreversible deformation(or accumulation of irreversible deformation),which are caused by the hydraulic hysteresis characteristics,should be considered together to explain the soil is softened by water.In addition,the phi b,which reflected the relationship between shear strength and matrix suction,and the effective stress parameters are gradually reduced after repeated wetting cycels.Thirdly,on the basis of analyzing the characteristic of Southeast Fujian granite residual soil,the influence mechanism of water content and dry density on shear strength parameters and the relativity between water content,dry density and shear strength parameters of three kinds of unsaturated residual soil(residual cohesive soil,residual sandy clay,residual gravelly clay)are analyzed.Then,the empirical formula between water content,dry density and cohesion of granite residual soil are established.Results show that the interrelationship between the logarithm of residual soil cohesion and water content is linear;the internal friction angle has a nonlinear damping trend with water content increases,and changes with a limit scope;when the residual soil had a high compactness,the curve of ?-? has a marked peak value in three kinds of residual soil;the effects between compactness and cohesion of residual soil can be expressed by a linear positive correlation formula.Forthly,the rainfall simulation system and real-time monitoring system were used to make a comprehensive and multiple parameters real-time monitoring of rainfall-induced unsaturated residual soil slope failure process,the real-time response of the soil water content(saturation)and matric suction as well as soil pressure and slope sliding force of soil slope with different loaction,different slope angles,different density and different rainfall conditions are summarized and analyzed,and point out that the areas of slope internal high stresses can be fully released after repeated rain,that is,a high level of residual slide force will be happened and the effect of soil softening will be even more significant.According to the experimental data which are re-disposed carefully,the relationship between the matric suction and water content of the SWCC curve transition zone and the laws of infiltration rate and wetting front under different working conditions are analyzed emphatically,results show that the suction logarithm and logarithmic volume water content have a linear connection through the soil parameters a and b,furthermore,both a and b has a linear relationship.Then,the mechanisms of soil parameters a and b caused by different working conditions are discussed.Next,the modified Lumb formula was proposed subsequently by comparing the scatter distribution of wetting front actual and semiempirical values.Combined with the response laws of different positions soil suctions under rainfall infiltration and the rule of body deformation,three stages of slope deformation development were put forward,namely initial creep stage,accelerated development stage and sliding failure stage.At the same time,it is pointed out that the selection of slope instability warning factor should set out from the instability mechanism of rainfall-induced landslides,and focus on the mechanics parameters change of slope key position.Then,it shows that the matric suction of slope key position can be choosed as the slope instability warning factor for the unsupported slope,while,not only the change characteristics of matric suction but also the supporting structure stresses must be considered together as the instability warning factor selection for the supported slope.Finally,based on the theory of unsaturated soil mechanics,the finite element method was used to calculate and analyze the slope seepage,deformation properties and stability under rainfall infiltration,the calculation results was utilized to study dynamic response mechanism of landslide due to rainfall and the relationship between safety factor and displacement under different real working conditions.In the initial stage of the heavy rainfall,the safety factor of the slope dropped rapidly,which can easily cause the landslide;the safety coefficient reduced further due to the continued infiltration of rainwater and landslide occurred after rainfall should be paid more attention;safety factor was affected and vibrated with water in different parts of the slope.From the perspective of safety stock,the concept of risk factor,which was based on the selection of stability factor as total factor of slope failure mechanism,was proposed to reflect the harmful degree of slope under groundwater level and rainfall condition variation.Then,the relationship between the risk factor andincremental displacement was established to describe the changes of surface displacement of slope measure point at different time periods.The above research and method will be usefull and meaningful as reference for the management and early warning of the similar projects.